Data encoding and decoding

Abstract

A data encoding method is provided, the method comprising: selectively encoding a flag when encoding image data having a source chroma format (such as 4:4:4 or 4:2:2) as frames represented in a bits

Description

BACKGROUND Field This disclosure relates to data encoding and decoding. Description of Related Art The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, is neither expressly or impliedly admitted as prior art against the present disclosure. There are several systems, such as video or image data encoding and decoding systems, which involve transforming video data into a frequency domain representation, quantising the frequency domain coefficients and then applying some form of entropy encoding to the quantised coefficients. This can achieve compression of the video data. A corresponding decoding or decompression technique is applied to recover a reconstructed version of the original video data. Efficient encoding and decoding of video content is required in applications such as entertainment or communication. In some situations, a decoding system may only be capable of supporting certain profiles. For example, a decoder may be capable only of supporting profiles with a certain chroma format, such as 4:2:0. However, there is a desire to provide content with higher levels of colour information. For example, high levels of colour information, such as a 4:4:4 or 4:2:2 format, may be desirable for certain situations. As such, there is a demand to improve distribution of content with high levels of colour information, while retaining compatibility with decoding systems with limited profiles. It is an aim of the present disclosure to address these issues. SUMMARY Aspects of the present disclosure are defined by the appended claims. Further respective aspects and features of the present disclosure are defined in the appended claims. In accordance with embodiments of the disclosure, an image can be efficiently encoded to generate a bitstream, where the different spatial regions within frames of the bitstream, having a first chroma format, can be used in order to reconstruct an image having a source chroma format. Thus, improved compatibility and distribution of high colour fidelity image data can be achieved. It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present technology. BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 schematically illustrates an audio / video (A / V) data transmission and reception system using video data compression and decompression; Figure 2 schematically illustrates a video display system using video data decompression; Figure 3 schematically illustrates an audio / video storage system using video data compression and decompression; Figure 4 schematically illustrates a video camera using video data compression; Figures 5 and 6 schematically illustrate storage media; Figure 7 provides a schematic overview of a video data compression and decompression apparatus; Figure 8 schematically illustrates a predictor; Figure 9 schematically illustrates source image data having a 4:4:4 format; Figure 10 schematically illustrates source image data having a 4:2:2 format; Figure 11 schematically illustrates an example encoding method; Figure 12 schematically illustrates an example multi-layer bitstream for source image data having a 4:2:2 format; Figure 13 schematically illustrates an example decoding method; Figure 14 schematically illustrates an example encoding method; Figure 15 schematically illustrates an example multi-layer bitstream for source image data having a 4:4:4 format; Figure 16 schematically illustrates an example Supplemental Enhancement Information message; Figure 17 schematically illustrates an example multi-layer bitstream for source image data having a 4:2:2 format; Figure 18 schematically illustrates an example multi-layer bitstream for source image data having a 4:4:4 format; Figure 19 schematically illustrates an example decoding method; Figure 20 schematically illustrates an example encoding method; Figure 21 schematically illustrates source image data having a 4:4:4: format; Figure 22A schematically illustrates source image data having a 4:2:2 format; Figure 22B schematically illustrates source image data having a 4:2:2 format and Figure 23 schematically illustrates an example decoding method. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, Figures 1-4 are provided to give schematic illustrations of apparatus or systems making use of the compression and / or decompression apparatus to be described below in connection with embodiments of the present technology. All of the data compression and / or decompression apparatus to be described below may be implemented in hardware, in software running on a general-purpose data processing apparatus such as a general-purpose computer, as programmable hardware such as an application specific integrated circuit (ASIC) or field programmable gate array (FPGA) or as combinations of these. In cases where the embodiments are implemented by software and / or firmware, it will be appreciated that such software and / or firmware, and non-transitory data storage media by which such software and / or firmware are stored or otherwise provided, are considered as embodiments of the present technology. Figure 1 schematically illustrates an audio / video data transmission and reception system using video data compression and decompression. In this example, the data values to be encoded or decoded represent image data. An input audio / video signal 10 is supplied to a video data compression apparatus 20 which compresses at least the video component of the audio / video signal 10 for transmission along a transmission route 30 such as a cable, an optical fibre, a wireless link or the like. The compressed signal is processed by a decompression apparatus 40 to provide an output audio / video signal 50. For the return path, a compression apparatus 60 compresses an audio / video signal for transmission along the transmission route 30 to a decompression apparatus 70. The compression apparatus 20 and decompression apparatus 70 can therefore form one node of a transmission link. The decompression apparatus 40 and decompression apparatus 60 can form another node of the transmission link. Of course, in instances where the transmission link is uni-directional, only one of the nodes would require a compression apparatus and the other node would only require a decompression apparatus. Figure 2 schematically illustrates a video display system using video data decompression. In particular, a compressed audio / video signal 100 is processed by a decompression apparatus 110 to provide a decompressed signal which can be displayed on a display 120. The decompression apparatus 110 could be implemented as an integral part of the display 120, for example being provided within the same casing as the display device. Alternatively, the decompression apparatus 110 maybe provided as (for example) a so-called set top box (STB), noting that the expression "set-top" does not imply a requirement for the box to be sited in any particular orientation or position with respect to the display 120; it is simply a term used in the art to indicate a device which is connectable to a display as a peripheral device. Figure 3 schematically illustrates an audio / video storage system using video data compression and decompression. An input audio / video signal 130 is supplied to a compression apparatus 140 which generates a compressed signal for storing by a store device 150 such as a magnetic disk device, an optical disk device, a magnetic tape device, a solid state storage device such as a semiconductor memory or other storage device. For replay, compressed data is read from the storage device 150 and passed to a decompression apparatus 160 for decompression to provide an output audio / video signal 170. It will be appreciated that the compressed or encoded signal, and a storage medium such as a machine-readable non-transitory storage medium, storing that signal, are considered as embodiments of the present technology. Figure 4 schematically illustrates a video camera using video data compression. In Figure 4, an image capture device 180, such as a charge coupled device (CCD) image sensor and associated control and read-out electronics, generates a video signal which is passed to a compression apparatus 190. A microphone (or plural microphones) 200 generates an audio signal to be passed to the compression apparatus 190. The compression apparatus 190 generates a compressed audio / video signal 210 to be stored and / or transmitted (shown generically as a schematic stage 220). The techniques to be described below relate primarily to video data compression and decompression. It will be appreciated that many existing techniques may be used for audio data compression in conjunction with the video data compression techniques which will be described, to generate a compressed audio / video signal. Accordingly, a separate discussion of audio data compression will not be provided. It will also be appreciated that the data rate associated with video data, in particular broadcast quality video data, is generally very much higher than the data rate associated with audio data (whether compressed or uncompressed). It will therefore be appreciated that uncompressed audio data could accompany compressed video data to form a compressed audio / video signal. It will further be appreciated that although the present examples (shown in Figures 1-4) relate to audio / video data, the techniques to be described below can find use in a system which simply deals with (that is to say, compresses, decompresses, stores, displays and / or transmits) video data. That is to say, the embodiments can apply to video data compression without necessarily having any associated audio data handling at all. Figure 4 therefore provides an example of a video capture apparatus comprising an image sensor and an encoding apparatus of the type to be discussed below. Figure 2 therefore provides an example of a decoding apparatus of the type to be discussed below and a display to which the decoded images are output. A combination of Figure 2 and 4 may provide a video capture apparatus comprising an image sensor 180 and encoding apparatus 190, decoding apparatus 110 and a display 120 to which the decoded images are output. Figures 5 and 6 schematically illustrate storage media, which store (for example) the compressed data generated by the apparatus 20, 60, the compressed data input to the apparatus 110 or the storage media or stages 150, 220. Figure 5 schematically illustrates a disc storage medium such as a magnetic or optical disc, and Figure 6 schematically illustrates a solid state storage medium such as a flash memory. Note that Figures 5 and 6 can also provide examples of non-transitory machine-readable storage media which store computer software which, when executed by a computer, causes the computer to carry out one or more of the methods to be discussed below. Therefore, the above arrangements provide examples of video storage, capture, transmission or reception apparatuses embodying any of the present techniques. Figure 7 provides a schematic overview of a video or image data compression and decompression apparatus, for encoding and / or decoding image data representing one or more images. A controller 343 controls the overall operation of the apparatus and, in particular when referring to a compression mode, controls a trial encoding processes by acting as a selector to select various modes of operation such as block sizes and shapes, and whether the video data is to be encoded losslessly or otherwise. The controller is considered to form part of the image encoder or image decoder (as the case may be). Successive images of an input video signal 300 are supplied to an adder 310 and to an image predictor 320. The image predictor 320 will be described below in more detail with reference to Figure 8. The image encoder or decoder (as the case may be) plus the intra-image predictor of Figure 8 may use features from the apparatus of Figure 7. This does not mean that the image encoder or decoder necessarily requires every feature of Figure 7 however. The adder 310 in fact performs a subtraction (negative addition) operation, in that it receives the input video signal 300 on a "+" input and the output of the image predictor 320 on a input, so that the predicted image is subtracted from the input image. The result is to generate a so-called residual image signal 330 representing the difference between the actual and predicted images. One reason why a residual image signal is generated is as follows. The data coding techniques to be described, that is to say the techniques which will be applied to the residual image signal, tend to work more efficiently when there is less "energy" in the image to be encoded. Here, the term "efficiently" refers to the generation of a small amount of encoded data; for a particular image quality level, it is desirable (and considered "efficient") to generate as little data as is practicably possible. The reference to "energy" in the residual image relates to the amount of information contained in the residual image. If the predicted image were to be identical to the real image, the difference between the two (that is to say, the residual image) would contain zero information (zero energy) and would be very easy to encode into a small amount of encoded data. In general, if the prediction process can be made to work reasonably well such that the predicted image content is similar to the image content to be encoded, the expectation is that the residual image data will contain less information (less energy) than the input image and so will be easier to encode into a small amount of encoded data. Therefore, encoding (using the adder 310) involves predicting an image region for an image to be encoded; and generating a residual image region dependent upon the difference between the predicted image region and a corresponding region of the image to be encoded. In connection with the techniques to be discussed below, the ordered array of data values comprises data values of a representation of the residual image region. Decoding involves predicting an image region for an image to be decoded; generating a residual image region indicative of differences between the predicted image region and a corresponding region of the image to be decoded; in which the ordered array of data values comprises data values of a representation of the residual image region; and combining the predicted image region and the residual image region. The remainder of the apparatus acting as an encoder (to encode the residual or difference image) will now be described. The residual image data 330 is supplied to a transform unit or circuitry 340 which generates a discrete cosine transform (DCT) representation of blocks or regions of the residual image data. The DCT technique itself is well known and will not be described in detail here. Note also that the use of DCT is only illustrative of one example arrangement. Other transforms which might be used include, for example, the discrete sine transform (DST). A transform could also comprise a sequence or cascade of individual transforms, such as an arrangement in which one transform is followed (whether directly or not) by another transform. The choice of transform may be determined explicitly and / or be dependent upon side information used to configure the encoder and decoder. In other examples a so-called “transform-skip” mode can selectively be used in which no transform is applied. Therefore, in examples, an encoding and / or decoding method comprises predicting an image region for an image to be encoded; and generating a residual image region dependent upon the difference between the predicted image region and a corresponding region of the image to be encoded; in which the ordered array of data values (to be discussed below) comprises data values of a representation of the residual image region. The output of the transform unit 340, which is to say (in an example), a set of DCT coefficients for each transformed block of image data, is supplied to a quantiser 350. Various quantisation techniques are known in the field of video data compression, ranging from a simple multiplication by a quantisation scaling factor through to the application of complicated lookup tables under the control of a quantisation parameter. The general aim is twofold. Firstly, the quantisation process reduces the number of possible values of the transformed data. Secondly, the quantisation process can increase the likelihood that values of the transformed data are zero. Both of these can make the entropy encoding process, to be described below, work more efficiently in generating small amounts of compressed video data. A data scanning process is applied by a scan unit 360. The purpose of the scanning process is to reorder the quantised transformed data so as to gather as many as possible of the non-zero quantised transformed coefficients together, and of course therefore to gather as many as possible of the zero-valued coefficients together. These features can allow so-called run-length coding or similar techniques to be applied efficiently. So, the scanning process involves selecting coefficients from the quantised transformed data, and in particular from a block of coefficients corresponding to a block of image data which has been transformed and quantised, according to a "scanning order" so that (a) all of the coefficients are selected once as part of the scan, and (b) the scan tends to provide the desired reordering. One example scanning order which can tend to give useful results is a diagonal order such as a so-called up-right diagonal scanning order. The scanning order can be different, as between transform-skip blocks and transform blocks (blocks which have undergone at least one spatial frequency transformation). The scanned coefficients are then passed to an entropy encoder (EE) 370. Again, various types of entropy encoding may be used. Two examples are variants of the so-called CABAC (Context Adaptive Binary Arithmetic Coding) system and variants of the so-called CAVLC (Context Adaptive Variable-Length Coding) system. In general terms, CABAC is considered to provide a better efficiency, and in some studies has been shown to provide a 10-20% reduction in the quantity of encoded output data for a comparable image quality compared to CAVLC. However, CAVLC is considered to represent a much lower level of complexity (in terms of its implementation) than CABAC. Note that the scanning process and the entropy encoding process are shown as separate processes, but in fact can be combined or treated together. That is to say, the reading of data into the entropy encoder can take place in the scan order. Corresponding considerations apply to the respective inverse processes to be described below. The output of the entropy encoder 370, along with additional data (mentioned above and / or discussed below), for example defining the manner in which the predictor 320 generated the predicted image, whether the compressed data was transformed or transformskipped or the like, provides a compressed output video signal 380. However, a return path 390 is also provided because the operation of the predictor 320 itself depends upon a decompressed version of the compressed output data. The reason for this feature is as follows. At the appropriate stage in the decompression process (to be described below) a decompressed version of the residual data is generated. This decompressed residual data has to be added to a predicted image to generate an output image (because the original residual data was the difference between the input image and a predicted image). In order that this process is comparable, as between the compression side and the decompression side, the predicted images generated by the predictor 320 should be the same during the compression process and during the decompression process. Of course, at decompression, the apparatus does not have access to the original input images, but only to the decompressed images. Therefore, at compression, the predictor 320 bases its prediction (at least, for inter-image encoding) on decompressed versions of the compressed images. The entropy encoding process carried out by the entropy encoder 370 is considered (in at least some examples) to be "lossless", which is to say that it can be reversed to arrive at exactly the same data which was first supplied to the entropy encoder 370. So, in such examples the return path can be implemented before the entropy encoding stage. Indeed, the scanning process carried out by the scan unit 360 is also considered lossless, so in the present embodiment the return path 390 is from the output of the quantiser 350 to the input of a complimentary inverse quantiser 420. In instances where loss or potential loss is introduced by a stage, that stage (and its inverse) may be included in the feedback loop formed by the return path. For example, the entropy encoding stage can at least in principle be made lossy, for example by techniques in which bits are encoded within parity information. In such an instance, the entropy encoding and decoding should form part of the feedback loop. In general terms, an entropy decoder 410, the reverse scan unit 400, an inverse quantiser 420 and an inverse transform unit or circuitry 430 provide the respective inverse functions of the entropy encoder 370, the scan unit 360, the quantiser 350 and the transform unit 340. For now, the discussion will continue through the compression process; the process to decompress an input compressed video signal will be discussed separately below. In the compression process, the scanned coefficients are passed by the return path 390 from the quantiser 350 to the inverse quantiser 420 which carries out the inverse operation of the scan unit 360. An inverse quantisation and inverse transformation process are carried out by the units 420, 430 to generate a compressed-decompressed residual image signal 440. The image signal 440 is added, at an adder 450, to the output of the predictor 320 to generate a reconstructed output image 460 (although this may be subject to so-called loop filtering and / or other filtering before being output - see below). This forms one input to the image predictor 320, as will be described below. Turning now to the decoding process applied to decompress a received compressed video signal 470, the signal is supplied to the entropy decoder 410 and from there to the chain of the reverse scan unit 400, the inverse quantiser 420 and the inverse transform unit 430 before being added to the output of the image predictor 320 by the adder 450. So, at the decoder side, the decoder reconstructs a version of the residual image and then applies this (by the adder 450) to the predicted version of the image (on a block by block basis) so as to decode each block. In straightforward terms, the output 460 of the adder 450 forms the output decompressed video signal 480 (subject to the filtering processes discussed below). In practice, further filtering may optionally be applied (for example, by a loop filter 565 shown in Figure 8 but omitted from Figure 7 for clarity of the higher level diagram of Figure 7) before the signal is output. The apparatus of Figures 7 and 8 can act as a compression (encoding) apparatus or a decompression (decoding) apparatus. The functions of the two types of apparatus substantially overlap. The scan unit 360 and entropy encoder 370 are not used in a decompression mode, and the operation of the predictor 320 (which will be described in detail below) and other units follow mode and parameter information contained in the received compressed bit-stream rather than generating such information themselves. Figure 8 schematically illustrates the generation of predicted images, and in particular the operation of the image predictor 320. There are two basic modes of prediction carried out by the image predictor 320: so-called intra-image prediction and so-called inter-image, or motion-compensated (MC), prediction. At the encoder side, each involves detecting a prediction direction in respect of a current block to be predicted, and generating a predicted block of samples according to other samples (in the same (intra) or another (inter) image). By virtue of the units 310 or 450, the difference between the predicted block and the actual block is encoded or applied so as to encode or decode the block respectively. (At the decoder, or at the reverse decoding side of the encoder, the detection of a prediction direction may be in response to data associated with the encoded data by the encoder, indicating which direction was used at the encoder. Or the detection may be in response to the same factors as those on which the decision was made at the encoder). Intra-image prediction bases a prediction of the content of a block or region of the image on data from within the same image. This corresponds to so-called l-frame encoding in other video compression techniques. In contrast to l-frame encoding, however, which involves encoding the whole image by intra-encoding, in the present embodiments the choice between intra- and inter- encoding can be made on a block-by-block basis, though in other embodiments the choice is still made on an image-by-image basis. Motion-compensated prediction is an example of inter-image prediction and makes use of motion information which attempts to define the source, in another adjacent or nearby image, of image detail to be encoded in the current image. Accordingly, in an ideal example, the contents of a block of image data in the predicted image can be encoded very simply as a reference (a motion vector) pointing to a corresponding block at the same or a slightly different position in an adjacent image. A technique known as “block copy” prediction is in some respects a hybrid of the two, as it uses a vector to indicate a block of samples at a position displaced from the currently predicted block within the same image, which should be copied to form the currently predicted block. Returning to Figure 8, two image prediction arrangements (corresponding to intra- and inter-image prediction) are shown, the results of which are selected by a multiplexer 500 under the control of a mode signal 510 (for example, from the controller 343) so as to provide blocks of the predicted image for supply to the adders 310 and 450. The choice is made in dependence upon which selection gives the lowest “energy” (which, as discussed above, may be considered as information content requiring encoding), and the choice is signalled to the decoder within the encoded output data-stream. Image energy, in this context, can be detected, for example, by carrying out a trial subtraction of an area of the two versions of the predicted image from the input image, squaring each pixel value of the difference image, summing the squared values, and identifying which of the two versions gives rise to the lower mean squared value of the difference image relating to that image area. In other examples, a trial encoding can be carried out for each selection or potential selection, with a choice then being made according to the cost of each potential selection in terms of one or both of the number of bits required for encoding and distortion to the picture. The actual prediction, in the intra-encoding system, is made on the basis of image blocks received as part of the signal 460 (as filtered by loop filtering; see below), which is to say, the prediction is based upon encoded-decoded image blocks in order that exactly the same prediction can be made at a decompression apparatus. However, data can be derived from the input video signal 300 by an intra-mode selector 520 to control the operation of the intra-image predictor 530. For inter-image prediction, a motion compensated (MC) predictor 540 uses motion information such as motion vectors derived by a motion estimator 550 from the input video signal 300. Those motion vectors are applied to a processed version of the reconstructed image 460 by the motion compensated predictor 540 to generate blocks of the inter-image prediction. Accordingly, the units 530 and 540 (operating with the estimator 550) each act as detectors to detect a prediction direction in respect of a current block to be predicted, and as a generator to generate a predicted block of samples (forming part of the prediction passed to the units 310 and 450) according to other samples defined by the prediction direction. The processing applied to the signal 460 will now be described. Firstly, the signal may be filtered by a so-called loop filter 565. Various types of loop filters may be used. One technique involves applying a "deblocking" filter to remove or at least tend to reduce the effects of the block-based processing carried out by the transform unit 340 and subsequent operations. A further technique involving applying a so-called sample adaptive offset (SAO) filter may also be used. In general terms, in a sample adaptive offset filter, filter parameter data (derived at the encoder and communicated to the decoder) defines one or more offset amounts to be selectively combined with a given intermediate video sample (a sample of the signal 460) by the sample adaptive offset filter in dependence upon a value of:(i) the given intermediate video sample; or (ii) one or more intermediate video samples having a predetermined spatial relationship to the given intermediate video sample. Also, an adaptive loop filter is optionally applied using coefficients derived by processing the reconstructed signal 460 and the input video signal 300. The adaptive loop filter is a type of filter which, using known techniques, applies adaptive filter coefficients to the data to be filtered. That is to say, the filter coefficients can vary in dependence upon various factors. Data defining which filter coefficients to use is included as part of the encoded output data-stream. The filtered output from the loop filter unit 565 in fact forms the output video signal 480 when the apparatus is operating as a decompression apparatus. It is also buffered in one or more image or frame stores 570; the storage of successive images is a requirement of motion compensated prediction processing, and in particular the generation of motion vectors. To save on storage requirements, the stored images in the image stores 570 may be held in a compressed form and then decompressed for use in generating motion vectors. For this particular purpose, any known compression I decompression system may be used. The stored images may be passed to an interpolation filter 580 which generates a higher resolution version of the stored images; in this example, intermediate samples (sub-samples) are generated such that the resolution of the interpolated image is output by the interpolation filter 580 is 4 times (in each dimension) that of the images stored in the image stores 570 for the luma channel of 4:2:0 and 8 times (in each dimension) that of the images stored in the image stores 570 for the chroma channels of 4:2:0. The interpolated images are passed as an input to the motion estimator 550 and also to the motion compensated predictor 540. The way in which an image is partitioned for compression processing will now be described. At a basic level, an image to be compressed is considered as an array of blocks or regions of samples. The splitting of an image into such blocks or regions can be carried out by a decision tree, such as that described in SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS Infrastructure of audio-visual services - Coding of moving video High efficiency video coding Recommendation ITU-T H.265 12 / 2016. Also: High Efficiency Video Coding (HEVC) algorithms and Architectures, Editors: Madhukar Budagavi, Gary J. Sullivan, Vivienne Sze; chapter 3; ISBN 978-3-319-06894-7; 2014 which are incorporated herein in their respective entireties by reference. In some examples, the resulting blocks or regions have sizes and, in some cases, shapes which, by virtue of the decision tree, can generally follow the disposition of image features within the image. This in itself can allow for an improved encoding efficiency because samples representing or following similar image features would tend to be grouped together by such an arrangement. In some examples, square blocks or regions of different sizes (such as 4x4 samples up to, say, 64x64 or larger blocks) are available for selection. In other example arrangements, blocks or regions of different shapes such as rectangular blocks or arrays (for example, vertically or horizontally oriented) can be used. Other non-square and non-rectangular blocks are envisaged. The result of the division of the image into such blocks or regions is (in at least the present examples) that each sample of an image is allocated to one, and only one, such block or region. Chroma Subsampling Chroma subsampling can be used in order to encode images with lower resolution while maintaining image quality. This will be explained with reference to Figures 9 and 10 of the present disclosure. Figure 9 of the present disclosure schematically illustrates source image data having a 4:4:4 format. A 4:4:4 format is an uncompressed format and therefore provides the highest levels of colour fidelity. Figure 9 relates to an example of a YCbCr colour space, having a 4:4:4 format. The YCbCr colour space comprises a luma signal (Y) which represents the brightness in the image, and two chroma signals (Cb and Cr) which represent the colour in the image. In a 4:4:4 format, there are the same number of luma samples as chroma samples in the image data. In the example of Figure 10, the luma samples are represented as a 4x4 block of samples. The first row comprises the samples Y00, Y01, Y02 and Y03. The second row comprises the samples Y10, Y11, Y12 and Y13. The third and fourth rows comprise the luma samples Y20 to Y23 and Y30 to Y33 respectively. In addition, there are also two separate blocks of 4x4 samples for each of Cb and Cr. Each luma sample has a corresponding sample in each of Cb and Cr. Therefore, when decoding the signal, luma sample Y00 is combined with chroma samples CbOO and CrOO in order to display the image. Figure 10 of the present disclosure schematically illustrates source image data having a 4:2:2 format. A 4:2:2 format is a compressed format. That is, in 4:2:2, chroma subsampling is performed such that the 4:2:2 signal will have half the sampling rate horizontally, but will maintain full sampling vertically. Similar to Figure 9, the example of Figure 10 is described with reference to a YCbCr colour space. In this example, a block of 4x4 luma samples are shown (again, Y00 to Y33 as described with reference to Figure 9). However, for each chroma component (Cb and Cr) there are now only 8 samples, arranged in a 2x4 block. This is because, for 4:2:2 chroma subsampling, the signal has half the sampling rate horizontally, but will maintain full sampling vertically (compared to the luma component). When decoding the signal, the first luma sample Y00 is combined with the chroma signals CbOO and CrOO in order to display the image. Then, the second luma sample Y01 is also combined with the chroma samples CbOO and CrOO. Accordingly, the same chroma samples (CbOO and CrOO respectively for the first and second chroma component) are combined with both the first and second luma samples (Y00 and Y01). This same approach then applies for the subsequent luma and chroma samples (e.g. Y02 and Y03 are each combined with Cb02 and Cr02). Thus, chroma subsampling enables images to be encoded while implementing less resolution for chroma information than for luma information. This takes advantage of the fact that the human visual system is more sensitive to changes in brightness than it is to changes in colour. Accordingly, it is possible to sample colour information at a lower resolution while maintaining image quality. The manner by which chroma subsampling is performed in order to generate a format such as 4:2:2 is not particularly limited. That is, in some examples, the subsampling may be performed by taking values directly from the 4:4:4 (uncompressed) format. Taking Figures 9 and 10 as an example, the Cb samples in the first row of the 4:2:2 source are - directly - the first and third samples from the first row of the 4:4:4 source (i.e. CbOO and Cb02). Thus, chroma subsampling is performed, in this example, by using values directly from the 4:4:4 source (uncompressed data). In other words, for the 4:2:2 example of Figure 10, the sample CbOO is applied to both the luma sample Y00 and Y01, while the sample Cb01 is discarded. However, in some examples, the chroma subsampling may be performed by predicting or generating a value for Cb in the 4:2:2 format from the values of the 4:4:4 source. That is, for example, the first sample of the first row of Cb (to be combined with luma samples Y00 and Y01) may be generated based on the values CbOO and Cb01 from the 4:4:4 source. In examples, CbOO in 4:2:2 may be generated based on an average of CbOO and Cb01 in the 4:4:4 source. In examples, CbOO in 4:2:2 may be generated based on a weighted average of CbOO and Cb01 in the 4:4:4 source. In examples, CbOO in 4:2:2 may be generated based on a weighted average of a number of different neighbouring samples in the 4:4:4 source (and is thus not particularly limited to the use of CbOO and Cb01 in the 4:4:4 source). As such, there are different ways of coding the chroma subsampling in 4:2:2. Any method of downsampling the chroma component from 4:4:4 can be applied as desired and the present disclosure is not particularly limited in this regard. While Figures 9 and 10 schematically illustrate 4:4:4 and 4:2:2 formats, it will be appreciated that chroma subsampling can also be performed in order to produce other formats, such as a 4:2:0 format. In a 4:2:0 format, the signal has half the sampling rate horizontally and half the sampling vertically. Therefore, the colour information is sampled at a lower resolution than the 4:2:2 format, while still maintaining luma image quality. Furthermore, while the example of Figures 9 and 10 has been described with reference to a YCbCr colour space, it will be appreciated that the present disclosure is not particularly limited in this regard; the YCbCr colour space is merely one example colour space which can be used in accordance with embodiments of the disclosure. Other colour spaces can be used as desired depending on the situation to which embodiments of the disclosure are applied. As explained in the Background, certain decoding systems may be limited to certain profiles. That is, a decoder system may be capable of supporting only 4:2:0 profiles. Despite this, there is an ever growing demand for the provision of image data which supports high fidelity colour information (such as 4:4:4 signals or 4:2:2 signals, for example). High fidelity colour information may be required for certain applications, such as medical imaging or entertainment. High fidelity formats may also be required for computer vision tasks (which may not have the same distinction between brightness and colour sensitivity as the human vision system). As such, there is a demand to improve distribution of content with high levels of colour information, while retaining compatibility with decoding systems with limited profiles. Accordingly, data encoding and decoding methods, apparatuses and computer programs are provided in accordance with embodiments of the disclosure. Example Encoding Of Multi-Layer Bitstream Consider, now, Figure 11 of the present disclosure. Figure 11 schematically illustrates an example method. The example method starts at step S1100 and proceeds to step S1102. In step S1102, the method comprises selectively encoding a flag when encoding image data having a 4:2:2 chroma format as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and a second layer, wherein the first layer comprises data in a 4:2:0 chroma format and the second layer, having the 4:2:0 chroma format, comprises data for reconstructing an image with the 4:2:2 chroma format when combined with the data in the first layer. The method proceeds to, and ends with, step S1104. Accordingly, in the example method of Figure 11, image data having a 4:2:2 chroma format is encoded as a multi-layer bitstream. The first layer of the bitstream can carry the image data in a 4:2:0 data format, while the second layer can carry additional data (e.g. enhancement data) which can be used in order to reconstruct the image data with the 4:2:2 chroma format, while the additional data is still coded using a 4:2:0 chroma format. By providing the 4:2:0 in the first layer of the bitstream, backward compatibility can be maintained. As an example, certain decoders may only be able to support a 4:2:0 data format (that is, certain 4:2:0 profiles). Furthermore, since the additional data for the 4:2:2 chroma format is included in the second layer - which also has the 4:2:0 chroma format - carriage of the 4:2:2 chroma format images using 4:2:0 coding becomes possible. Thus, improved compatibility and distribution of 4:2:2 image data can be achieved, since even decoders limited to 4:2:0 profiles can reconstruct 4:2:2 image data from the multilayer bitstream. In addition, while an example method of encoding has been described with reference to Figure 11 of the present disclosure, it will be appreciated that the present disclosure is not particularly limited in this regard. An encoding apparatus is also provided in accordance with embodiments of the present disclosure. The encoding apparatus may comprise circuitry which is configured in order to perform an encoding method (such as that described with reference to Figure 11 of the present disclosure). In examples, the encoding apparatus may be part of a compression apparatus such as that described with reference to Figure 7 of the present disclosure. 4:2:2 Source - Example Situation An encoding method will now be described with reference to an example situation of a YCbCr colour space. However, as previously mentioned, it will be appreciated that the embodiments of the disclosure are not particularly limited to a YCbCr colour space. That is, embodiments of the disclosure can be applied more generally to any colour space. For example, embodiments of the disclosure may also be applied to an RGB colour space. The choice of colour space which is used will depend on the situation to which the embodiments of the disclosure are applied. Accordingly, the use of YCbCr in this example is merely illustrative and is not limiting to embodiments of the disclosure. Consider, now, Figure 12 of the present disclosure. Figure 12 schematically illustrates an example multi-layer bitstream for source image data having a 4:2:2 format. In this example, the source image data, being the image data to be encoded, has a 4:2:2 data format. That is, there are three components of the image data. In this example, the image data has a YCbCr 4:2:2 format. Therefore, the three components are a luma component (Y) and two chroma components (Cb and Cr). In this example, there are 16 luma samples. That is, the luma is defined by a 4x4 block of luma samples, which range from 00 to 03 on the first row, 10 to 13 on the second row, 20 to 23 on the third row and 30 to 33 on the fourth row. For each of the chroma components, Cb and Cr, there are half as many chroma samples horizontally (compared to the luma), but a full number of samples vertically (compared to the luma). Accordingly, the Cb component is defined by a 2x4 block of chroma samples. Likewise, the Cr component is defined by a 2x4 block of chroma samples. This is illustrated in the example of Figure 12. As has been described with reference to Figures 9 and 10 of the present disclosure, the manner by which the chroma sub-sampling is performed in order to generate the Cr and Cb components in the 4:2:2 format is not particularly limited. In some examples, each chroma sample in the 4:2:2 format may correspond directly to a chroma sample in the 4:4:4 format. That is, in some examples, chroma sub-sampling may be performed by selecting a subsample of the chroma samples in the 4:4:4 format and using that sub-sample of the chroma samples directly as the chroma values in the 4:2:2 format. On the other hand, in some examples, the chroma samples in the 4:2:2 format may be generated from the chroma samples in the 4:4:4 format as part of the chroma sub-sampling process. Nevertheless, in the example of Figure 12 of the present disclosure, each chroma sample in the 4:2:2 format corresponds directly to a chroma sample in the 4:4:4 format. As such, the first chroma sample on the first row is sample 00 (corresponding to the first chroma sample of a 4:4:4 block). The second sample on the first row is sample 02 (corresponding to the third chroma sample of the 4:4:4 block). Thus half sampling of the chroma values in the horizontal direction is achieved. Once the 4:2:2 source has been generated (e.g. by downsampling from 4:4:4) it is desired that the 4:2:2 data be encoded for distribution. However, certain decoder implementations may be limited to a 4:2:0 profile and therefore cannot natively decode a 4:2:2 image. Therefore, according to the present disclosure, the 4:2:2 image data can be encoded as a multi-layer bitstream, where each layer of the multi-layer bitstream has a 4:2:0 format. As has been described with reference to the example method of Figure 11 of the present disclosure, this enables a 4:2:2 image data to be distributed even when a decoder implementation is capable only of supporting 4:2:0 profiles, since the 4:2:2 image data can be reconstructed from across the different layers of the multi-layer bitstream. In particular, the first layer (4:2:0 LayerO) of the multi-layer bitstream includes a complete image in the 4:2:0 format. That is, the luma samples are included in full within the first layer. On the other hand, the chroma samples have been sub-sampled from the 4:2:2 image data. As has been mentioned, in 4:2:0 there are half as many chroma samples horizontally (compared to the luma), and half as many chroma of samples vertically (compared to the luma). Accordingly, the Cb component is defined by a 2x2 block of chroma samples. Likewise, the Cr component is defined by a 2x2 block of chroma samples. This is illustrated in the example of Figure 12 (first layer). In other words, in this first layer, the data for the second component (Cb) and the third component (Cr) include a sub-set of the samples for the second component and the third component included in the image data. Accordingly, for the Cb component, the samples 00 and 02 are included on the first row (the same as for the first row in 4:2:2). However, the second row has the samples 20 and 22 (corresponding to the third row in the 4:2:2). However, while, in this example, the chroma subsampling from the 4:2:2 source to the 4:2:0 of the first layer of the multi-layer bitstream has been performed by subsampling of the chroma components from the 4:2:2 source, it will be appreciated that the present disclosure is not particularly limited in this regard. More generally, any method of downsampling the chroma components from the 4:2:2 source to the 4:2:0 of the first layer can be used as desired. For example, the downsampling may be performed by generating the samples for the chroma components in 4:2:0 from the 4:2:2 source. Thus, returning to the example of Figure 12, it will be appreciated the luma samples are fully included within the first layer while the chroma samples have been sub-sampled at half as many samples in both the horizontal and vertical direction (compared to 4:4:4). Therefore, the first layer of the bitstream carries the 4:2:0 signal. A decoder can thus use the first layer of the multi-layer bitstream to produce image data having the 4:2:0 format if desired. In this example, however, the bitstream is a multi-layer bitstream comprising a first layer (4:2:0 LayerO) and a second layer (4:2:0 Layerl). The second layer is an enhancement layer, which can be used, in combination with the first layer, in order to reconstruct image data having the 4:2:2 format (even though each layer of the multi-layer bitstream is in the 4:2:0 format). That is, the second layer of the multi-layer bitstream does not include any additional luma values. The luma values are fully included within the first layer. Instead, the second layer includes chroma values which can be combined with the luma values from the first layer in order to reconstruct the image data having the 4:2:2 format. Within the second layer, the additional chroma values are not included within the area typically reserved for chroma values. Instead, the chroma values are packaged within an area corresponding to an area reserved for the luma component in the first layer. Within the second layer, the area typically reserved for the chroma component is left blank (i.e. has a zero value). As these values are blank, they have minimal impact on the size of the bitstream (as it is very efficient for an encoder to encode these values). In effect, the second (enhancement) layers are, in this example, effectively of a 4:0:0 format, with the chorma information being provided in the area which is typically reserved for luma. Accordingly, in some examples, the second (enhancement) layers may be of a 4:2:0 format (with the chroma channels being unused), however, in other examples, the second (enhancement) layers may be of a 4:0:0 format. Utilizing the area reserved for luma values for transmission of the chroma values in the second (enhancement) layer enables the additional chroma values to be included in the second layer while ensuring that the second layer can also have the 4:2:0 format (and can thus be decoded by a decoded that may only be able to support a 4:2:0 data format / 4:2:0 profiles. For example, the 8 Cb values and the 8 Cr values which are required for the second layer would not fit within the area reserved for chroma values in the second layer (which could hold only 4 Cb values and 4 Cr values) without changing the 4:2:0 profile. However, these values can be provided within the area reserved for the luma values; since the luma values are provided in the first layer of the multi-layer bit stream, they do not need to be provided in the second (enhancement) layer. That is, in the example of Figure 12, the data for reconstructing an image with the 4:2:2 chroma format, when combined with the data in the first layer, thus comprises the complete chroma samples of the 4:2:2 image data (which have not been included within the first layer of the multi-layer bitstream). These additional chroma samples are packaged, in the second layer, within the area reserved for the luma component in the first layer. Indeed, in the example of Figure 12, the chroma samples for Cb and Cr are packaged side by side within the second layer. This is done for a number of reasons. Firstly, each chroma component, in 4:2:2, is defined by a 2x4 block of samples. Accordingly, the samples for Cb and Cr, when placed side by side, fit within 4x4 block which is typically reserved for luma components. As such, these samples can be packaged within the second layer using a same profile as required for 4:2:0. In addition, packing the samples Cb and Cr side by side in this manner also reduces latency when decoding. When decoding, a decoder may typically decode starting at the first value in the first row. The next value to be decoded will be the second value in the first row and so on, until the final value in the first row is decoded. Then, the decoder will proceed to the second row. This process will continue until all rows are decoded. If the decoder is to produce only an image having 4:2:0 data, the decoder can produce this image from the data included only in the first layer. However, if the decoder is to produce an image having 4:2:2 data, then the decoder will be required to use information from both the first layer (the base layer) and the second layer (the enhancement layer) in order to produce the image. That is, the decoder will combine the luma values from the first layer with the chroma values from the second layer in order to produce the image having the 4:2:2 format. In other words, the first layer (4:2:0 LayerO) is a base layer carrying a 4:2:0 signal and the second layer (4:2:0 Layerl) is a higher layer carrying enhancement information to reconstruct a 4:2:2 signal. For sample 00, this means that the decoder must decode the first Y sample from the first layer. Furthermore, the decoder must decode the first sample from the luma area of the second layer: this is Cb sample 00. However, at this stage, the decoder still does not have the information for Cr sample 00 (with this information required in order to complete the reproduction of sample 00). As Cb and Cr components are packaged side by side within the second layer, the first Cr sample (Cr sample 00) is included as the third value of the first row within the second layer. Therefore, the decoder must decode only the first value of the first layer and the first three values of the second layer before it has the complete YCrCb values for the sample 00. Indeed, once the first row of the first layer and the first row of the second layer has been decoded, the decoder has complete YCrCb values for the first row of image data in the 4:2:2 format. Accordingly, this arrangement has low latency compared to a different arrangement of packing the Cb and Cr components in the second layer. For example, consider a situation whereby the Cb and Cr samples were packed vertically within the second layer (such that the Cb components were stacked on top of the Cb components). In this example, the chroma values included in the second layer would form a 2x8 block of samples. The first 4 rows would include the Cb samples and the remaining 4 rows would then include the Cr samples. With this arrangement, a decoder would only obtain Cr sample 00 once the first sample of the fifth row in the second layer had been decoded. Accordingly, until this sample had been decoded, the decoder would be unable to reconstruct an image having the 4:2:2 format, even for sample 00. Compared with this arrangement, a side by side packing of the Cb and Cr samples therefore provides lower latency decoding. Thus, in examples, in the first layer, data for a first component, a second component and a third component are included in the 4:2:0 chroma format and, in the second layer, data for the second component and the third component are included within an area corresponding to an area reserved for the first component in the first layer. As previously mentioned, the method embodiments of the disclosure comprises selectively encoding a flag when image data having a 4:2:2 chroma format is encoded as a multi-layer bitstream. This flag can be used, by the decoder, for the decoder to identify that the bitstream is a multi-layer bitstream and can be used for the reconstruction of the 4:2:2 chroma format. The flag may, optionally, be included within a Supplemental Enhancement Information (SEI) message. The SEI message may also include additional information to instruct the decoder how to reconstruct the 4:2:2 signal from the multi-layer bitstream. For example, information concerning the number of layers of the multi-layer bitstream and the manner of packing the chroma information within the enhancement layer(s) of the multi-layer bitstream may also be included within the SEI message. The present disclosure is not particularly limited in this regard. Further discussion concerning the format of an example SEI message will be described later (with reference to Figure 16 of the present disclosure). It will be appreciated that the present disclosure is not particularly limited to the specific number of layers and arrangement of samples within those layers which has been schematically illustrated in Figure 12 of the present disclosure. The number of layers and arrangement of samples within those layers may vary depending on the situation to which the embodiments of the disclosure are applied. However, in this way, it will be appreciated that improved compatibility and distribution of 4:2:2 image data can be achieved, since even decoders limited to 4:2:0 profiles can reconstruct 4:2:2 image data from the multi-layer bitstream. Example Decoding of Multi-Layer Bitstream Consider, now, the example of Figure 13 of the present disclosure. Figure 13 schematically illustrates an example decoding method. The example method starts at step S1300 and proceeds to step S1302. In step S1302, the method comprises selectively decoding a flag when decoding image data having a 4:2:2 chroma format, encoded as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and a second layer, wherein the first layer comprises data in a 4:2:0 chroma format and the second layer, having the 4:2:0 chroma format, comprises data for reconstructing an image with the 4:2:2 chroma format when combined with the data in the first layer. The method proceeds to, and ends with, step S1306. Accordingly, in the example method of Figure 13, image data having a 4:2:2 chroma format is decoded from a multi-layer bitstream, where each layer of the multi-layer bitstream has a 4:2:0 format. That is, the first layer of the bitstream can carry the image data in a 4:2:0 data format, while the second layer can carry additional data (e.g. enhancement data) which can be used in order to reconstruct the image data with the 4:2:2 chroma format, while the additional data is still coded using a 4:2:0 chroma format. By providing the 4:2:0 in the first layer of the bitstream, backward compatibility can be maintained. For example, certain decoders may only be able to support a 4:2:0 data format (that is, certain 4:2:0 profiles). Furthermore, since the additional data for the 4:2:2 chroma format is included in the second layer - which also has the 4:2:0 chroma format - carriage of the 4:2:2 chroma format images using 4:2:0 coding becomes possible. Thus, improved compatibility and distribution of 4:2:2 image data can be achieved, since even decoders limited to 4:2:0 profiles can reconstruct 4:2:2 image data. In addition, while an example method of decoding has been described with reference to Figure 11 of the present disclosure, it will be appreciated that the present disclosure is not particularly limited in this regard. A decoding apparatus is also provided in accordance with embodiments of the present disclosure. The decoding apparatus may comprise circuitry which is configured in order to perform a decoding method (such as that described with reference to Figure 13 of the present disclosure). In examples, the decoding apparatus may be part of a decompression apparatus such as that described with reference to Figure 7 of the present disclosure. Delta Values In the example which has been described with reference to Figure 12 of the present disclosure, a flag is selectively encoded when encoding image data as a multi-layer bitstream, comprising a first layer (the base layer) and a second layer (the enhancement layer). The enhancement layer contains information which can be combined with the base layer in order to reconstruct the data with a 4:2:2 format. The additional transmission of the information within the enhancement layer can increase the size of the encoded bitstream. However, the inventors have realized that there is an opportunity reduce the size of the bitstream when encoding a multi-layer bitstream in this manner, which in turn improves the coding efficiency. Example Encoding of Multi-Layer Bitstream Consider, now, Figure 14 of the present disclosure. Figure 14 schematically illustrates an example encoding method. The example encoding method starts at step S1400 and proceeds to step S1402. In step S1402, the method comprises selectively encoding a flag when encoding image data having a source chroma format as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and one or more second layers, wherein the first layer comprises data in a first chroma format and the one or more second layers, having the first format, comprise data for reconstructing an image with the source chroma format when combined with the data in the first layer; wherein the flag indicates that the data in the one or more second layers is defined relative to the data in the first layer. The method then proceeds to, and ends with, step S1404. Accordingly, in the example method of Figure 14, image data having a source chroma format is encoded as a multi-layer bitstream comprising a first layer and one or more second layers. The first layer of the bitstream can carry the image data in a first chroma format (e.g. as a base layer) while the one or more second layers, also having this first chroma format, can carry additional data (e.g. enhancement data) which can be used in order to reconstruct the image data with the source chroma format. In this way, a decoder able only to natively decode an image with a first chroma format (e.g. 4:2:0) can nevertheless decode and reconstruct an image having a source chroma forma (e.g. 4:4:4) through use of the additional information from the enhancement layers. Therefore, similar to that as has been described with reference to Figures 11 to 13 of the present disclosure, improved compatibility and distribution of image data with higher levels of colour fidelity can be achieved, even when a decoder is limited to only certain decoding profiles. However, in addition, the method of Figure 14 of the present disclosure further comprises selectively encoding the flag when encoding image data having a source chroma format as a multi-layer bitstream, wherein the flag indicates that the data in the one or more second layers is defined relative to the data in the first layer. In examples, the values in the one or more second layers may be defined in terms of an offset from the values in the first layer. By defining the values in the one or more second layers relative to the values in the first layer in this manner, the values in the one or more second layers typically become smaller in magnitude. That is, the enhancement data is signalled only as an offset from the base data which is present in the first layer. As the values in the one or more second layers become smaller, it becomes easier to encode these values, thus reducing the size of the resultant bitstream. As such, a further compression of the multi-layer bitstream can be performed, improving coding efficiency. While an example method of encoding has been described with reference to Figure 14 of the present disclosure, it will be appreciated that the present disclosure is not particularly limited in this regard. An encoding apparatus is also provided in accordance with embodiments of the present disclosure. The encoding apparatus may comprise circuitry which is configured in order to perform an encoding method (such as that described with reference to Figure 14 of the present disclosure). In examples, the encoding apparatus may be part of a compression apparatus such as that described with reference to Figure 7 of the present disclosure. 4:4:4 Source - Example Situation An encoding method will now be described with reference to an example situation of a YCbCr colour space. However, as previously mentioned, it will be appreciated that the embodiments of the disclosure are not particularly limited to a YCbCr colour space. That is, embodiments of the disclosure can be applied more generally to any colour space. For example, embodiments of the disclosure may also be applied to an RGB colour space. Alternatively, embodiments of the disclosure may be applied to a YUV colour space. The choice of colour space which is used will depend on the situation to which the embodiments of the disclosure are applied. Consider, now, Figure 15 of the present disclosure. Figure 15 schematically illustrates an example multi-layer bitstream for source image data having a 4:4:4 format. In this example, the source image data has a 4:4:4 format and the first format is a 4:2:0 format. In other words, this example illustrates an example situation where 4:4:4 data is encoded in a 4:2:0 multi-layer bitstream. However, the present disclosure is not particularly limited in this regard and can more generally be applied to any suitable source image data format and first format as desired. A number of different example situations will also be described later. The 4:4:4 data of Figure 15 comprises a 4x4 block of luma data 1500 and two 4x4 blocks of chroma data 1502 and 1504. In this example, the source format of the image data is therefore 4:4:4. It is desired that the 4:4:4 data is encoded and transmitted to a decoding device. For example, the decoding device may be part of a computer vision system, for which use of 4:4:4 data is of high importance. However, the decoder may be able only to decode data of a first format. That is, the decoder may be limited to certain decoding profiles and may only be able to decode data corresponding to those decoding profiles. As such, the decoder may not natively be able to decode image data of the source format (here, 4:4:4 data). Indeed, in this example, the decoder is able only to decode image data of a 4:2:0 format. Therefore, in this example, the first format is 4:2:0. Therefore, in accordance with embodiments of the disclosure, the 4:4:4 source will be encoded as a multi-layer bitstream, where each layer of that multi-layer bitstream has the first format (namely, the 4:2:0 format). Accordingly, the decoder will be able to decode the multilayer bitstream, since it is in the 4:2:0 format. Moreover, since the encoder sets a flag when encoding the 4:4:4 source as a multi-layer bitstream, the decoder can identify this flag and use the information from across the multi-layer bitstream (namely, the base layer and the enhancement layers) in order to reconstruct the image data in 4:4:4 format. As such, the image data with the 4:4:4 format can be signalled to the decoder even when the decoder is only able to decode image data in 4:2:0. Furthermore, in this example, the flag - selectively encoded by the encoder - further indicates that the data in the one or more second layers is defined relative to the data in the first layer. This further improves coding efficiency, as will now be described with reference to the multi-layer bitstream schematically illustrated in Figure 15. Consider, now, the first layer of the multi-layer bitstream schematically illustrated in Figure 15 (namely 4:2:0 LayerO). As explained, this layer (and indeed all layers of the multilayer bitstream) conforms to the first format - 4:2:0 in this example. Indeed, this first layer is a base layer of the multi-layer bitstream; it contains a complete representation of the image data in the 4:2:0 format. That is, the complete block of 4x4 luma samples is included within this first layer. Furthermore, a 2x2 block of Cb samples and a 2x2 block of Cr samples is included within this first layer of the multi-layer bitstream. Thus, a complete 4:2:0 image data is included within this first layer of the multi-layer bitstream. A decoder could, therefore, decode this first layer and use this first layer (and only this first layer) in order to produce image data having the 4:2:0 format. Concerning the 2x2 block of Cb samples and the 2x2 block of Cr samples which are included in the first layer, it is noted that these samples have been generated by downsampling the Cb and Cr samples from the 4:4:4 source. The present disclosure is not particularly limited to any specific way of downsampling the 4:4:4 source to produce the 4:2:0 samples. However, in this example, the downsampling has been performed by direct sampling from the 4:4:4 source. That is, as previously noted, the 4:2:0 format has half the sampling rate horizontally and half the sampling rate vertically as compared to the 4:4:4 source. Accordingly, in this example, the Cb and Cr samples include sample 00, 02, 20 and 22 from the 4:4:4 source. More specifically, the Cb and Cr samples for 4:2:0 are constructed by sampling half of the values horizontally from the 4:4:4 source and half of the values vertically from the 4:4:4 source. By means of an example, it will be appreciated that each of the first two luma samples of the first row (00 and 01) and also the first two luma samples of the second row (10 and 11) will be combined with the first Cb sample in the first row (00) and the first Cr sample in the first row (00) when decoding the image data. Likewise, the final two samples in the third row (22 and 23) and the final two samples of the fourth row (32 and 33) of the luma samples will be combined with the final Cb sample in the second row (22) and the final Cr sample in the second row (22) when decoding the image data. In this way, the decoder can decode image data having the 4:2:0 format from the first layer of the multi-layer bit stream (4:2:0 LayerO). In examples, the same 00 chroma sample from 4:2:0 can be used as the predictor for 4 samples. In other examples, one can upsample the 4:2:0 to 4:4:4 using interpolation and use the 4 full resolution chroma samples from that as the predictors for the 4 chroma samples. In examples, the predictor could also be derived by encoding and then decoding the 4:2:0 to maximise quality of the decoded 4:4:4 chroma. However, additionally, the data from the one of more second layers can be used, with the data from the first layer, in order to reconstruct the image data having the source (4:4:4) format. In this example, there are two second (enhancement) layers. The first enhancement layer is 4:2:0 Layerl and the second enhancement layer is 4:2:0 Layer2. These are, respectively, the second and third layers of the multi-layer bitstream. Of these two different layers, in this example, the first enhancement layer, 4:2:0 Layerl, is used in order to provide data for the Cb component while the second enhancement layer, 4:2:0 Layer2, is used in order to provide data for the Cr component. Thus, in this example, each of the one of more second (enhancement) layers corresponds to an individual chroma component. The data from the first and second enhancement layer can be combined with the data in the first layer in order to reconstruct an image with the source chroma format (4:4:4 in this example). Taking the first enhancement layer, 4:2:0 Layerl, as an example, the Cb data from this enhancement layer can be combined with the Cb data from the first layer in order to reconstruct the complete 4x4 block of Cb samples from the 4:4:4 source. As has been described with reference to Figure 12 of the present disclosure, the Cb samples (data for the Cb component) are included within an area corresponding to an area reserved for the first component (the Y samples) in the first layer. This means that the Cb components can be packaged within the first enhancement layer (4:2:0 Layerl) using a same profile as used for the base layer (4:2:0 LayerO); this is because the Cb samples fit within the area reserved for the Y samples. The area typically reserved for the Cb and Cr samples is left blank within this first enhancement layer. However, since these areas are left blank, they can be encoded very efficiently and therefore do not have any significant impact on the resultant bitstream. Indeed, as has previously been mentioned (with respect to Figure 12 of the present disclosure) in some examples, the second (enhancement) layers may actually be coded as a 4:0:0 format, if the chroma channels are unused. Thus, in some examples, the base layer and the enhancement layers may have a different format, provided that decoders are able to decode the format of both the base and enhancement layers. It will be appreciated that if the 4:4:4 source image data is to be reconstructed by the decoder, the decoder must therefore have access to the full 4x4 Cb samples of the 4:4:4 source. The decoder therefore reconstructs these Cb samples from the base layer (first layer of the multi-layer bitstream) and the first enhancement layer (second layer of the multi-layer bitstream). In this example, the data in the first enhancement layer is defined relative to the data in the base layer. Accordingly, each Cb sample in the first enhancement layer corresponds to a difference value (or delta value) between the corresponding value in the 4:4:4 source and an associated value in the base layer. More generally, for each of the one or more second layers, each chroma sample is defined as a difference value between the value of that chroma sample in the image data and the value of a corresponding sample in the base layer. Take the first value of the first row of the Cb data of the first enhancement layer as an example. This Cb sample corresponds to value 00 of the Cb samples 1502 in the 4:4:4 source. The associated value in the base layer is Cb sample 00 (the first sample of the first row of the Cb samples in the base layer). The difference between these two values is zero, since the 00 sample from the 4:4:4 source was used directly as the 00 sample in the base layer. Therefore, in the first enhancement layer, the first sample in the first row has a zero value. In other words, this sample was already included within the base layer and therefore does not need to be included again within the first enhancement layer. Since this value is zero, it can be encoded very efficiently and has little impact on the resulting bitstream. In this example, it will be appreciated that the third sample in the first row, the first sample in the third row and the third sample in the third row will also be zero, as these samples (02, 20 and 22) are already included in the base layer (so the difference value is zero). Again, this enables these values to be encoded with little impact on the resulting bitstream. The remaining values are non-zero; however, they are indicated relative to the data in the first layer. This makes the values smaller and easier to encode (resulting in a smaller bitstream). Take the second sample of the first row of the Cb data of the first enhancement layer as an example. This Cb sample corresponds to value 01 of the Cb samples 1502 in the 4:4:4 source. The associated value in the base layer is Cb sample 00 (the first sample of the first row of the Cb samples in the base layer). This is because said value is the value that would be used with luma sample 01 in a 4:2:0 format. Therefore, a plurality of samples in the enhancement layer share a corresponding sample in the first layer (e.g. samples 00, 01, 10 and 11 in the enhancement layer share a corresponding sample 00 in the base layer). Accordingly, the second sample in the first row of the Cb data of the first enhancement layer is defined as the difference between these two values (sample 01 of the Cb sample in the 4:4:4 source and the Cb sample 00 in the base layer). This difference value will be small, since the value of sample 00 in the 4:4:4 source and the value of sample 01 in the 4:4:4 source will be similar to each other (corresponding to a similar part of the image). That is, changes in colour often happen quite slowly across an image, such that chroma samples corresponding to a same image region will (likely) be quite similar to each other. Since this difference value is (or will likely be) small, it becomes easier to encode for the encoder, such that the resulting bitstream also becomes smaller. Furthermore, a decoder can use this information from the first enhancement layer, combined with the data from the base layer, in order to reconstruct the full 4x4 block of Cb samples from the 4:4:4 source. That is, the decoder must, for each of the 4x4 samples, add the difference value from the first enhancement layer to the associated value from the base layer to reconstruct the full 4x4 block of Cb samples from the 4:4:4 source. Furthermore, while this process has been described for the first chroma component (Cb) using the first enhancement layer, it will be appreciated that the same process also applies to the second chroma component (Cr) using the second enhancement layer. As such, it is possible to reconstruct the 4:4:4 source from the multi-layer bitstream, where the multi-layer bitstream comprises a first layer and one or more second layers, wherein the first layer comprises data in the 4:2:0 format and the one or more second layers, having the 4:2:0 format, comprise data for reconstructing an image with the 4:4:4 format when combined with the data in the first layer. Furthermore, as indicated by the flag which is selectively encoded, the data in the one or more second layers can be defined relative to the data in the first layer thus exploiting similarity between the values of the samples in the 4:4:4 source in order to reduce the size of the resultant bitstream. Notably, this example has been described with reference to an example whereby the values used in the first layer of the multi-layer bitstream (i.e. the values in the base layer) are sampled directly from the 4:4:4 source. This means that certain values in the enhancement layer will be zero, as the difference between these values and the corresponding samples in the 4:4:4 source will be zero (i.e. the samples 00, 02, 20 and 22 in this example). However, the present disclosure is not particularly limited in this regard and can, more generally, be applied to any situation regardless of the method used in order to perform chroma subsampling to downsample the 4:4:4 source to the 4:2:0 format for the base layer (first layer of the multi-layer bitstream). In some examples, the chroma sub-sampling may be performed by generating corresponding values from the 4:4:4 source. For example, the first value in the first row of the Cb data in the 4:2:0 LayerO may be generated based on an average (or a weighted average) of the samples 00, 01, 10 and 11 in the 4:4:4 source. In this case, the values within the first enhancement layer would still be defined as a difference between the corresponding value in the 4:4:4 source and the associated value in the base layer. While this would not result in any zero values within the first enhancement layer, as none of the values in the base layer would correspond directly to a value in the 4:4:4 source, the values in the first enhancement layer would still be small (defined as a difference value) and a corresponding reduction in the size of the bitstream would still be achieved. In some examples, the spatial relationship between, say, the 4:2:0 chroma samples and, say, the 4:4:4 chroma samples may affect the compression efficiency which can be achieved. For example, if the 4:2:0 sample 00 is coincident with the 4:4:4 sample 00 (as schematically illustrated in the example of Figure 15) then the offset between these two values will be (near) zero. However, if, in examples, the 4:2:0 chorma sample is offset to be, say, midway between the 4:4:4 sample 00 and 01 (or 00 and 10) then the offset (or delta) between these two values (delta 00) will generally be larger. On the other hand, the delta 01 (or, alternatively, delta 10) will be reduced. Accordingly, the offset (or delta) values which are determined will have a different relative magnitude depending upon the spatial relationship between the 4:2:0 chorma samples and the 4:4:4 chroma samples. This will, in turn, affect the compression efficiency which can be achieved. Nevertheless, it will be appreciated that these delta values will still be smaller than the original values of the 4:4:4 chroma samples and therefore a reduction in the size of the bitstream would still be achieved. Accordingly, an improvement in coding efficiency will be achieved for any method of chroma downsampling which is used. Example SEI Message As previously mentioned, the method of embodiments of the disclosure comprises selectively encoding a flag when encoding image data a source chroma format as a multilayer bitstream. This flag can be used, by the decoder, for the decoder to identify that the bitstream is a multi-layer bitstream and can be used for the reconstruction of the source chroma format. Furthermore, the flag indicates that the data in the one or more second layers is defined relative to the data in the first layer. Accordingly, this flag can be used, by the decoder, in order to identify how to reconstruct the source chroma format from the layers of the multi-layer bitstream. In examples, flags may be one bit or multi-bit. In examples, the flag may be a binary flag. The encoding or presence of a flag may be conditional on another flag being encoded or present. In examples, flags may be part of another syntax element, for example, a predetermined bit or bits of another syntax element. The another syntax element may be related or unrelated to the purpose of the flag. In examples, the flag may, optionally, be included within a Supplemental Enhancement Information (SEI) message. The SEI message may also include additional information to further instruct the decoder how to reconstruct the source signal from the multi-layer bitstream. For example, information concerning the number of layers of the multi-layer bitstream and the manner of packing the chroma information within the enhancement layer(s) of the multi-layer bitstream may also be included within the SEI message. The present disclosure is not particularly limited in this regard. An enhanced SEI message may take the form as illustrated in Figure 16 of the present disclosure. That is, Figure 16 of the present disclosure schematically illustrates an example Supplemental Enhancement Information message. The present disclosure is not particularly limited to the example SEI message which is illustrated in Figure 16 of the present disclosure. However, the example SEI message of Figure 16 of the present disclosure provides an example of one such SEI message which could be used in order to provide a flag in accordance with embodiments of the disclosure. In particular, it is suggested to include within the SEI message a flag such as value_delta_flag[i] 1600. The value of value_delta_flag[i] may be set to a first value, such as 1, if the data in a given second (enhancement) layer is defined relative to the data in the base layer. Alternatively, the value of value_delta_flag[i] may be set to a second value, such as 0, if the data in a given second (enhancement) layer is not defined relative to the data in the base layer (and instead includes, directly, the values from the source. Accordingly, based on the value of this flag - which can be set during the encoding process - the decoder can identify, for any given layer of the multi-layer bitstream, whether or not the values of that layer are defined relative to the data in the base layer. Indeed, in some examples, a first enhancement layer (corresponding, say, to a first chroma component) may be defined relative to the base layer while a second enhancement layer (corresponding, say, to a second chroma component) may provide original values (and may not, therefore, be defined relative to the base layer). However, while the example of Figure 16 provides one example of SEI messaging for the flag, it will be appreciated that the present disclosure is not particularly limited in this respect and, more generally, any suitable messaging of the flag of embodiments of the disclosure may be provided. 4:2:2 Source - Example Situation The example of Figure 15 of the present disclosure has been described with reference to an example situation of the use of 4:4:4 source image format and a 4:2:0 first format (being the format of the individual layers of the multi-layer bit stream). However, the present disclosure is not particularly limited in this regard. More generally, the method of embodiments of the disclosure may be used for efficient coding of any high fidelity colour format in a native format of a decoder. As an example, it will be appreciated that embodiments of the present disclosure may be applied to an example situation of 4:2:2 source being encoded as a multi-layer bitstream, where the layers of the bitstream are in a 4:2:0 format. Consider, now, Figure 17 of the present disclosure. Figure 17 schematically illustrates an example multi-layer bitstream for source image data having a 4:2:2 format. In this example, the layers of the multi-layer bitstream have a 4:2:0 format. The example of Figure 17 is similar to the example which has been described with reference to Figure 12 of the present disclosure. Therefore, details of the example of Figure 17 which are the same as the example situation of Figure 12 will not be repeated again, for brevity of disclosure. The example of Figure 17 differs from the example of Figure 12 insofar that, in the example of Figure 12, the values within the second layer (the enhancement layer) are original values corresponding to the values in the 4:2:2 source. However, in the example of Figure 17, the values are defined as relative to the data in the first layer. Accordingly, the flag is set to indicate that the data in the second layer is defined relative to the data in the first layer. Accordingly, since the data in the second layer is defined relative to the data of the first layer, the size of the resulting bitstream can be reduced, further improving coding efficiency. In this example, the first layer of the multi-layer bitstream (the base layer) contains the luma information and the chroma information (for both Cb and Cr) which is required for the decoder to produce an image in the 4:2:0 format. The samples of Cb and Cr which are present in the base layer have been generated by downsampling of the corresponding Cb and Cr samples of the 4:2:2 source. In this example, the chroma downsampling has been performed by directly sampling the samples from the 4:2:2 source. That is, a value in the base layer (such as Cb 00) corresponds directly to a value in the 4:2:2 source (again, Cb 00, for example). Furthermore, in this example, the additional information for the two chroma components (Cb and Cr in this example) is combined within a single second layer (4:2:0 Layerl). Indeed, in this example, the Cb and Cr samples are packaged side by side within the second layer (in the area corresponding to the area reserved for the luma samples in the first layer). As explained, in this example, the values in this second (enhancement) layer are defined as relative to the data of the first layer. This means that each chroma sample is defined as a difference value between the value of that chroma sample in the (source) image data and the value of a corresponding sample in the first layer. Accordingly, in this example, the first row and the third row of the second layer become zero valued, since these values have already been provided in the first layer. As an example, the first sample of the first row in the second layer corresponds to the value 00 of the Cb component in the source image data and has a corresponding value 00 in the first layer. The difference between these two values is zero, such that the sample becomes zero valued in the first layer. Likewise, the third sample of the first row in the second layer corresponds to the value 00 of the Cr component in the source image data and has a corresponding value 00 in the first layer. The difference between these two values is zero, such that the sample becomes zero valued in the first layer. However, the values of the second and fourth rows of the enhancement layer are not zero in this example. Instead, these samples contain additional information which, when combined with the information from the first layer, can be used by the decoder to reconstruct the image in a 4:2:2 format. As these values are defined relative to the values in the first layer, these values become much smaller than the corresponding values in the example of Figure 12 of the present disclosure. For example, Cb 00 (from the 4:2:2 source) has already been included within the first layer of the multi-layer bitstream. Cb 10 has not been included within the first layer of the multi-layer bitstream and must therefore be included within the second layer, if the decoder is to be able to reconstruct the 4:2:2 source. However, instead of including the value of Cb 10 directly within the second layer, the value Cb 10 - Cb 00 is included in the second layer in this example. In other words, the value of Cb 10 in the second layer is included as an offset (or delta value) from Cb 00 which has already been included in the first layer. Since the values of Cb 10 and Cb 00 are likely to be very similar (since colour typically varies gradually over an image) the difference value (Cb 10 - Cb 00) is likely to be much smaller than the original value of Cb 10. This means that the difference value is much easier to encode, which reduces the size of the resultant bitstream, thus improving the coding efficiency. It will be appreciated that a similar process applies for all those values within the second layer (4:2:0 Layerl)- including those values for both the first chroma component (Cb) and the second chroma component (Cr). For each sample in the second layer, the value of that sample is defined as relative to the data in the first layer. While this example has been described with reference to a YCbCr colour space, it will be appreciated that the present disclosure is not particularly limited in this regard. More generally, embodiments of the present disclosure can be applied to any suitable colour space as desired. 4:4:4 Source - Example Situation Embodiments of the present disclosure may also be applied to a situation whereby the source data format is 4:4:4 and the first data format is 4:2:2. That is, the embodiments of the present disclosure may also be used in order to achieve 4:4:4 decoding on systems that support only 4:2:2 profiles. Consider, now, the example of Figure 18 of the present disclosure. Figure 18 of the present disclosure schematically illustrates an example multi-layer bitstream for source image data having a 4:4:4 format. This example is similar to the example of Figure 15 of the present disclosure. Accordingly, details which have already been described with reference to Figure 15 of the present disclosure are not repeated here, for brevity of the disclosure. In particular, Figure 18 is similar to the example which has been described with reference to Figure 15 of the present disclosure, in that Figure 18 shows an example situation where a 4:4:4 source is encoded as a multi-layer bitstream. However, the example of Figure 18 differs from the example of Figure 15 in that the multi-layer bitstream has a 4:2:2 format (whereas, the example described with reference to Figure 15 related to a multi-layer bitstream having a 4:2:0 format). As such, the example of Figure 18 shows an example where a 4:4:4 source is encoded as a multi-layer bitstream with a 4:2:2 format, such that a decoder able only to decode images with a 4:2:2 format is still able to reconstruct a 4:4:4 image from the multilayer bitstream. In other words, in this example, the source chroma format is a 4:4:4 chroma format and the first chroma format (being the format of the layers of the multi-layer bitstream) is a 4:2:2 chroma format. In this example, the first layer of the multi-layer bitstream (the base layer) contains complete information which can be used to construct an image having a 4:2:2 format. Taking YCbCr as an example, the base layer in Figure 18 contains a 4x4 block of Y, and a 2x4 block of each of Cb and Cr (having been sampled from the 4:4:4 source). Accordingly, the base layer is in a 4:2:2 format, since the horizontal information of Cb and Cr has been sampled at half the rate of Y, while full sampling is maintained vertically). In this example, two second layers (enhancement layers) are provided. The first of these enhancement layers is for the Cr component of the image while the second of these enhancement layers is for the Cb component of the image. However, while this arrangement is shown in this example, it will be appreciated that the present disclosure is not particularly limited in this regard. In examples, multiple image components can be included within a single enhancement layer. Alternatively, in examples, there may be many more enhancement layers (i.e. the number of enhancement layers is not particularly limited to 2). The number and configuration of the enhancement layers may vary depending on the colour space, the source format and the first format, for example. As has been explained, these enhancement layers are configured such that they comprise data for reconstructing an image with the source chroma format when combined with the data in the first layer. Furthermore, in this example, the data in the one or more second layers is defined relative to the data in the first layer. This reduced the size of the resultant bitstream (by reducing the size of the values which are to be encoded) and thus improves coding efficiency. Taking the Cr component as an example (corresponding to the second enhancement layer, 4:2:2 Layer2 in this example), it is noted that the base layer (4:2:2 LayerO) of the multilayer bitstream already contains information for the Cr component in a 4:2:2 format. That is, the base layer contains all of the samples of the first column (00, 10, 20 and 30) and all of the samples of the third column (02, 12, 22 and 32) of the Cr component from the 4:4:4 source. Therefore, the additional information which is required in the enhancement layer to enable a 4:4:4 image to be reconstructed is the information relating to the second column (01, 11, 21 and 31) and the fourth column (03, 13, 23 and 33) of the Cr component from the 4:4:4 source. This information is thus included in the second enhancement layer (4:2:2 Layer2) as an offset between the 4:4:4 source and the corresponding sample which has been included in the base layer. Since, in this example, the chorma subsampling to produce the 4:2:2 image from the 4:4:4 source directly sampled the values from the 4:4:4 source, this means that the first column of the Cr component in the base layer corresponds directly to the first column of the Cr component in 4:4:4, while the second column of the Cr component in 4:4:4 corresponds directly to the third column of the Cr component in 4:4:4. Accordingly, in the second enhancement layer, the first and third column are zero valued, as there is no difference between the values in 4:4:4 and the values which have already been included in the base layer. However, for the second and fourth column, the samples are defined by the difference between the corresponding value in the 4:4:4 source and the associated value in the base layer. As an example, the second sample in the first row of the Cr component in the second enhancement layer corresponds to the difference between the second sample in the first row of the Cr component in the 4:4:4 source (01) and the associated value in the base layer, which is the first sample in the first row (00). By encoding this additional information as the difference value (i.e. 01-00) the values contained within the enhancement layer become much smaller than if those values were signalled directly (i.e. 01). Therefore, the coding efficiency is improved. In this way, a decoder, limited only to 4:2:2 profiles, can nevertheless reconstruct a 4:4:4 image from the multi-layer bitstream. The 4:2:2 image data can be obtained directly from the base layer and the additional values required for the 4:4:4 image can be obtained from the corresponding enhancement layer. When these values are included as a difference value (as in the example of Figure 18) the decoder can, on the basis of the flag, obtain the necessary values by combining the difference value from the enhancement layer with the associated value from the base layer. As such, it is possible to reconstruct the 4:4:4 source from the multi-layer bitstream, where the multi-layer bitstream comprises a first layer and one or more second layers, wherein the first layer comprises data in the 4:2:2 format and the one or more second layers, having the 4:2:2 format, comprise data for reconstructing an image with the 4:4:4 format when combined with the data in the first layer. Furthermore, as indicated by the flag which is selectively encoded, the data in the one or more second layers can be defined relative to the data in the first layer thus exploiting similarity between the values of the samples in the 4:4:4 source in order to reduce the size of the resultant bitstream. Video Capture, Transmission, Display and / or Storage Apparatus Consider a situation where a video capture, transmission, display and / or storage apparatus is capable of coding only a certain data format, such a 4:2:0 data format. Typically, that video capture, transmission, display and / or storage apparatus may be limited to use only of data in a 4:2:0 format. However, for certain tasks, such as computer vision tasks, it may be necessary, or at least desirable, that the data is coded in a 4:4:4 data format. Accordingly, embodiments of the present disclosure can be used in order to achieve efficient 4:4:4 coding on such a video capture, transmission, display and / or storage apparatus. This can be achieved through the multi-layer bitstream of the present disclosure, wherein the multi-layer bitstream comprises a first layer and one or more second layers, wherein the first layer comprises data in a first chroma format and the one or more second layers, having the first format, comprise data for reconstructing an image with the source chroma format when combined with the data in the first layer. In particular, by selectively encoding a flag when encoding image data having a source chroma format as a multi-layer bitstream, wherein the flag indicates that the data in the one or more second layers is defined relative to the data in the first layer, said 4:4:4 coding can be efficiently achieved. Thus, improved compatibility and distribution of high colour fidelity image data can be achieved, even when a video capture, transmission, display and / or storage apparatus is capable of coding only a certain data format (such as 4:2:0 data). Example Decoding of Multi-Layer Bitstream Consider, now, Figure 19 of the present disclosure. Figure 19 schematically illustrates an example decoding method. The example decoding method starts at step S1900 and proceeds to step S1902. In step S1902, the method comprises selectively decoding a flag when decoding image data having a 4:2:2 chroma format, encoded as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and a second layer, wherein the first layer comprises data in a 4:2:0 chroma format and the second layer, having the 4:2:0 chroma format, comprises data for reconstructing an image with the 4:2:2 chroma format when combined with the data in the first layer. The method then proceeds to, and ends with, step S1904. Accordingly, with the example method of Figure 19, image data having a source chroma format can be decoded. The source chroma format can be decoded by reconstruction of the data from across the layers of the multi-layer bit stream. In particular, the first layer of the bitstream can be used to decode the image data in a first chroma format (e.g. as a base layer) while the one or more second layers, also having this first chroma format, can be used to obtain additional data (e.g. enhancement data) which can be used in order to reconstruct the image data with the source chroma format. In this way, the decoder, able only to decode an image with the first chroma format can nevertheless decode and reconstruct an image having a source chroma format through use of the additional information from the enhancement layers. Therefore, improved compatibility and distribution of image data with higher levels of colour fidelity can be achieved, even when a decoder is limited to only certain decoding profiles. Furthermore, the decoder decodes a flag which indicates that the data in the one or more second layers is defined relative to the data in the first layer. The decoder can therefore reconstruct the image data with the source format, even though the values have only been defined relative to the values in the first layer of the multi-layer bitstream. This reduces the size of the (encoded) bitstream which is required to transmit the image data to the decoder, and thus improves coding efficiency. While an example method of decoding has been described with reference to Figure 19 of the present disclosure, it will be appreciated that the present disclosure is not particularly limited in this regard. A decoding apparatus is also provided in accordance with embodiments of the present disclosure. The decoding apparatus may comprise circuitry which is configured in order to perform a decoding method (such as that described with reference to Figure 19 of the present disclosure). In examples, the decoding apparatus may be part of a decompression apparatus such as that described with reference to Figure 7 of the present disclosure. Example Encoding - Spatial Regions Figures 11 to 19 of the present disclosure have been described with reference to an example situation of coding of image data having a source chroma format as a multi-layer bitstream, with layers having a first chroma format. However, in alternative embodiments of the disclosure, the image data may not be encoded as a multi-layer bitstream at all. Rather, in embodiments of the disclosure, image data having a source chroma format may be encoded using different spatial regions within frames of a bitstream, with those frames having a first chroma format. The image data having the source chroma format may be reconstructed from the different spatial regions, thus enabling improved compatibility and distribution of image data having the source chroma format. Consider, now, Figure 20 of the present disclosure. Figure 20 schematically illustrates an example encoding method. The example encoding method starts at step S20000 and proceeds to step S20002. In step S20002, the method comprises selectively encoding a flag when encoding image data having a source chroma format as frames represented in a bitstream having a first chroma format, wherein the frames represented in the bitstream comprise a first spatial region and two second spatial regions, the first spatial region comprising data of a first component and the two second spatial regions comprising data of a second and a third component for reconstructing an image with the source chroma format when combined with the data in the first spatial region; wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in each of the frames. The method then proceeds to, and ends with, step S20004. Accordingly, in the example of Figure 20, image data having a source chroma format is encoded as frames represented in a bitstream, the frames having a first chroma format. Each of these frames comprises a number of different spatial regions. In a first spatial region, data of a first component (e.g. a Y component) is provided, while the two second spatial regions each provide data of a second and third component (e.g. Cb and Cr). In other words, all components for reconstructing image data having the source chroma format can be provided within a frame of image data having a first chroma format (different from the source chroma format) such that the image data having the source chroma format can be reconstructed. In this way, a decoder able only to natively decode an image with a first chroma format (e.g. 4:2:0) can nevertheless decode and reconstruct image data having a source chroma format (e.g. 4:4:4) through use of the additional information from the different spatial regions. Accordingly, improved compatibility and distribution of image data with higher levels of colour fidelity can be achieved, even when a decoder is limited only to certain decoding profiles. In examples, a post-process may be applied by the decoder. In examples, the postprocess may split the frame into three separate frames comprising each of the colour components which have been arranged in the frames of the bitstream. Consider the example of Figure 21 of the present disclosure. In this example, three colour components (Y, Cb and Cr) have been packaged into the frame of the bitstream. Each colour component has width W and height H. Therefore, since there are three colour components arranged side by side in this example, the frame of the bitstream has a width 3W and height H. The decoder postprocess may split the 3WxH frame into three separate frames, of size WxH each comprising one of the colour components (i.e. one WxH frame for Y, one WxH frame for Cb and one WxH frame for Cr). Furthermore, in examples, a similar function may be applied by an encoder pre-process. For example, the encoder pre-process (applied prior to the encoding) may package the three separate frames of each colour component in a single frame prior to encoding. It will be appreciated that since the different spatial regions are arranged side by side in the frames, latency when decoding can be further reduced. That is, when decoding, a decoder may typically decode starting at the first value in the first row. The next value to be decoded will be the second value in the first row and so on, until the final value in the first row is decoded. Then, the decoder will proceed to the second row. This process will continue until all rows are decoded. Accordingly, by arranging (or packing) the different spatial regions side by side in the frames, latency when decoding can be reduced. This will be discussed in more detail later. While an example method of encoding has been described with reference to Figure 20 of the present disclosure, it will be appreciated that the present disclosure is not particularly limited in this regard. Furthermore, an encoding apparatus is also provided in accordance with embodiments of the present disclosure. The encoding apparatus may comprise circuitry which is configured in order to perform an encoding method (such as that described with reference to Figure 20 of the present disclosure). Furthermore, in examples, the encoding apparatus may be part of a compression apparatus such as that described with reference to Figure 7 of the present disclosure. 4:4:4 Source - Example Situation An encoding method will now be described with reference to an example situation of a YCbCr colour space. However, as previously mentioned, it will be appreciated that the embodiments of the disclosure are not particularly limited to a YCbCr colour space. That is, embodiments of the disclosure can be applied more generally to any colour space. For example, embodiments of the disclosure may be applied to an RGB colour space. Alternatively, embodiments of the disclosure may be applied to a YUV colour space. The choice of colour space which is used will depend on the situation to which the embodiments of the disclosure are applied. Consider, now, Figure 21 of the present disclosure. Figure 21 of the present disclosure schematically illustrates source image data having a 4:4:4 format. More specifically, Figure 21 schematically illustrates how source image data having a 4:4:4 format can be encoded within frames of a bitstream, where those frames have a 4:2:0 or a 4:0:0 chroma format. In this example, the source image data, being the image data to be encoded, has a 4:4:4 image data format. That is, in this example, there are three components of the image data. More specifically, in this example, the image data has a YCbCr 4:4:4 format. Therefore, the three components are a luma component (Y) and two chroma components (Cb and Cr). Indeed, in this example, there are 16 luma samples. That is, the luma is defined by a 4x4 block of luma samples, which range from 00 to 03 on the first row, 10 to 13 on the second row, 20 to 23 on the third row and 30 to 33 on the fourth row. For each of the chroma components, Cb and Cr, there are - in the source format - an equal number of chroma samples as there are luma samples. That is, the Cb component is defined by a 4x4 block of chroma samples. Likewise, the Cr component is defined by a 4x4 block of chroma samples. This is illustrated in Figure 21. Certain decoder implementations may be limited to a profile other than the 4:4:4 profile and may therefore not be able to natively decode a 4:4:4 image. Therefore, according to the present disclosure, the 4:4:4 image data can be encoded as frames within the bitstream, where those frames have a format such as 4:2:0 (or 4:0:0). This enables the 4:4:4 image data to be distributed even when a decoder implementation is capable only of supporting 4:4:0 (or 4:0:0) profiles, since the image data having the 4:4:4 format can be reconstructed from the different regions within the frames of the bitstream. In particular, consider the example of an encoded frame of the bitstream which is schematically illustrated in Figure 21. In this example, the three components are arranged within an area of the frame which is typically reserved for the Y component in the 4:2:0 format. That is, the area of the frame which is typically reserved for the Y component in the 4:2:0 format instead comprises three distinct spatial regions (a first spatial region and two second spatial regions). The first of these spatial regions is occupied by the luma samples of the 4:4:4 source. That is, the first spatial region in the example frame of Figure 21 is occupied by the luma samples 00 to 03 (on the first row), 10 to 13 (on the second row), 20 to 23 (on the third row) and 30 to 33 (on the fourth row). Then, each of the second spatial region is occupied by the Cb chroma samples and Cr chroma samples respectively. That is, the Cb chroma samples occupy the fifth to the eighth positions on the first, second, third and fourth row. Furthermore, the Cr chroma samples occupy the ninth to the twelfth positions on the first, second, third and fourth row. Thus, all samples of the 4:4:4 source (i.e. all luma samples and all chroma samples of the 4:4:4 source) are arranged within different spatial regions of the area of the frame which are typically reserved for the luma components in 4:2:0 (or 4:0:0) chroma format. Therefore, the frame, having the 4:2:0 or 4:0:0 format, carries the information which is necessary for reconstructing the image data having the 4:4:4 format. In this example, each of the two second spatial regions thus corresponds to an individual one of the second and third components. That is, one of the second spatial regions is occupied by the samples of the first chroma component (e.g. Cb) while the other of the second spatial regions is occupied by the samples of the second chroma component (e.g. Cr). Furthermore, in this example, a size of each of the two second spatial regions is, individually, the same as a size of the first spatial region. For example, the 4x4 block of luma samples is matched in size by the 4x4 block of Cb components and the 4x4 block of Cr components. However, the present disclosure is not particularly limited in this regard, as will be explained with reference to Figure 22A of the present disclosure later. A decoder can thus use the frame of the bitstream to produce image data having the 4:4:4 format if desired. Within each frame of the bitstream, the area typically reserved for the chroma components may be left blank (as these values have been instead included within different spatial regions of the area typically reserved for the luma component). As these values are blank (i.e. zero value) they have minimal impact on the size of the bitstream (as it is very efficient for an encoder to encode these values). Therefore, in effect, the frame of the bitstream in a 4:2:0 format with those areas typically reserved for the chroma components left blank is, effectively, a 4:0:0 format. As noted with reference to Figure 20 of the present disclosure, the data for reconstructing the image with the 4:4:4 chroma format can thus be completely provided within a frame having a 4:2:0 (or 4:0:0) chroma format, leading to improved compatibility and distribution of image data with higher levels of colour fidelity can be achieved, even when a decoder is limited only to certain decoding profiles. Furthermore, as has been noted with reference to Figure 20 of the present disclosure, the chroma samples for Cb and Cr are each, in turn, packaged side by side with the luma Y samples within the frame. Arranging the samples side by side in this manner also reduces latency when decoding. In examples, a side by side arrangement of the samples comprises side by side arrangement of the samples in a horizontal direction (as illustrated in Figure 21 of the present disclosure). In examples, a side by side arrangement of the samples comprises side by side arrangement of the samples in an intended decoding direction in each of the frames (such that those samples are side by side in a direction in which the decoder decodes the samples). When decoding, a decoder may typically decode starting at the first value on the first row. The next value to be decoded will be the second value in the first row and so on, until the final value in the first row is decoded. Then, the decoder will proceed to the second row. This process will continue until all rows are decoded. If the decoder is to produce an image having 4:4:4 data, then the decoder will be required to use all the samples contained within each frame of the bitstream in order to produce the image. That is, the decoder will need to combine the luma values from the first spatial area with the chroma values from the two second spatial regions in order to produce the image having the 4:4:4 format. For sample 00, this means that the decoder must decode the first Y sample from the frame (located in the first sample of the first row of the frame). However, at this stage, the decoder still does not have information for the Cb sample 00 and the Cr sample 00 (with this information being required in order to complete the reproduction of sample 00). As the Y, Cb and Cr samples are packaged side by side within the frame, the first Cb sample (Cb sample 00) is included as the fifth sample of the first row and the first Cr sample (Cr sample 00) is located as the ninth value of the first row. Therefore, the decoder must only decode the first nine values before it has complete YCbCr values for the sample 00. Moreover, once the first row of the frame has been decoded, the decoder will have complete YCbCr values for the first row of image data in the 4:4:4 format. Accordingly, this arrangement has low latency compared to a different arrangement of packing the Cb and Cr components in the frame. For example, consider a situation whereby the Cb and Cr samples were packed vertically within the frame (such that the Cb components were stacked on top of the Cr components). In this example, the luma and chroma values included in the frame would form a 4x12 block of samples. The first 4 rows would include the luma samples and the remaining 8 rows would then include the Cb and Cr samples respectively. With this arrangement, a decoder would only obtain Cb sample 00 once the first sample of the fifth row of the frame had been decoded. Furthermore, the decoder would only obtain Cr sample 00 once the first sample in the ninth row of the frame had been decoded. Accordingly, until this sample had been decoded, the decoder would be unable to reconstruct an image having the 4:4:4 format, even for sample 00. Compared with an arrangement such as this, a side by side packing of the Cb and Cr samples with the Y samples in the frame therefore provides lower latency decoding. As previously mentioned, embodiments of the disclosure comprises selectively encoding a flag, wherein the flag indicates that the first spatial region (e.g. the region containing the luma component) and each of the two second spatial regions (e.g. the regions containing the chroma components) are arranged side by side in the frames of the bitstream. Accordingly, this flag can be used, by the decoder, for the decoder to identify that the bitstream is a bitstream which can be used for reconstruction of the 4:4:4 chroma format. The flag may, optionally, be included within a Supplemental Enhancement Information (SEI) message. The SEI message may also include additional information to instruct the decoder how to reconstruct the 4:4:4 signal from the frame of the bitstream. The present disclosure is not particularly limited in this regard. In examples, a flag may be one bit or multi-bit. In examples, the flag may be a binary flag. The encoding or presence of a flag may be conditional on another flag being encoded or present. In examples, flags may be part of another syntax element, for example, a predetermined bit or bits of another syntax element. The another syntax element may be related or unrelated to the purpose of the flag. Accordingly, the present disclosure is not particularly limited in this regard. It will be appreciated that the present disclosure is not particularly limited to the specific number and arrangement of samples which has been schematically illustrated in Figure 21 of the present disclosure. However, in accordance with embodiments of the disclosure, it will be appreciated that improved compatibility and distribution of 4:4:4 image data can be achieved, since even decoders limited to 4:2:0 (or event 4:0:0) profiles can reconstruct 4:4:4 image data from the multi-layer bitstream. Moreover, as the different samples are arranged side by side within each frame of the bitstream, this distribution of 4:4:4 image data can be achieved in a low latency environment. 4:2:2 Source - Example Situation While an example of coding image data having a 4:4:4 source format has been described with reference to Figure 21 of the present disclosure, it will be appreciated that the present disclosure is not particularly limited in this regard. Embodiments of the disclosure may also be applied to image data having a different source chroma format. Indeed, in examples, embodiments of the present disclosure may be applied to image data having a 4:2:2 format as a source chroma format. Consider, now, Figure 22A of the present disclosure. Figure 22A of the present disclosure schematically illustrates source image data having a 4:2:2 format. More specifically, Figure 22A schematically illustrates how source image data having a 4:2:2 format can be encoded within frames of a bitstream, where those frames have a 4:2:0 or a 4:0:0 chroma format. In this example, the source image data, being the image data to be encoded, has a 4:2:2 image data format. That is, in this example, there are three components of the image data. The three components are, in this example, a luma component (Y) and two chroma components (Cb and Cr). In this example, there are 16 luma samples. That is, the luma is defined by a 4x4 block of luma samples, which range from 00 to 03 on the first row, 10 to 13 on the second row, 20 to 23 on the third row and 30 to 33 on the fourth row. For each of the chroma components, Cb and Cr, there are half as many chroma samples horizontally (compared to the luma), but a full number of samples vertically (compared to the luma). Accordingly, the Cb component is defined by a 2x4 block of chroma samples. Likewise, the Cr component is defined by a 2x4 block of chroma samples. This is illustrated in the example of Figure 22A. As has been described with reference to Figures 9 and 10 of the present disclosure, the manner by which the chroma sub-sampling is performed in order to generate the Cr and Cb components in the 4:2:2 format is not particularly limited. In some examples, each chroma sample in the 4:2:2 format may correspond directly to a chroma sample in the 4:4:4 format. That is, in some examples, chroma sub-sampling may be performed by selecting a subsample of the chroma samples in the 4:4:4 format and using that sub-sample of the chroma samples directly as the chroma values in the 4:2:2 format. On the other hand, in some examples, the chroma samples in the 4:2:2 format may be generated from the chroma samples in the 4:4:4 format as part of the chroma sub-sampling process. Nevertheless, in the example of Figure 22A of the present disclosure, each chroma sample in the 4:2:2 format corresponds directly to a chroma sample in a 4:4:4 format. As such, the first chroma sample on the first row is sample 00 (corresponding to the first chroma sample of a 4:4:4 block). The second sample on the first row is sample 02 (corresponding to the third chroma sample of the 4:4:4 block). Thus half sampling of the chroma values in the horizontal direction is achieved. Once the 4:2:2 source has been generated (e.g. by downsampling from 4:4:4) it is desired that the 4:2:2 data be encoded for distribution. However, certain decoder implementations may be limited to a 4:2:0 profile and therefore cannot natively decode a 4:2:2 image. Therefore, according to the present disclosure, the 4:2:2 image data can be encoded as frames within a bitstream, where each of the frames in the bitstream has a 4:2:0 (or 4:0:0) format. As has been described with reference to the example method of Figure 20 of the present disclosure, this enables a 4:2:2 image to be distributed even when a decoder implementation is capable of supporting 4:2:0 (or 4:0:0) profiles, since the image data can be reconstructed from across the different spatial regions of the frames of the bitstream. In particular, consider the example of the encoded frame of the bitstream which is schematically illustrated in Figure 22A. In this example, the three components are arranged within an area of the frame which is typically reserved for the Y component in the 4:2:0 format. That is, the area of the frame which is typically reserved for the Y component in the 4:2:0 format instead comprises three distinct spatial regions (a first spatial region and two second spatial regions). The first of these spatial regions is occupied by the luma samples of the 4:2:2 source. That is, the first spatial region in the example frame of Figure 22A is occupied by the luma samples 00 to 03 (on the first row), 10 to 13 (on the second row), 20 to 23 (on the third row) and 30 to 33 (on the fourth row). Then, one of the second spatial regions is occupied by the Cb chroma samples (such that the Cb chroma samples occupy the fifth and sixth positions on the first, second, third and fourth row). The other second spatial region is occupied by the Cr chroma samples (such that the Cr chroma samples occupy the seventh and eighth positions on the first, second, third and fourth row). Thus, all samples of the 4:2:2 source (i.e. all luma samples and all chroma samples of the 4:2:2 source) are arranged within different spatial regions of the area of the frame which are typically reserved for the luma components in 4:2:0 (or 4:0:0) chroma format. Therefore, the frame, having the 4:2:0 or 4:0:0 format, carries the information which is necessary for reconstructing the image data having the 4:2:2 format. In this example, each of the two second spatial regions thus corresponds to an individual one of the second and third components. That is, one of the second spatial regions is occupied by the samples of the first chroma component (e.g. Cb) while the other of the second spatial regions is occupied by the samples of the second chroma component (e.g. Cr). Furthermore, in this example, a size of each of the two second spatial regions (those regions comprising the Cb and Cr samples) are individually, different in size to the first spatial region (the region containing Y samples). This may arise since the 4:2:2 source comprises half as many chroma samples horizontally compared to the luma samples. Accordingly, for this same reason, the combined size of the two second spatial regions is the same as the size of the first spatial region in this example. However, in some examples, it may be advantageous to maintain a size of the two second spatial regions such that they are the same size, individually, as the size of the first spatial region. Accordingly, in some examples, zero values may be included within each of the second spatial regions in order to maintain a size of the second spatial regions compared to the first spatial region. For example, a vertical column of zero values (or other blank data) may interlaced with the Cb and Cr samples, such that the two second spatial regions are also formed by 4x4 blocks (i.e. such that the second spatial regions have the same size as the first spatial region). This is schematically illustrated in Figure 22B of the present disclosure (which illustrates a source image having a 4:2:2 format). This enables a decoder to decode the frames of the bitstream in a same way regardless of whether the source image format has a 4:4:4 image format or a 4:2:2 image format, and thus further improves the efficiency of coding (and reduces the information which must be signalled to the decoder in the bitstream). A decoder can thus use the frame of the bitstream to produce image data having the 4:2:2 format if desired. In examples, this flag can be used, by the decoder, for the decoder to identify that the bitstream is a bitstream which can be used for reconstruction of the 4:2:2 chroma format. In examples, the flag (or flags) may indicate specifically the source format which can be reconstructed from the frames of the bitstream (e.g. whether the frame of the bitstream can be used in order to reconstruct a 4:4:4 format or a 4:2:2 format). In examples, the flag may also indicate a manner by which the decoder can reconstruct the source format and / or an arrangement or configuration of the frames of the bitstream. For example, the flag may indicate that the frames contain the information which can be used for reconstruction of the source format in a manner such as illustrated in the example of Figure 22A (i.e. without zero packing) or in a manner such as illustrated in the example of Figure 22B (i.e. with zero packing). In examples, the bitstream may also include additional information to instruct the decoder how to reconstruct the 4:2:2 signal from the frame of the bitstream. The present disclosure is not particularly limited in this regard. Within each frame of the bitstream, the area typically reserved for the chroma components is left blank (as these values have been instead included within different spatial regions of the area typically reserved for the luma component). As these values are blank (i.e. zero value) they have minimal impact on the size of the bitstream (as it is very efficient for an encoder to encode these values). Therefore, in effect, the frame of the bitstream in a 4:2:0 format with those areas typically reserved for the chroma components left blank is, effectively, a 4:0:0 format. As noted with reference to Figure 20 of the present disclosure, the data for reconstructing the image with the 4:2:2 chroma format is thus completely provided within a frame having a 4:2:0 (or 4:0:0) chroma format, leading to improved compatibility and distribution of image data with higher levels of colour fidelity can be achieved, even when a decoder is limited only to certain decoding profiles. Furthermore, as has been noted with reference to Figure 20 of the present disclosure, the chroma samples for Cb and Cr are packaged side by side with the luma Y samples within the frame. Arranging the samples side by side in this manner also reduces latency when decoding, as has been described in detail with reference to Figure 21 of the present disclosure. As previously mentioned, embodiments of the disclosure comprises selectively encoding a flag, wherein the flag indicates that the first spatial region (e.g. the region containing the luma component) and each of the two second spatial regions (e.g. the regions containing the chroma components) are arranged (packaged or present) side by side in the frames of the bitstream. Accordingly, this flag can be used, by the decoder, for the decoder to identify that the bitstream is a bitstream which can be used for reconstruction of the 4:2:2 chroma format. The flag may, optionally, be included within a Supplemental Enhancement Information (SEI) message. The SEI message may also include additional information to instruct the decoder how to reconstruct the 4:2:2 signal from the frame of the bitstream. The present disclosure is not particularly limited in this regard. In examples, a flag may be one bit or multi-bit. In examples, the flag may be a binary flag. The encoding or presence of a flag may be conditional on another flag being encoded or present. In examples, flags may be part of another syntax element, for example, a predetermined bit or bits of another syntax element. The another syntax element may be related or unrelated to the purpose of the flag. Accordingly, the present disclosure is not particularly limited in this regard. It will be appreciated that the present disclosure is not particularly limited to the specific number and arrangement of samples which has been schematically illustrated in Figure 22 of the present disclosure. However, in accordance with embodiments of the disclosure, it will be appreciated that improved compatibility and distribution of 4:2:2 image data can be achieved, since even decoders limited to 4:2:0 (or event 4:0:0) profiles can reconstruct 4:2:2 image data from the multi-layer bitstream. Moreover, as the different samples are arranged side by side within each frame of the bitstream, this distribution of 4:2:2 image data can be achieved in a low latency environment. Example Decoding - Spatial Regions Consider, now, Figure 23 of the present disclosure. Figure 23 schematically illustrates an example decoding method. The example decoding method starts at step S23000 and proceeds to step S23002. In step S23002, the method comprises selectively decoding a flag when decoding image data having a source chroma format as frames represented in a bitstream having a second chroma format, wherein the frames represented in the bitstream comprise a first spatial region and two second spatial regions, the first spatial region comprising data of a first component and the two second spatial regions comprising data of a second and a third component for reconstructing an image with the source chroma format when combined with the data in the first spatial region; and wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in each of the frames. The method then proceeds to, and ends with, step S23004. In examples, the decoder may also use the flag in order to identify the source format to be reconstructed from the bitstream (e.g. 4:4:4, 4:2:2, or 4:2:0). Accordingly, in the example of Figure 23, image data having a source chroma format can be decoded from frames represented in a bitstream, the frames having a first chroma format. Each of these frames comprises a number of different spatial regions (e.g. within an area which is typically reserved for a first component). Furthermore, since these spatial regions are arranged side by side in each frame, lower latency when decoding can be achieved. While an example method of decoding has been described with reference to Figure 23 of the present disclosure, it will be appreciated that the present disclosure is not particularly limited in this regard. Furthermore, an decoding apparatus is also provided in accordance with embodiments of the present disclosure. The decoding apparatus may comprise circuitry which is configured in order to perform a decoding method (such as that described with reference to Figure 23 of the present disclosure). Furthermore, in examples, the decoding apparatus may be part of a decompression apparatus such as that described with reference to Figure 7 of the present disclosure. Video Capture, Transmission, Display and / or Storage Apparatus Consider, again, a situation where a video capture, transmission, display and / or storage apparatus is capable of coding only a certain data format, such a 4:2:0 data format. Typically, that video capture, transmission, display and / or storage apparatus may be limited to use only of data in a 4:2:0 format. However, for certain tasks, such as computer vision tasks, it may be necessary, or at least desirable, that the data is coded in a 4:4:4 (or even 4:2:2) data format. Accordingly, embodiments of the present disclosure can be used in order to achieve efficient and low latency 4:4:4 (or even 4:2:2) coding on such a video capture, transmission, display and / or storage apparatus. This can be achieved through the bitstream of the present disclosure, wherein the image data, having a source chroma format, is encoded as frames represented in the bitstream, wherein the frames represented in the bitstream comprise a first spatial region and two second spatial regions, the first spatial region comprising data of a first component and the two second spatial regions comprising data of a second and a third component for reconstructing an image with the source chroma format when combined with the data in the first spatial region; and wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in each of the frames. In particular, by selectively encoding a flag in this manner when encoding image data having a source chroma format such as a 4:4:4 (or 4:2:2), coding can be efficiently achieved with low latency. Thus, improved compatibility and distribution of high colour fidelity image data can be achieved, even when a video capture, transmission, display and / or storage apparatus is capable of coding only a certain data format. Additional Embodiments In addition, embodiments of the present disclosure are defined in accordance with the following numbered clauses: 1) A data encoding method, the method comprising: selectively encoding a flag when encoding image data having a source chroma format as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and one or more second layers, wherein the first layer comprises data in a first chroma format and the one or more second layers, having the first format, comprise data for reconstructing an image with the source chroma format when combined with the data in the first layer; and wherein the flag indicates that the data in the one or more second layers is defined relative to the data in the first layer. 2) The data encoding method according to clause 1, wherein the source chroma format is a 4:4:4 chroma format and the first chroma format is a 4:2:0 chroma format. 3) The data encoding method according to clause 1, wherein the source chroma format is a 4:4:4 chroma format and the first chroma format is a 4:2:2 chroma format. 4) The data encoding method according to clause 1, wherein the source chroma format is a 4:2:2 chroma format and the first chroma format is a 4:2:0 chroma format. 5) The data encoding method according to any preceding clause, wherein the source chroma format and the first chroma format are a YCbCr chroma format or a RGB chroma format. 6) The data encoding method according to any preceding clause, wherein each of the one or more second layers corresponds to an individual chroma component. 7) The data encoding method according to any preceding clause, wherein the first layer is a base layer carrying a first chroma signal and each of the one or more second layers is a higher layer carrying enhancement information to reconstruct source chroma format signal. 8) The data encoding method according to clause 7, wherein in the base layer, data for a first component, a second component and a third component are included in the first chroma format and wherein, in a first higher layer, data for the second component is included within an area corresponding to an area reserved for the first component in the first layer and wherein, in a second higher layer, data for the third component is included within an area corresponding to an area reserved for the first component in the first layer. 9) The data encoding method according to either clause 7 or clause 8, wherein in the base layer, the data for the second component and the third component include a sub-set of the samples for the second component and the third component included in the image data. 10) The data encoding method according to clause 9, wherein the sub-set of samples comprise a 00, 02, 20 and 22 sample. 11) The data encoding method according to clause 7 or clause 8, wherein in the base layer, the data for the second component and the third component are generated from the samples for the second component and the third component in the image data. 12) The data encoding method according to any preceding clause, wherein defining the data in the one or more second layers relative to the data in the first layer comprises defining the data in the second layer as a difference value relative to the data in the first layer. 13) The data encoding method according to clause 12, wherein for each of the one or more second layers, each chroma sample is defined as a difference value between the value of that chroma sample in the image data and the value of a corresponding sample in the first layer. 14) The data encoding method according to clause 13, wherein a plurality of samples in each of the one or more second layers share a corresponding sample in the first layer. 15) The data encoding method according to any preceding clause, wherein the flag is selectively encoded within a Supplemental Enhancement Information (SEI) message. 16) A data encoding apparatus, the apparatus comprising circuitry configured to: selectively encode a flag when encoding image data having a source chroma format as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and one or more second layers, wherein the first layer comprises data in a first chroma format and the one or more second layers, having the first format, comprise data for reconstructing an image with the source chroma format when combined with the data in the first layer; and wherein the flag indicates that the data in the one or more second layers is defined relative to the data in the first layer. 17) A data decoding method, the method comprising: selectively decoding a flag when decoding image data having a source chroma format, encoded as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and one or more second layers, wherein the first layer comprises data in a first chroma format and the one or more second layers, having the first format, comprise data for reconstructing an image with the source chroma format when combined with the data in the first layer; and wherein the flag indicates that the data in the one or more second layers is defined relative to the data in the first layer. 18) A data decoding apparatus, the apparatus comprising circuitry configured to: selectively decode a flag when decoding image data having a source chroma format, encoded as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and one or more second layers, wherein the first layer comprises data in a first chroma format and the one or more second layers, having the first format, comprise data for reconstructing an image with the source chroma format when combined with the data in the first layer; and wherein the flag indicates that the data in the one or more second layers is defined relative to the data in the first layer. 19) A data encoding method, the method comprising: selectively encoding a flag when encoding image data having a 4:2:2 chroma format as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and a second layer, wherein the first layer comprises data in a 4:2:0 chroma format and the second layer, having the 4:2:0 chroma format, comprises data for reconstructing an image with the 4:2:2 chroma format when combined with the data in the first layer. 20) The data encoding method according to clause 19, wherein in the first layer, data for a first component, a second component and a third component are included in the 4:2:0 chroma format and wherein, in the second layer, data for the second component and the third component are included within an area corresponding to an area reserved for the first component in the first layer. 21) The data encoding method according to clause 20, wherein in the first layer, the data for the second component and the third component include a sub-set of the samples for the second component and the third component included in the image data. 22) The data encoding method according to clause 21, wherein the sub-set of samples comprise a 00, 02, 20 and 22 sample. 23) The data encoding method according to clause 20, wherein in the first layer, the data for the second component and the third component are generated from the samples for the second component and the third component in the image data. 24) The data encoding method according to any of clauses 20 to 23, wherein the data for the second component and the data for the third component are packaged side by side within the second layer. 25) The data encoding method according to any of clauses 19 to 24, wherein the first component is a luma component and the second component and the third component are chroma components. 26) The data encoding method according to any of clauses 19 to 25, wherein the method wherein the flag indicates that the data in the second layer is defined relative to the data in the first layer. 27) The data encoding method according to any of clauses 19 to 26, wherein the first layer is a base layer carrying a 4:2:0 signal and the second layer is a higher layer carrying enhancement information to reconstruct a 4:2:2 signal. 28) The data encoding method according to any of clauses 19 to 27, wherein the image data has a YCbCr 4:2:2 chroma format or a RGB 4:2:2 chroma format. 29) The data encoding method according to any of clauses 19 to 28, wherein the flag is selectively encoded within a Supplemental Enhancement Information (SEI) message. 30) A data decoding method, the method comprising: selectively decoding a flag when decoding image data having a 4:2:2 chroma format, encoded as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and a second layer, wherein the first layer comprises data in a 4:2:0 chroma format and the second layer, having the 4:2:0 chroma format, comprises data for reconstructing an image with the 4:2:2 chroma format when combined with the data in the first layer. 31) A data encoding apparatus, the apparatus comprising circuitry configured to: selectively encode a flag when encoding image data having a 4:2:2 chroma format as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and a second layer, wherein the first layer comprises data in a 4:2:0 chroma format and the second layer, having the 4:2:0 chroma format, comprises data for reconstructing an image with the 4:2:2 chroma format when combined with the data in the first layer. 32) A data decoding apparatus, the apparatus comprising circuitry configured to: selectively decode a flag when decoding image data having a 4:2:2 chroma format, encoded as a multi-layer bitstream, wherein the multi-layer bitstream comprises a first layer and a second layer, wherein the first layer comprises data in a 4:2:0 chroma format and the second layer, having the 4:2:0 chroma format, comprises data for reconstructing an image with the 4:2:2 chroma format when combined with the data in the first layer. 33) An encoded data stream encoded by the data encoding method of any of clauses 1 to 16 or the encoding method of clauses 19 to 29. 34) Computer software which, when implemented by a computer, causes the computer to perform the data encoding method of any of clauses 1 to 16 or clauses 19 to 29, or the data decoding method of clause 17 or clause 30. 35) A non-transitory computer readable storage medium which stores the computer software of clause 34. 36) Video data capture, transmission, display and / or storage apparatus comprising the data encoding apparatus of clause 31 or the data decoding apparatus of clause 32. Furthermore, embodiments of the present disclosure may also be arranged in accordance with the following numbered paragraphs: 1) A data encoding method, the method comprising: selectively encoding a flag when encoding image data having a source chroma format as frames represented in a bitstream having a first chroma format, wherein the frames represented in the bitstream comprise a first spatial region and two second spatial regions, the first spatial region comprising data of a first chroma component and the two second spatial regions comprising data of a second and a third chroma component for reconstructing an image with the source chroma format when combined with the data in the first spatial region; and wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in each of the frames. 2) The data encoding method of paragraph number 1, wherein the source chroma format is a 4:4:4 chroma format and the first chroma format is a 4:2:0 chroma format; or wherein the source chroma format is a 4:4:4 chroma format and the first chroma format is a 4:0:0 chroma format; or wherein the source chroma format is a 4:2:2 chroma format and the first chroma format is a 4:2:0 chroma format; or wherein the source chroma format is a 4:2:2 chroma format and the first chroma format is a 4:0:0 chroma format. 3) The data encoding method according to paragraph number 1 or 2, wherein each of the two second spatial regions corresponds to an individual one of the second and third chroma components. 4) The data encoding method according to any preceding numbered paragraph, wherein a size of each of the two second spatial regions is, individually, the same as a size of the first spatial region. 5) The data encoding method according to any preceding numbered paragraph, wherein zero values are included within each of the second spatial regions in order to maintain a size of the second spatial regions compared to the first spatial region. 6) The data encoding method according to any of paragraphs numbered 1 to 4, wherein a size of each of the two second spatial regions is, individually, different to a size of the first spatial region. 7) The data encoding method according to any of paragraphs numbered 1 to 4 or 6, wherein a size of the two second spatial regions combined is the same as a size of the first spatial region. 8) The data encoding method according to any preceding numbered paragraph, wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in an intended decoding direction in each of the frames. 9) The data encoding method according to any preceding numbered paragraph, wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in a horizontal direction in each of the frames. 10) The data encoding method according to any preceding numbered paragraph, wherein each of the first spatial region and the two second spatial regions are located within an area reserved for the first chroma component in the source chroma format. 11) The data encoding method according to any preceding numbered paragraph, wherein the flag is selectively encoded within a Supplemental Enhancement Information (SEI) message. 12) A data encoding apparatus, the apparatus comprising circuitry configured to: selectively encode a flag when encoding image data having a source chroma format as frames represented in a bitstream having a second chroma format, wherein the frames represented in the bitstream comprise a first spatial region and two second spatial regions, the first spatial region comprising data of a first component and the two second spatial regions comprising data of a second and a third component for reconstructing an image with the source chroma format when combined with the data in the first spatial region; and wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in each of the frames. 13) A data decoding method, the method comprising: selectively decoding a flag when decoding image data having a source chroma format as frames represented in a bitstream having a second chroma format, wherein the frames represented in the bitstream comprise a first spatial region and two second spatial regions, the first spatial region comprising data of a first component and the two second spatial regions comprising data of a second and a third component for reconstructing an image with the source chroma format when combined with the data in the first spatial region; and wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in each of the frames. 14) A data decoding apparatus, the apparatus comprising circuitry configured to: selectively decode a flag when decoding image data having a source chroma format as frames represented in a bitstream having a second chroma format, wherein the frames represented in the bitstream comprise a first spatial region and two second spatial regions, the first spatial region comprising data of a first component and the two second spatial regions comprising data of a second and a third component for reconstructing an image with the source chroma format when combined with the data in the first spatial region; and wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in each of the frames. 15) An encoded data stream encoded by the data encoding method of any of numbered paragraphs 1 to 11 or the data encoding apparatus of numbered paragraph 12. 16) Computer software which, when implemented by a computer, causes the computer to perform the data encoding method of numbered any of numbered paragraphs 1 to 11, or the data decoding method of numbered paragraph 13. 17) A non-transitory computer readable storage medium which stores the computer software of numbered paragraph 16. 18) Video data capture, transmission, display and / or storage apparatus comprising the data encoding apparatus of numbered paragraph 12 or the data decoding apparatus of numbered paragraph 14. In so far as embodiments of the disclosure have been described as being implemented, at least in part, by software-controlled data processing apparatus, it will be appreciated that a non-transitory machine-readable medium carrying such software, such as an optical disk, a magnetic disk, semiconductor memory or the like, is also considered to represent an embodiment of the present disclosure. Similarly, a data signal comprising coded data generated according to the methods discussed above (whether or not embodied on a non-transitory machine-readable medium) is also considered to represent an embodiment of the present disclosure. It will be apparent that numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended clauses, the technology may be practised otherwise than as specifically described herein. It will be appreciated that the above description for clarity has described embodiments with reference to different functional units, circuitry and / or processors. However, it will be apparent that any suitable distribution of functionality between different functional units, circuitry and / or processors may be used without detracting from the embodiments. Described embodiments may be implemented in any suitable form including hardware, software, firmware or any combination of these. Described embodiments may optionally be implemented at least partly as computer software running on one or more data processors and / or digital signal processors. The elements and components of any embodiment may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the disclosed embodiments may be implemented in a single unit or may be physically and functionally distributed between different units, circuitry and / or processors. Although the present disclosure has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in any manner suitable to implement the technique. Furthermore, in the present disclosure, the wording “in dependence upon” defines a relationship by which one value or parameter can be calculated from another value or parameter. That is, “A is calculated in dependence upon B” means that the value of B wholly or in part, directly or indirectly, contributes to a determination or calculation of A. Thus, when A is calculated in dependence upon B, A is at least in part based on a value of B or a value associated with B. Thus, the wording “in dependence upon” and “based on” may be used interchangeable within the present disclosure.

Claims

1. A data encoding method, the method comprising:selectively encoding a flag when encoding image data having a source chroma format as frames represented in a bitstream having a first chroma format, wherein the frames represented in the bitstream comprise a first spatial region and two second spatial regions, the first spatial region comprising data of a first component and the two second spatial regions comprising data of a second and a third component for reconstructing an image with the source chroma format when combined with the data in the first spatial region;and wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in each of the frames.

2. The data encoding method of claim 1, wherein the source chroma format is a 4:4:4 chroma format and the first chroma format is a 4:2:0 chroma format; or wherein the source chroma format is a 4:4:4 chroma format and the first chroma format is a 4:0:0 chroma format; or wherein the source chroma format is a 4:2:2 chroma format and the first chroma format is a 4:2:0 chroma format; or wherein the source chroma format is a 4:2:2 chroma format and the first chroma format is a 4:0:0 chroma format.

3. The data encoding method according to claim 1, wherein each of the two second spatial regions corresponds to an individual one of the second and third components.

4. The data encoding method according to claim 1, wherein a size of each of the two second spatial regions is, individually, the same as a size of the first spatial region.

5. The data encoding method according to claim 1, wherein zero values are included within each of the second spatial regions in order to maintain a size of the second spatial regions compared to the first spatial region.

6. The data encoding method according to claim 1, wherein a size of each of the two second spatial regions is, individually, different to a size of the first spatial region.

7. The data encoding method according to claim 1, wherein a size of the two second spatial regions combined is the same as a size of the first spatial region.

8. The data encoding method according to claim 1, wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in an intended decoding direction in each of the frames.

9. The data encoding method according to claim 1, wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in a horizontal direction in each of the frames.

10. The data encoding method according to claim 1, wherein each of the first spatial region and the two second spatial regions are located within an area reserved for the first component in the source chroma format.

11. The data encoding method according to claim 1, wherein the flag is selectively encoded within a Supplemental Enhancement Information (SEI) message.

12. A data encoding apparatus, the apparatus comprising circuitry configured to: selectively encode a flag when encoding image data having a source chroma format as frames represented in a bitstream having a second chroma format, wherein the frames represented in the bitstream comprise a first spatial region and two second spatial regions, the first spatial region comprising data of a first component and the two second spatial regions comprising data of a second and a third component for reconstructing an image with the source chroma format when combined with the data in the first spatial region;and wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in each of the frames.

13. A data decoding method, the method comprising:selectively decoding a flag when decoding image data having a source chroma format as frames represented in a bitstream having a second chroma format, wherein the frames represented in the bitstream comprise a first spatial region and two second spatial regions, the first spatial region comprising data of a first component and the two second spatial regions comprising data of a second and a third component for reconstructing an image with the source chroma format when combined with the data in the first spatial region;and wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in each of the frames.

14. A data decoding apparatus, the apparatus comprising circuitry configured to:selectively decode a flag when decoding image data having a source chroma format as frames represented in a bitstream having a second chroma format, wherein the frames represented in the bitstream comprise a first spatial region and two second spatial regions, the first spatial region comprising data of a first component and the two second spatial regions comprising data of a second and a third component for reconstructing an image with the source chroma format when combined with the data in the first spatial region;and wherein the flag indicates that the first spatial region and each of the two second spatial regions are arranged side by side in each of the frames.

15. An encoded data stream encoded by the data encoding method of claim 1 or the data encoding apparatus of claim 12.

16. Computer software which, when implemented by a computer, causes the computer to perform the data encoding method of claim 1, or the data decoding method of claim 13.

17. A non-transitory computer readable storage medium which stores the computer software of claim 16.

18. Video data capture, transmission, display and / or storage apparatus comprising the data encoding apparatus of claim 12 or the data decoding apparatus of claim 14.s

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