Method and device for coding and decoding a data stream representing at least one image

Abstract

To provide a coding method and a method for decoding a coded data stream representative of at least one image that is split into blocks.SOLUTION: For at least one block of an image, which is called a current block, an item of information indicating a coding mode of the current block is decoded from a data stream (E42). When the coding mode of the current block corresponds to a first coding mode, the current block is decoded (E43) using a first determined quantization step (E430) to dequantize, in a transform domain, a prediction residue associated with the current block. When the coding mode of the current block corresponds to a second coding mode, the current block is decoded (E44) using a second determined quantization step (E441) to dequantize, in the spatial domain, a prediction residue associated with the current block. The first quantization step and the second quantization step are determined according to the same quantization parameter.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The field of the invention is that of coding and decoding images or sequences of images, in particular video streams.

[0002] More particularly, the present invention relates to the compression of an image or sequence of images using a block representation of the image.

[0003] The invention may be applied in particular to image or video coding, and corresponding decoding, performed in current or future encoders (JPEG, MPEG, H.264, HEVC, etc., and modifications thereof). [Background technology]

[0004] Digital images and sequences of images take up a lot of space in terms of memory, which makes it necessary to compress these images when transmitting them in order to avoid congestion problems on the networks used for this transmission.

[0005] Many techniques for compressing video data are already known. Among these, the HEVC compression standard ("High Efficiency Video Coding, Coding Tools and Specification", Matthias Wien, Signals and Communication Technology, 2015) proposes performing prediction of pixels of a current image with respect to other pixels belonging to the same image (intra prediction) or to previous or subsequent images (inter prediction).

[0006] More specifically, intra prediction exploits spatial redundancy within an image. To do this, the image is divided into blocks of pixels. The blocks of pixels are then predicted using already reconstructed information corresponding to previously coded / decoded blocks in the current image, according to the scanning order of the blocks in the image.

[0007] Furthermore, in conventional schemes, coding of a current block is performed using a prediction of the current block, called a predictor block, and a prediction residual or "residual block" corresponding to the difference between the current block and the predictor block. The resulting residual block is then transformed, for example using a DCT (Discrete Cosine Transform) type transform. The coefficients of the transformed residual block are then quantized, coded by entropy coding, and transmitted to a decoder, which can reconstruct the current block by adding this residual block to the predictor block.

[0008] Decoding is performed image by image and block by block for each image. For each block, the corresponding element of the stream is read. Inverse quantization and inverse transformation of the coefficients of the residual block is performed. A block prediction is then calculated to obtain a predictor block, and the current block is reconstructed by adding the prediction (i.e., the predictor block) to the decoded residual block.

[0009] In U.S. Patent No. 9,253,508, a DPCM (Differential Pulse Code Modulation) coding technique for coding a block in intra mode is integrated into an HEVC encoder. Such a technique consists in predicting a set of pixels of an intra block by another set of pixels of the same block that has been previously reconstructed. In U.S. Patent No. 9,253,508, the set of pixels of the intra block to be coded corresponds to a row, or a column, or a row and a column, of the block, and the intra prediction used to predict the set of pixels is one of the directional intra predictions defined in the HEVC standard.

[0010] However, such a technique is not optimal. Indeed, the reconstruction of a set of pixels of an intrablock either corresponds to the addition of a prediction residual in the case of lossless coding, which therefore exhibits a significantly smaller compression ratio, or corresponds to the addition of a prediction residual after inverse transformation and / or inverse quantization of the other set of pixels that serve for the prediction. Therefore, such a technique does not allow each pixel of an intrablock to be predicted using a local prediction function, and the predicted pixel to be reconstructed before predicting subsequent pixels. Indeed, this technique requires that a certain set of pixels (e.g., rows / columns of a block) be reconstructed in order to predict another set of pixels. In other words, with each prediction and reconstruction of a part of the block, several pixels of the block are predicted and reconstructed.

[0011] Furthermore, U.S. Pat. No. 9,253,508 does not explain, for example, how conventional intra prediction modes and DPCM prediction modes coexist, as defined in the HEVC standard, nor how to determine the quantization step for blocks coded by DPCM intra prediction as described in U.S. Pat. No. 9,253,508, which is adapted to a rate / distortion optimization mechanism that aims to minimize the D+λR function (where R represents the rate for coding the image, D represents the associated distortion, and λ represents the Lagrangian).

[0012] Therefore, new coding and decoding methods are needed to improve the compression of image or video data. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] U.S. Patent No. 9,253,508 [Non-patent literature]

[0014] [Non-Patent Document 1] "High Efficiency Video Coding, Coding Tools and Specification", Matthias Wien, Signals and Communication Technology, 2015 Summary of the Invention [Means for solving the problem]

[0015] The present invention improves upon the prior art. To this end, the invention relates to a method for decoding a coded data stream representing at least one image divided into blocks, the decoding method comprising, for at least one block of the image, called the current block: - decoding an item of information indicating the coding mode of the current block; - decoding the current block, comprising determining a first quantization step for inverse quantizing, in a transform domain, a prediction residual associated with the current block when the coding mode of the current block corresponds to a first coding mode; - decoding the current block, when a coding mode of the current block corresponds to a second coding mode, comprising determining a second quantization step for inverse quantizing in the spatial domain a prediction residual associated with the current block; The first quantization step and the second quantization step are determined according to the same quantization parameter.

[0016] According to the present invention, it is thus possible to define a first quantization step for quantizing / dequantizing the prediction residual in the transform domain and a second quantization step for quantizing / dequantizing the prediction residual in the spatial domain, and the first and second quantization steps are determined by the same quantization parameter.Therefore, the first and second quantization steps can be concatenated and used together.

[0017] Advantageously, according to the present invention, the quantization steps used in the different coding modes available for the current block are consistent, particularly for the rate-distortion optimization mechanism applied to the current block; however, when some blocks are coded according to a first coding mode and other blocks are coded according to a second coding mode, the quantization steps are similarly consistent between blocks, particularly for the rate-distortion optimization mechanism applied to the image. Indeed, if the quantization steps are not concatenated together, one of the two quantization steps may be significantly larger than the other. A block coded according to a coding mode using a significantly larger quantization step will then be significantly compressed, unlike other blocks coded according to other coding modes using smaller quantization steps, which will result in significant spatial variability of visual quality in the image and, therefore, a lower visual quality for the entire image.

[0018] According to the invention, a first coding mode corresponds to coding of a block in the transform domain. In other words, according to this first coding mode, the prediction residual of the current block, calculated in the spatial domain, i.e. between the original block and the predicted block, is transformed, for example by a DCT transform, to provide a block of transform coefficients. This block of transform coefficients is then quantized using a first quantization step.

[0019] The second coding mode corresponds to coding of the block in the spatial domain, in other words, according to this second coding mode, the prediction residual of the current block calculated in the spatial domain is directly quantized without undergoing any transformation operation.

[0020] According to one particular embodiment of the invention, when the coding mode of the current block corresponds to the second coding mode, the decoding of the current block comprises, for each pixel of the current block: - obtaining a prediction of said pixel from another previously decoded pixel, said other previously decoded pixel belonging to said current block or to a previously decoded block of an image; - decoding a prediction residual associated with said pixel; - dequantizing the prediction residual associated with the pixel using the second quantization step; - reconstructing said pixel by adding a prediction of said pixel to a dequantized prediction residual.

[0021] According to this particular embodiment of the invention, in accordance with the second coding mode, the prediction of the current block is obtained by local prediction of each pixel of the current block using previously decoded pixels of the current block or pixels of previously decoded neighboring blocks, and the pixel prediction is thus improved.

[0022] The invention also relates to a method for coding a coded data stream representing at least one image divided into blocks, the coding method comprising, for at least one block of the image, called the current block, - coding an item of information indicating the coding mode of the current block; - coding the current block, comprising determining a first quantization step for quantizing a prediction residual associated with the current block in a transform domain when the coding mode of the current block corresponds to a first coding mode; - coding the current block, when a coding mode of the current block corresponds to a second coding mode, comprising determining a second quantization step for quantizing a prediction residual associated with the current block in the spatial domain; The first quantization step and the second quantization step are determined according to the same quantization parameter.

[0023] According to one particular embodiment of the invention, when the coding mode of the current block corresponds to the second coding mode, the coding of the current block comprises, for each pixel of the current block: - obtaining a prediction of said pixel from another previously decoded pixel, said other previously decoded pixel belonging to said current block or to a previously decoded block of an image; - obtaining a prediction residual associated with said pixel from a prediction of said pixel; - quantizing the prediction residual associated with the pixel using the second quantization step; - coding the quantized prediction residual.

[0024] According to the present invention, the current block can be coded / decoded by conventional intra prediction (first coding mode), e.g., spatial prediction according to any of the prediction modes defined in the HEVC standard, or by a new intra prediction (second coding mode) based on the use of pixels of the current block that have been previously decoded.

[0025] According to this new intra prediction, hereafter also referred to as ILR (In-Loop Residual), each pixel is predicted, then a residual is calculated for the predicted pixel, and the residual is quantized. The quantized residual is then dequantized and added to the pixel's prediction to reconstruct the pixel's value and obtain its decoded value. Such a decoded value of the pixel can then be used to predict other pixels of the current block.

[0026] Advantageously, such intra prediction methods can predict pixels of the current block using spatially closer pixels than the reference pixels used in conventional intra prediction, and the prediction is thus improved.

[0027] According to another particular embodiment of the above mentioned decoding or coding method, the second quantization step is determined according to the first quantization step.

[0028] According to this particular embodiment of the invention, the quantization parameter then corresponds to the first quantization step, the two quantization steps being dependent on each other.

[0029] In one variation of this particular embodiment, where δ1 corresponds to the first quantization step and a, b, c, d, and e are predetermined parameters, the second quantization step is a*δ1+b or

[0030]

number

[0031] is determined by.

[0032] According to another particular embodiment of the above mentioned decoding or coding method, the first quantization step is taken from a quantization step table predetermined for values ​​of the quantization parameter.

[0033] According to another particular embodiment of the above-mentioned decoding or coding method, the quantization parameter corresponds to a Lagrangian λ used to optimize a D+λ*R function, where R corresponds to the rate of the data stream and D corresponds to the associated distortion of the decoded image.

[0034] According to another particular embodiment of the above mentioned decoding or coding method, the quantization parameter is coded in or decoded from the data stream.

[0035] According to another particular embodiment, the decoding of the current block according to the second coding mode comprises: - decoding another prediction residual associated with the current block; and - inverse quantizing the other prediction residual associated with the current block using the first quantization step; - reconstructing the current block from reconstructed pixels of the current block and the dequantized other prediction residuals.

[0036] According to this particular embodiment of the present invention, when a current block is coded according to a second coding mode, a second prediction residual is coded / decoded, pixel by pixel, relative to the current block from the prediction obtained when coding the first prediction residual. An additional feature of this particular embodiment corresponds to the decoding step of the block coded according to the first coding mode. Thus, it is possible to maintain similar coding / decoding for both coding modes, especially at the entropy encoder / decoder level, thus further improving compression performance.

[0037] The invention also relates to a stream or signal of coded data representing at least one image, said image being divided into blocks, the coded data stream comprising, for at least one block of the image, currently called a block: - an information item indicating the coding mode of the current block; - a prediction residual associated with the current block, the prediction residual comprising coefficients transformed and quantized using a first quantization step when the coding mode of the current block corresponds to a first coding mode; - a prediction residual associated with the current block, the prediction residual comprising prediction residual values ​​quantized in the spatial domain using a second quantization step when a coding mode of the current block corresponds to a second coding mode; The first quantization step and the second quantization step are determined according to the same quantization parameter.

[0038] According to one particular embodiment of the invention, the coded data stream further comprises said quantization parameter.

[0039] The present invention also relates to a decoding device configured to implement the decoding method according to any one of the specific embodiments defined above. This decoding device can naturally have various characteristics related to the decoding method according to the invention. The characteristics and advantages of this decoding device are therefore the same as those of the decoding method and will not be further detailed.

[0040] The decoding device specifically performs, for at least one block of the image, referred to as the current block: - decoding an item of information indicating the coding mode of the current block; - decoding the current block by determining a first quantization step for inverse quantizing, in a transform domain, a prediction residual associated with the current block when the coding mode of the current block corresponds to a first coding mode; - a processor configured to decode the current block by determining a second quantization step for inverse quantizing a prediction residual associated with the current block in the spatial domain when a coding mode of the current block corresponds to a second coding mode; The first quantization step and the second quantization step are determined according to the same quantization parameter.

[0041] According to one particular embodiment of the invention, such a decoding device is included in a terminal.

[0042] The present invention also relates to a coding device configured to implement the coding method according to any one of the specific embodiments defined above. This coding device may of course have various characteristics related to the coding method according to the invention. The characteristics and advantages of this coding device are therefore the same as those of the coding method and will not be further detailed.

[0043] The coding device specifically determines for at least one block of the image, called the current block: - coding an item of information indicating the coding mode of the current block; - coding the current block by determining a first quantization step for quantizing a prediction residual associated with the current block in a transform domain when a coding mode of the current block corresponds to a first coding mode; - a processor configured to code the current block by determining a second quantization step for quantizing a prediction residual associated with the current block in the spatial domain when a coding mode of the current block corresponds to a second coding mode; The first quantization step and the second quantization step are determined according to the same quantization parameter.

[0044] According to one particular embodiment of the invention, such a coding device is comprised in a terminal or a server.

[0045] The decoding method, respectively the coding method according to the invention can be implemented in various ways, in particular in hardwired or software form. According to one particular embodiment of the invention, the decoding method, respectively the coding method is implemented by a computer program. The invention also relates to a computer program comprising instructions for implementing the decoding method or the coding method according to any one of the particular embodiments described above, when said program is executed by a processor. Such a program can use any programming language. The program can be downloaded from a communication network and / or recorded on a computer-readable medium.

[0046] The program may use any programming language and may be in the form of source code, object code, or an intermediate code between source code and object code, such as in partially compiled form, or in any other desired form.

[0047] The present invention also relates to a computer-readable storage medium or data medium comprising instructions for a computer program as described above. The aforementioned storage medium can be any entity or device capable of storing a program. For example, the medium can comprise a storage means such as a memory. Alternatively, the storage medium can represent a transmissible medium, such as an electrical or optical signal, which can be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention can in particular be downloaded over an Internet-type network.

[0048] Alternatively, the recording medium may represent an integrated circuit in which the program is embedded, the circuitry being adapted for performing, or for use in the performance of, the method in question.

[0049] Other characteristics and advantages of the invention will appear more clearly on reading the following description of one particular embodiment, given as a brief illustrative and non-limiting example, and the accompanying drawings, in which: [Brief explanation of the drawings]

[0050] [Figure 1] 3 illustrates steps of a coding method according to one particular embodiment of the invention; [Figure 2] 4A and 4B illustrate example positions of neighboring blocks of a current block for determining an intra-prediction mode, according to one particular embodiment of the present invention. [Figure 3] FIG. 2 illustrates example locations of reference pixels used to predict pixels of a current block, according to one particular embodiment of the invention. [Figure 4]3 illustrates steps of a decoding method according to one particular embodiment of the invention; [Figure 5] 1 illustrates an example signal comprising coded data representing at least one block of an image, according to one particular embodiment of the invention; [Figure 6] 1 shows a simplified structure of a coding device adapted to implement a coding method according to any one of the particular embodiments of the present invention; [Figure 7] FIG. 2 shows a simplified structure of a decoding device adapted to implement a decoding method according to any one of the particular embodiments of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0051] general principle The general principle of the present invention is to jointly determine a first quantization step used to quantize the transform coefficients of the prediction residual and a second quantization step used to quantize the values ​​of the prediction residual directly in the spatial domain (i.e., without any transform operation). To this end, the first and second quantization steps depend on the same quantization parameter.

[0052] According to different particular embodiments of the invention, the quantization parameter may correspond to one or the other of the first and second quantization steps, or to a quantization parameter determined by the encoder or by the user, or to the Lagrangian used during rate / distortion optimization in the encoder.

[0053] The quantization parameters may also be coded in the data stream and transmitted to the decoder.

[0054] When the quantization parameter corresponds to a first quantization step, the second quantization step may be determined according to a linear or polynomial function of the first quantization step, or a more general function.

[0055] The present invention thus improves compression performance when different coding modes, which perform coding of prediction residuals with and without transform operations, respectively, are used in the same system for coding / decoding an image or a sequence of images.

[0056] Embodiment 1 illustrates the steps of a coding method according to one particular embodiment of the invention. For example, given a sequence of images I1, I2, ..., I Nb are coded in the form of a coded data stream STR according to one particular embodiment of the invention, for example, by a coding device such as that described below with reference to FIG.

[0057] A sequence of images I1, I2, ..., I Nb is provided as input to the coding method, where Nb is the number of images in the sequence to be coded. The coding method outputs a coded data stream STR representing the sequence of images provided as input.

[0058] In a known scheme, a sequence of images I1, I2, ..., I Nb The coding of I is done picture by picture according to a coding order that is previously established and known to the encoder. For example, the pictures are coded in the temporal order I1, I2, ..., I Nb or in another order, e.g., I1, I3, I2, ..., I Nb It can be coded as:

[0059] In step E0, a sequence of images I1, I2, ..., I Nb Image I to be coded j is divided into blocks, for example, of size 32x32 or 64x64 pixels or larger. Such blocks may be further divided into square or rectangular sub-blocks, for example, 16x16, 8x8, 4x4, 16x8, 8x16...

[0060] In an optional step E10, according to one particular embodiment of the invention, image I j In data coded for a sequence of images, or in data coded for a sequence of images, a quantization parameter QP is coded in the data stream STR.

[0061] Then, in step E1, image I j The first block or sub-block to be coded is X b But image I j For example, it may be the first block in the lexicographical scan order of the image.

[0062] In step E2, the encoder calculates the current block X b You will then need to select the coding mode for coding the

[0063] According to the particular embodiment described herein, the encoder performs the following steps: b , select a coding mode for coding , from a first coding mode M1 and a second coding mode M2. Additional coding modes (not described here) may be used.

[0064] According to a particular embodiment described herein, the first coding mode M1 corresponds to coding of the current block by conventional intra prediction, e.g., as defined according to the HEVC standard, and the second coding mode M2 ​​corresponds to In-Loop Residual (ILR) predictive coding.

[0065] The principles of the present invention can be extended to other types of coding modes, whether for the first coding mode M1 or the second coding mode M2. For example, the first coding mode can correspond to any type of coding mode (such as inter-picture predictive coding, spatial prediction with template matching coding, etc.) that uses a transformation of the prediction residual before quantizing the coefficients resulting from the transformation operation. The second coding mode can correspond to any type of coding mode that uses quantization of the values ​​of the prediction residual in the spatial domain, i.e., does not perform any transformation operation, e.g., DCT, of the prediction residual.

[0066] In step E2, the encoder may perform a rate / distortion optimization to determine the best coding mode for coding the current block. During this rate / distortion optimization, additional coding modes different from the first and second coding modes, e.g., inter-mode coding modes, may be tested. During this rate / distortion optimization, the encoder may test the current block X with different available coding modes to determine the rate and distortion associated with each coding mode. b and selects the coding mode that offers the best rate / distortion compromise according to, for example, a D+λR function, where R is the rate required to code the current block according to the coding mode being evaluated, D is the distortion measured between the decoded block and the original current block, and λ is a Lagrange multiplier, for example, input by a user or specified in the encoder.

[0067] At step E20, an item of information indicating the coding mode selected for the current block is coded in the data stream STR.

[0068] Currently Block X bIf the current block X is coded according to the first coding mode M1, the method proceeds to step E21 for coding the block according to M1. b If is coded according to the second coding mode M2, the method proceeds to step E22 for coding the block according to M2.

[0069] A step E21 for coding a block according to a first coding mode M1 according to one particular embodiment of the invention is described below. According to the particular mode described here, the first coding mode corresponds to conventional intra prediction, such as that defined in the HEVC standard.

[0070] In step E210, a quantization step δ is determined. For example, the quantization step δ can be set by a user or calculated using a quantization parameter that sets a compromise between compression and quality and is input by the user or specified by the encoder. Such a quantization parameter can therefore be the parameter λ used in the rate-distortion cost function D+λ.R, where D represents the distortion introduced by coding and R represents the rate used for coding. This function is used to make coding choices, and typically a method of coding an image that minimizes this function is sought.

[0071] As a variant, the quantization parameter may be QP, which corresponds to the quantization parameter conventionally used in the AVC or HEVC standard. Thus, in the HEVC standard, the quantization step δ1 is determined by the equation δ1=levelScale[QP%6]<<(QP / 6)), where levelScale[k]={40, 45, 51, 57, 64, 72} for k=0..5.

[0072] In step E211, a prediction for the current block is determined using a conventional intra prediction mode, in which each pixel to be predicted is calculated only from decoded pixels originating from neighboring blocks (reference pixels) located above and to the left of the current block. The way in which pixels are predicted from the reference pixels depends on a prediction mode, which is transmitted to the decoder and chosen by the encoder from a predetermined set of modes known to the encoder and the decoder.

[0073] Thus, in HEVC, there are 35 possible prediction modes, namely, 33 modes that interpolate reference pixels in 33 different angular directions, as well as two other modes: DC mode, in which each pixel of the predicted block is generated from the average of reference pixels, and PLANAR mode, which performs planar and omnidirectional interpolation. This "traditional intra prediction" is well known and is also used in the ITU-T H.264 standard (which has only 9 different modes) and in the experimental JEM software, available at the Internet address (https: / / jvet.hhi.fraunhofer.de / ), which has 67 different prediction modes. In all cases, traditional intra prediction emphasizes the two aspects mentioned above: predicting pixels from neighboring blocks and transmitting the optimal prediction mode to the decoder.

[0074] In step E211, the encoder selects one of the available prediction modes from a predetermined list of prediction modes. One way to select is, for example, to evaluate all prediction modes and to retain the prediction mode that minimizes a cost function, such as classically the rate-distortion cost.

[0075] In step E212, the prediction mode chosen for the current block is coded from the neighboring blocks of the current block. b current block X for coding the prediction mode of b Adjacent Block Ab and B b An example of the position is shown below.

[0076] In step E212, the intra prediction mode chosen for the current block is coded using the intra prediction modes associated with the neighboring blocks.

[0077] According to one variant of the particular embodiment of the invention described herein, the intra prediction mode associated with a block is: - the intra-prediction mode used to predict the block if the block was coded using a conventional intra-prediction coding mode; - the intra-prediction mode associated with the block, if the block is coded with a coding mode different from conventional intra-prediction, an example of such an association is explained below with reference to step E229.

[0078] Therefore, the technique described in the HEVC standard for coding the prediction mode of the current block can be used. In the example in FIG. 2, such a technique is used for block A, which is located above the current block. b The intra prediction mode m associated with A , and block B located just to the left of the current block b The intra prediction mode m associated with B The purpose of this study is to identify the m A and m B Depending on the value of , a list containing three intra prediction modes, called MPM (representing most dominant modes), and a list containing 32 other prediction modes, called non-MPM, are created.

[0079] According to a variant of a particular embodiment described herein, the mechanism for creating the MPM list specified according to the HEVC standard is adapted to take into account the association of an intra-prediction mode with a block (step E229 described below) during its coding when this block is not coded according to a conventional intra-coding mode. According to this variant, the neighboring blocks of the current block always have an intra-prediction mode associated with them, regardless of whether this block is coded according to an intra-coding mode or according to another coding mode (ILR, inter, etc.).

[0080] The rest of the mechanism for creating the MPM list remains similar to that specified according to the HEVC standard. A and m B are both equal to the same intra prediction mode, and that intra prediction mode is a DC prediction mode or a PLANAR prediction mode, then the MPM list contains the following prediction modes: MPM[0]=PLANAR, MPM[1]=DC, MPM[2]=A(26), which corresponds to the angular prediction mode at index 26 in the HEVC intra prediction mode table.

[0081] m A and m B are both equal to the same angular prediction mode m(n), then the MPM list comprises the following prediction modes: MPM[0]=m(n), MPM[1]=m(2+(n+29)mod32), MPM[2]=m(2+(n-1)mod32).

[0082] m A and m B If different, the MPM list comprises the following prediction modes: MPM[0]=m B , MPM[1]=m A , MPM[2]=Mlast, where Mlast is defined by: -m Bis not equal to the PLANAR mode and m A If is not equal to PLANAR mode, Mlast is equal to PLANAR mode. - Otherwise, m B is not equal to DC mode and m A If is not equal to DC mode, Mlast is equal to DC mode. - Otherwise, Mlast is equal to the angular mode A(26).

[0083] The non-MPM list comprises all other intra-prediction modes not included in the MPM list.

[0084] According to another variant of the particular embodiment described herein, the neighboring block A is generated according to the mechanism for creating the MPM list specified according to the HEVC standard. b or B b If one of the is not coded according to an intra prediction mode, then by default the DC prediction mode is assigned to it. The rest of the mechanism for creating the MPM list remains similar to that specified according to the HEVC standard and described above.

[0085] According to the HEVC standard, to code the intra prediction mode of the current block, a syntax element, i.e. A binary indicator is transmitted indicating whether the prediction mode to be coded for the current block is in the MPM list; If the prediction mode of the current block belongs to the MPM list, an index in the MPM list corresponding to the prediction mode of the current block is coded; If the prediction mode of the current block does not belong to the MPM list, an index in the non-MPM list corresponding to the prediction mode of the current block is coded.

[0086] In step E213, a prediction residual R for the current block is constructed.

[0087] In step E213, in a conventional manner, a predicted block P is constructed according to the prediction mode chosen in step E211. A prediction residual R is then obtained by calculating the difference for each pixel between the predicted block P and the original current block.

[0088] In step E214, the prediction residual R is T is converted to

[0089] In step E214, a block R comprising the transformation coefficients is T A frequency transform is applied to the residual block R to generate . The transform may be, for example, a DCT type transform. The transform to be used is defined as a predefined set of transforms E T It is possible to choose between and inform the decoder which transformation to use.

[0090] In step E215, the transformed residual block R T is quantized using, for example, a quantization step scalar quantization δ. This means that the quantized transformed prediction residual block R TQ Generate.

[0091] At step E216, the quantized block R TQ The coefficients of are coded by an entropy encoder. For example, entropy coding specified in the HEVC standard may be used.

[0092] According to the embodiment variant described here, in step E217, the prediction mode determined in step E211 is associated with the current block.

[0093] In the known method, the current block is divided into quantized blocks R TQThe current block is decoded by dequantizing the coefficients of the current block and then applying an inverse transform to the dequantized coefficients to obtain a decoded prediction residual. A prediction is then added to the decoded prediction residual to reconstruct the current block and obtain a decoded version of it. The decoded version of the current block can then be used later to spatially predict other neighboring blocks of the image or to predict blocks of other images by inter-image prediction.

[0094] A step E22 for coding a block according to a second coding mode M2 ​​according to one particular embodiment of the invention is described below. According to the particular embodiment described here, the second coding mode corresponds to ILR predictive coding.

[0095] In step E220, a local predictor PL for the current block is determined. According to the coding mode described here, the pixels of the current block are predicted by previously reconstructed pixels of its neighboring blocks or of the current block itself.

[0096] Preferably, for prediction, a pixel as close as possible to the pixel to be predicted is chosen. This is why it is called a local predictor. The local predictor PL may also be incorporated into the prediction mode of the current block associated with the second coding mode M2. According to this interpretation, in a particular embodiment described here, the first coding mode uses a first group of intra prediction modes, for example the intra prediction modes defined by the HEVC standard, and the second coding mode, here the ILR mode, uses a second group of prediction modes that is different from the first group of intra prediction modes. According to an implementation variant described later, the correspondence between these two groups of prediction modes can be determined.

[0097] The local predictor PL may be unique or may be selected from a predetermined set of local predictors (second group of prediction modes).

[0098] According to one embodiment variant, four local predictors are defined. Thus, as illustrated in Figure 3 showing a current block Xb, if the current pixel to be predicted from the current block is called X, the pixel located immediately to the left of X is called A, the pixel located immediately to the left and above X is called B, and the pixel located immediately above X is called C. Four local predictors PL1, PL2, PL3, PL4 can be defined as follows: PL1(X)=min(A,B) if C≧max(A,B) max(A,B) if C≦min(A,B) A+BC Other cases PL2(X)=A PL3(X)=B PL4(X)=C Here, min(A,B) corresponds to a function that returns the minimum value between the value of A and the value of B, and max(A,B) corresponds to a function that returns the maximum value between the value of A and the value of B.

[0099] In step E220, the local predictor PL to be used for the current block is determined. In other words, the same local predictor, i.e. the same prediction function, will be used for all pixels of the current block. To this end, several implementation variants are possible.

[0100] The coding of the current block using each of the predictors may be simulated (similar to the optimization for choosing a coding mode for the current block), and a local predictor is selected that optimizes a cost function (e.g., minimizes the D+λ.R function, where R is the rate used to code the block, D is the distortion of the decoded block relative to the original block, and λ is a parameter set by the user).

[0101] Otherwise, to limit the complexity of selecting a local predictor for the current block, the texture orientation of previously coded pixels is analyzed, for example, previously coded pixels in blocks located above or to the left of the current block are analyzed using a Sobel-type operator. If the orientation is determined to be horizontal, the local predictor PL2 is selected; If the orientation is determined to be vertical, the local predictor PL3 is selected; If the orientation is determined to be diagonal, the local predictor PL4 is selected; If the orientation is not clear, the local predictor PL1 is selected.

[0102] A syntax element is coded in the data stream STR to indicate to the decoder which local predictor was used to predict the current block.

[0103] In step E221, a quantization step δ2 is determined, which, according to the particular embodiment described here, depends on the same quantization parameter as the quantization step δ1 to be determined in step E210 if the current block is coded according to the first coding mode.

[0104] According to one variant, the quantization step δ2 can be determined according to the quantization step δ1, for example, δ2=a1*δ1+b1, where a1 and b1 are predetermined integers or real numbers, or δ2=c1*δ1 2 A linear or polynomial function may be used, such as a1 + d1 * δ1 + e1, where c1, d1, and e1 are predetermined integers or real numbers. For example, a1 = 2, b1 = 10, c1 = 0.1, d1 = 2, and e1 = 10. Other values ​​are possible.

[0105] According to another variant, the quantization step δ2 can be determined from a parameter λ that sets a compromise between compression and quality. This Lagrangian parameter λ, like δ1, can be entered by the user, or specified by the encoder, or determined from the quantization parameter QP mentioned previously.

[0106] According to another variant, the quantization step δ2 may be set directly by the user, the quantization step δ1 then depending on δ2.

[0107] In step E222, a prediction residual R1 is calculated for the current block. For this purpose, once a local predictor has been chosen, for each current pixel of the current block: - To obtain a prediction value PRED, the current pixel X of the current block is predicted by the selected local predictor PL using either already reconstructed pixels outside the block (and therefore available with their decoded values) or previously reconstructed pixels within the current block, or both. In all cases, the predictor PL uses previously reconstructed pixels. In Figure 3, it can be seen that the pixels of the current block located in the first row and / or first column of the current block will use already reconstructed pixels outside the block (pixels in grey in Figure 3) and possibly also already reconstructed pixels of the current block as reference pixels (for constructing the prediction value PRED). For other pixels of the current block, the reference pixels used to construct the prediction value PRED are located inside the current block. The difference DIFF between PRED and X is quantized to a value Q(X) by a quantization step scalar quantizer δ2 with Q(X)=ScalarQuant(DIFF)=ScalarQuant(δ2,X-PRED), the scalar quantizer being, for example,

[0108]

number

[0109] Q(X) is the quantized residual associated with X. Q(X) is calculated in the spatial domain, i.e., directly from the difference between the predicted value PRED of pixel X and the original value of X. Such a quantized residual Q(X) for pixel X is then stored in a quantized prediction residual block R1 to be coded later. Q is stored in the - The decoded prediction P1(X) of X is calculated by adding the dequantized value of the quantized residual Q(X) to the prediction PRED. The decoded prediction P1(X) of X is thus obtained by P1(X) = PRED + ScalarDequant(δ2,Q(X)). For example, the closest scalar quantization inverse function is given by ScalarDequant(Δ,x) = Δ×x.

[0110] The decoded prediction value P1(X) thus makes it possible to predict possible pixels in the current block that are still to be processed. Moreover, the block P1 comprising the decoded / reconstructed values ​​of the pixels of the current block is an ILR predictor (rather than a conventional intra predictor) of the current block.

[0111] The sub-steps described above are performed for all pixels of the current block in a scan order that ensures that the pixels used for prediction, chosen from PL1, ..., PL4, are available.

[0112] According to one implementation variant, the scanning order of the current block is lexicographic, ie from left to right and top to bottom.

[0113] According to another implementation variant, several scanning orders of the current block, e.g. - lexicographical order, or - Scan the first column from top to bottom, then the column just to the right of that, etc., or Alternating diagonal scans can be used.

[0114] According to another variant, it is possible to simulate the coding costs associated with each of the scanning orders and to choose the best scanning order for the current block from a rate / distortion perspective, and then code an item of information representing the chosen scanning order for the current block.

[0115] At the end of step E222, the quantized residual block R1 Q This quantized residual block R1 Q must be coded for transmission to the decoder. The predictor P1 for the current block has also been determined.

[0116] In step E223, the quantized residual block R1 Q is coded for transmission to the decoder. Any known technique may be used to code the quantized coefficients of the conventional prediction residual, such as the method described in HEVC. For example, according to one embodiment, the quantized residual block R1 Q The values ​​of are coded in the data stream STR using an entropy encoder.

[0117] According to one particular embodiment of the invention, it is possible to determine and code an additional prediction residual R2 from the ILR predictor obtained for the current block. However, coding of the additional prediction residual R2 is optional. Simply, the ILR predictor is determined from its predicted version P1 and the quantized residual R1. Q It is indeed possible to code the current block by:

[0118] To code the additional prediction residual R2 for the current block, the following steps are performed.

[0119] In step E224, the predictor P1 and the original current block X b The difference R2 between the two is calculated and the additional residual R2: R2 = X b-P1. The following steps correspond to the conventional coding steps for this residual R2.

[0120] In step E225, coefficient R2 T The residual R2 is transformed using a frequency transform to produce a block of

[0121] The transformation may be, for example, a DCT type transformation. The transformation to be used is selected from a predetermined set of transformations E T2 It is possible to choose from the set E to match specific statistics of the residual R2. T2 Set E T may be different from

[0122] In step E226, coefficient R2 T is quantized using, for example, a quantization step scalar quantization δ. This means that the block R2 TQ Generate.

[0123] The quantization step δ can be set by the user. The quantization step δ can also be calculated using another parameter λ, which sets a compromise between compression and quality and is input by the user or the encoder. For example, the quantization step δ can correspond to the quantization step δ or can be determined similarly to the quantization step δ.

[0124] In step E227, the quantization block R2 TQ The coefficients of are then transmitted in a coded manner. For example, the coding specified in the HEVC standard may be used.

[0125] In a known manner, the quantization block R2 TQ The current block is decoded by dequantizing the coefficients of P and then applying an inverse transform to the dequantized coefficients to obtain a decoded prediction residual. The prediction P is then used to reconstruct the current block and its decoded version Xrec is added to the decoded prediction residual to obtain the decoded version X of the current block. rec can then be used later to spatially predict other neighboring blocks of the image or to predict blocks of other images by inter-image prediction.

[0126] According to one particular embodiment of the invention, a conventional intra prediction mode is determined and associated with the current block. For this purpose, in step E228, an intra prediction mode is determined for the current block from a list of intra prediction modes available in conventional intra coding modes.

[0127] Several implementation variations are possible.

[0128] According to one variant, a list of MPM prediction modes is created for the current block according to the mechanism described above for coding according to the first coding mode M1, and according to this variant, the prediction mode is determined as the first mode in the MPM list.

[0129] According to another variant, the prediction mode may be predetermined, for example a prediction mode that is defined by default or transmitted in the data stream. For example, the predetermined prediction mode is the PLANAR mode of the HEVC standard.

[0130] According to another variant, the decoded current block X rec According to this variant, all prediction modes are determined as the intra prediction mode that best approximates the current decoded block X rec and the predicted block obtained by the evaluated prediction mode. The intra prediction mode that results in the smallest distortion is selected.

[0131] According to another variant, the prediction mode is determined as the intra prediction mode that best approximates the current predicted block P1 obtained by ILR prediction. According to this variant, all prediction modes are evaluated by measuring the distortion between the current predicted block P1 and the prediction block obtained by the evaluated prediction mode. The intra prediction mode that results in the smallest distortion is selected.

[0132] According to another variant, when there are several possible local predictors for an ILR-coded block, the intra-prediction mode may depend on the local predictor chosen to predict the current block, e.g., an association table indicates which intra-prediction mode should be associated with the current block according to the chosen local predictor.

[0133] In step E229, the determined intra-prediction mode is associated with the current block. The idea here is to associate an intra-prediction mode, but not to transmit it to the decoder. The method for determining the intra-mode to be associated with the current block is reproducible in the decoder and does not require the transmission of any information.

[0134] Although several implementation variations have been described above, the encoder and decoder must of course implement the same variations.

[0135] According to a particular embodiment described herein, the intra prediction mode associated with the current block is then determined as: - predict the intra-prediction mode of a subsequent block that will be coded in conventional intra mode, or - When appropriate, it may be used to determine the intra-prediction mode to be associated with a subsequent block that is to be coded according to another coding mode different from the conventional intra-prediction mode, which may be an ILR coding mode or other (e.g., inter) coding mode.

[0136] In step E23, it is checked whether the current block is the last block of the image to be processed by the coding method, taking into account the previously defined scanning order. If yes, the method proceeds to coding the next image of the video (step E25), if any. If no, in step E24, a subsequent block of the image to be processed is selected according to the previously defined scanning order of the images, and the coding method proceeds to step E2, where the selected block becomes the current block to be processed.

[0137] FIG. 4 shows a sequence of images I1, I2, ..., I to be decoded according to one particular embodiment of the invention. Nb 1 shows the steps of a method for decoding a stream of coded data STR representing

[0138] For example, the data stream STR has been generated via the coding method illustrated in relation to figure 1. The data stream STR is provided as input to a decoding device DEC, as will be explained in relation to figure 7.

[0139] The decoding method decodes the stream picture by picture, and each picture is decoded block by block.

[0140] At step E40, the image I to be decoded is j is subdivided into blocks. Each block undergoes a decoding operation in a series of steps, which are detailed below. The blocks can be of the same size or of different sizes.

[0141] In an optional step E401, according to one particular embodiment of the invention, a quantization parameter QP is read from the data stream STR.

[0142] In step E41, image I j The first block or sub-block X to be decoded b But image I jis selected as the current block according to a predetermined scanning order of the image. For example, it may be the first block in the lexicographical scanning order of the image.

[0143] In step E42, an item of information indicating the coding mode for the current block is read from the data stream STR. According to a particular embodiment described herein, this item of information indicates whether the current block is coded according to a first coding mode M1 or a second coding mode M2. According to a particular embodiment described herein, the first coding mode M1 corresponds to conventional intra-predictive coding of the current block, for example as defined according to the HEVC standard, and the second coding mode M2 ​​corresponds to in-loop residual (ILR) predictive coding.

[0144] In other particular embodiments, the item of information read from the stream STR may also indicate the use of other coding modes for coding the current block (not described here).

[0145] The step E43 for decoding the current block when it is coded according to the first coding mode M1 is described below.

[0146] In step E430, the quantization step δ1 is determined. For example, the quantization step δ1 is determined from the quantization parameter QP read in step E401, or as done in the encoder. For example, the quantization step δ1 may be calculated using the quantization parameter QP read in step E401. For example, the quantization parameter QP may be a quantization parameter conventionally used in the AVC or HEVC standard. Thus, in the HEVC standard, the quantization step δ1 is determined by the equation δ1=levelScale[QP%6]<<(QP / 6)), where levelScale[k]={40, 45, 51, 57, 64, 72} for k=0..5.

[0147] In step E431, the prediction mode chosen for coding the current block is decoded from the neighboring blocks. To this end, the intra prediction mode chosen for the current block is coded using the intra prediction modes associated with the neighboring blocks of the current block, as was done in the encoder.

[0148] According to one variant of the particular embodiment of the invention described herein, the intra prediction mode associated with a block is: - the intra-prediction mode used to predict the block if the block was coded using a conventional intra-prediction coding mode; - the intra-prediction mode associated with the block, if the block is coded with a coding mode different from conventional intra-prediction, an example of such an association is explained below with reference to steps E449 and E450.

[0149] The construction of both MPM and non-MPM lists is strictly similar to that done during coding. According to the HEVC standard, the following types of syntax elements are used: A binary indicator indicating whether the prediction mode to be coded for the current block is in the MPM list is decoded; If the prediction mode of the current block belongs to the MPM list, an index in the MPM list corresponding to the prediction mode of the current block is coded; If the prediction mode of the current block does not belong to the MPM list, an index in the non-MPM list corresponding to the prediction mode of the current block is coded.

[0150] The binary indicator and the prediction mode index are thus read for the current block from the data stream STR to decode the intra-prediction mode of the current block.

[0151] In step E432, the decoder constructs a prediction block P for the current block from the decoded prediction mode.

[0152] In step E433, the decoder extracts the quantization blocks R from the data stream STR, for example using the decoding specified in the HEVC standard. TQ Decode the coefficients of

[0153] At step E434, the decoded block R TQ is dequantized using, for example, a quantization step scalar dequantization δ. This means that the dequantized coefficients R TQD Generate a block of

[0154] At step E435, the decoded prediction residual block R TQDI The dequantized coefficients R TQD An inverse frequency transform is applied to the blocks of . The transform may be, for example, an inverse DCT type transform. By decoding an indicator from the data stream STR, the transform to be used can be selected from a predefined set of transforms E TI It is possible to choose from.

[0155] At step E436, the decoded current block X rec , X rec =P+R TQDI the prediction block P obtained in step E432 and the decoded residual block R obtained in step E435, TQDI From this, the current block is reconstructed.

[0156] According to an embodiment variant described here, in step E437, the intra prediction mode decoded in step E431 is associated with the current block.

[0157] The step E44 for decoding the current block when it is coded according to the second coding mode M2 ​​is described below.

[0158] In step E440, the local predictor PL to be used to predict the pixels of the current block is determined. If only one predictor is available, the local predictor is set by default, for example at the decoder level, and no syntax element needs to be read from the stream STR to determine it.

[0159] If several local predictors are available, such as the predictors PL1-PL4 described above, a syntax element is decoded from the data stream STR to identify which local predictor was used to predict the current block, and the local predictor is so determined from the decoded syntax element.

[0160] In step E441, the quantization step δ2 is determined in the same way as it was done in the encoder.

[0161] In step E442, the quantized residual R1 is extracted from the data stream STR. Q Any known technique may be used to decode the quantized coefficients of the conventional prediction residual, such as the methods described in HEVC.

[0162] In step E443, the dequantized residual block R1 QD quantize residual block R1 using quantization step δ2 to generate Q is inverse quantized.

[0163] In step E444, the dequantized residual block R1 QD Once obtained, a prediction block P1 is constructed using the local predictor PL determined at step E440.

[0164] In step E444, each pixel of the current block is predicted and reconstructed as follows: - To obtain a prediction value PRED, the current pixel X of the current block is predicted by a selected predictor PL using either already reconstructed pixels outside the block or previously reconstructed pixels of the current block, or both. In all cases, the predictor PL uses previously decoded pixels. The decoded prediction P1(X) of the current pixel X is P1(X)=PRED+R1 QD The dequantized value of the prediction residual, R1, is QD is calculated by adding to the predicted value PRED.

[0165] These steps are performed for all pixels of the current block, in a scan order that guarantees that the pixels used for prediction, chosen from PL1, . . . , PL4, are available.

[0166] For example, the scan order is lexicographical (left to right, then rows from top to bottom).

[0167] According to one particular embodiment of the invention, a prediction block P1 comprising decoded prediction values ​​P1(X) of each pixel of the current block is calculated based on the decoded current block X rec is formed here.

[0168] According to another particular embodiment of the invention, it is now taken into account that an additional prediction residual has been coded for the current block. rec To reconstruct the decoded version of {tilde over (x)}, it is necessary to decode this additional prediction residual.

[0169] For example, this other particular embodiment may be activated or not by default at the encoder and decoder levels. Alternatively, an indicator may be coded in the data stream together with block-level information to indicate whether additional prediction residuals are coded for each block coded according to the ILR coding mode. Alternatively, an indicator may be coded in the data stream together with image or sequence-level information to indicate whether additional prediction residuals are coded for all blocks of an image or sequence of images coded according to the ILR coding mode.

[0170] When additional prediction residuals are coded for the current block, in step E445, the quantized prediction residual R2 TQ are decoded from the data stream STR using means adapted to those implemented in an encoder, for example means implemented in an HEVC decoder.

[0171] At step E446, the quantization coefficient R2 TQ is dequantized using, for example, a quantization step scalar dequantization δ1. This means that the dequantized coefficient R2 TQD Generate a block of

[0172] At step E447, the decoded prediction residual block R2 TQDI To generate the block R2 TQD An inverse frequency transform is applied to

[0173] The inverse transform may for example be an inverse DCT type transform.

[0174] The transformations to be used are defined in a given set of transformations E T2It is possible to choose from the set E to match particular statistics of the residual R2 and decode an item of information that informs the decoder which transformation should be used. T2 Set E T is different.

[0175] In step E448, the prediction block P1 obtained in step E444 is decoded to obtain the prediction residual R2 TQDI The current block is reconstructed by adding to

[0176] According to one particular embodiment of the invention, a conventional intra-prediction mode is determined and associated with the current block. For this purpose, in step E449, an intra-prediction mode is determined for the current block from a list of intra-prediction modes available in conventional intra-coding modes. This determination is performed in the same way as it is performed at encoder level.

[0177] In step E450, the determined intra prediction mode is associated with the current block.

[0178] In step E45, it is checked whether the current block is the last block of the image to be processed by the decoding method, taking into account the previously defined scanning order. If yes, the method proceeds to decoding the next image of the video (step E47), if any. If no, in step E46, the subsequent block of the image to be processed is selected according to the previously defined scanning order of the images, and the decoding method proceeds to step E42, with the selected block becoming the current block to be processed.

[0179] 5 shows an example signal STR comprising coded data representing at least one block of an image according to one particular embodiment of the present invention. For example, the signal STR may comprise a quantization parameter QP for determining the quantization steps δ1 and δ2 described above. When coding video, this quantization parameter QP may be coded at the block level or at the image or image sequence level.

[0180] The signal STR comprises, for a block of the image, a coding indicator TY indicating the coding mode for that block. When the TY indicator indicates that the block is coded according to a second coding mode, here the ILR mode, the signal then includes a quantized prediction residual R1 Q and possibly the quantized transform prediction residual R2 TQ When several local predictors are possible for the current block, the signal also comprises a local predictor PL coding indicator.

[0181] When the indicator TY indicates that the block is coded according to a first coding mode, here a conventional intra-prediction mode, the signal then includes the quantized transform prediction residual R TQ , a binary indicator i indicating whether the prediction mode to be coded for the current block is in the MPM list. MPM , and an index idx indicating the index of the current block prediction mode in the corresponding list. MPM Equipped with.

[0182] FIG. 6 shows a simplified structure of a coding device COD adapted to implement a coding method according to any one of the particular embodiments of the invention.

[0183] According to one particular embodiment of the invention, the steps of the coding method are implemented by computer program instructions. For this purpose, the coding device COD has a computer of standard architecture and in particular comprises a processing unit UT equipped with a memory MEM, for example a processor PROC, and driven by a computer program PG stored in the memory MEM. The computer program PG comprises instructions for implementing the steps of the coding method as described above, when the program is executed by the processor PROC.

[0184] During initialization, the code instructions of the computer program PG are loaded, for example, into a RAM memory (not shown) before being executed by the processor PROC, which in particular performs the steps of the coding method described above according to the instructions of the computer program PG.

[0185] FIG. 7 shows a simplified structure of a decoding device DEC adapted to implement the decoding method according to any one of the particular embodiments of the invention.

[0186] According to one particular embodiment of the invention, the decoding device DEC comprises a computer of standard architecture and in particular comprises a processing unit UT0 equipped with a memory MEM0, for example a processor PROC0, and driven by a computer program PG0 stored in the memory MEM0, the computer program PG0 comprising instructions for carrying out the steps of the decoding method as described above, when the program is executed by the processor PROC0.

[0187] During initialization, the code instructions of the computer program PG0 are loaded, for example, into a RAM memory (not shown) before being executed by the processor PROC0. In particular, the processor PROC0 of the processing unit UT0 performs the steps of the decoding method described above according to the instructions of the computer program PG0. [Explanation of symbols]

[0188] I1, I2, ..., I Nb Sequence I j image X b Current Block STR Data Stream QP quantization parameter M1 First coding mode M2 Second coding mode A b and B b Adjacent blocks PG Computer Program COD coding device PROC Processor DEC Decryption Device UT Processing Unit

Claims

1. 1. A method for decoding a coded data stream representing at least one image, wherein the image is divided into blocks, and the method for decoding comprises, for at least one block of the image, called a current block: - a step (E42) of decoding an item of information indicating the coding mode of the current block, wherein a first coding mode corresponds to a type of coding mode that uses a transformation of the prediction residual and a second coding mode corresponds to a type of coding mode that does not use a transformation of the prediction residual; - a step (E43) of decoding the current block, comprising, when the coding mode of the current block corresponds to the first coding mode, determining a first quantization step (E430) for inverse quantizing, in the transform domain, the prediction residual associated with the current block; - a step (E44) of decoding the current block, comprising, when the coding mode of the current block corresponds to the second coding mode, determining a second quantization step (E441) for inverse quantizing in the spatial domain the prediction residual associated with the current block; - obtaining a quantization parameter; Equipped with - the first quantization step and the second quantization step are determined according to the quantization parameter; When the coding mode of the current block corresponds to the second coding mode, the decoding of the current block comprises, for at least one pixel of the current block: - obtaining a prediction of said pixel from another pixel, said other pixel belonging to said current block of said image; - decoding a prediction residual associated with said pixel; - reconstructing said pixel from said prediction of said pixel and said prediction residual associated with said pixel using said second quantization step; The method for decoding further comprises:

2. 2. The method for decoding of claim 1, wherein, when the coding mode of the current block corresponds to the second coding mode, coefficients of the prediction residual associated with the current block are decoded by an entropy decoder that was used to decode the coefficients of the prediction residual associated with the current block when the coding mode of the current block corresponds to the first coding mode.

3. A method for coding a coded data stream representing at least one image, said image being divided into blocks, said method for coding comprising the steps of: - coding (E20) an item of information indicating a coding mode of the current block, wherein a first coding mode corresponds to a type of coding mode that uses a transformation of a prediction residual and a second coding mode corresponds to a type of coding mode that does not use a transformation of a prediction residual; - a step (E21) of coding the current block, when the coding mode of the current block corresponds to the first coding mode, comprising determining a first quantization step (E210) for quantizing a prediction residual associated with the current block in a transform domain; - a step (E22) of coding the current block, when the coding mode of the current block corresponds to the second coding mode, comprising determining a second quantization step (E221) for quantizing a prediction residual associated with the current block in the spatial domain; - obtaining a quantization parameter; Equipped with - the first quantization step and the second quantization step are determined according to the quantization parameter; When the coding mode of the current block corresponds to the second coding mode, the step of coding the current block includes, for at least one pixel of the current block: - obtaining a prediction of said pixel from another pixel, said other pixel belonging to said current block of said image; - obtaining a prediction residual associated with said pixel from said prediction of said pixel; - coding the quantized prediction residual; A method for coding comprising:

4. 4. The method for coding of claim 3, wherein, when the coding mode of the current block corresponds to the second coding mode, coefficients of the prediction residual associated with the current block are coded by an entropy encoder that was used to code the coefficients of the prediction residual associated with the current block when the coding mode of the current block corresponds to the first coding mode.

5. 5. Method for decoding or coding according to claim 1, wherein the second quantization step is determined according to the first quantization step.

6. δ 1 corresponds to the first quantization step, and a, b, c, d, and e are predetermined parameters, the second quantization step is a*δ 1 +b or [Equation 1] 6. The method for decoding or coding according to claim 5, wherein the value is determined by:

7. 5. A method for decoding or coding according to claim 1, wherein the first quantization step is obtained from a quantization step table predetermined for the value of the quantization parameter.

8. 3. A method for decoding according to claim 1, wherein the quantization parameter corresponds to a Lagrangian λ used to optimize a D+λ*R function, where R corresponds to the rate of the coded data stream and D corresponds to the associated distortion of the decoded image.

9. 5. A method for decoding or coding according to any one of claims 1 to 4, wherein the quantization parameter is coded in or decoded from the coded data stream.

10. the decoding of the current block in the second coding mode includes: - decoding another prediction residual associated with the current block; - inverse quantizing the other prediction residual associated with the current block using the first quantization step; - reconstructing the current block from the reconstructed pixels of the current block and the dequantized further prediction residual; Further provided with Method for decoding according to any one of claims 1 to 2.

11. 1. A device for decoding a coded data stream representing at least one image, said image being divided into blocks, said device for decoding determining for at least one block of said image, called a current block: - decoding an item of information indicating a coding mode of the current block, wherein a first coding mode corresponds to a type of coding mode that uses a transformation of a prediction residual and a second coding mode corresponds to a type of coding mode that does not use a transformation of a prediction residual; - decoding the current block by determining a first quantization step for inverse quantizing the prediction residual associated with the current block in a transform domain when the coding mode of the current block corresponds to the first coding mode; - decoding the current block by determining a second quantization step for inverse quantizing the prediction residual associated with the current block in the spatial domain when the coding mode of the current block corresponds to the second coding mode; - a processor (PROC0) configured to obtain a quantization parameter; - the first quantization step and the second quantization step are determined according to the quantization parameter; When the coding mode of the current block corresponds to the second coding mode, the processor performs, for at least one pixel of the current block: - obtaining a prediction of said pixel from another pixel, said another pixel belonging to said current block of said image; - decoding a prediction residual associated with said pixel; - reconstructing said pixel from said prediction of said pixel and said prediction residual associated with said pixel using said second quantization step; the device is further configured to decode the current block by

12. 1. A device for coding a coded data stream representing at least one image, said image being divided into blocks, said device for coding performing, for at least one block of said image, called a current block, - coding an item of information indicating a coding mode of the current block, wherein a first coding mode corresponds to a type of coding mode that uses a transformation of a prediction residual, and a second coding mode corresponds to a type of coding mode that does not use a transformation of a prediction residual; - coding the current block by determining a first quantization step for quantizing a prediction residual associated with the current block in a transform domain when the coding mode of the current block corresponds to the first coding mode; - coding the current block by determining a second quantization step for quantizing a prediction residual associated with the current block in the spatial domain when the coding mode of the current block corresponds to the second coding mode; - a processor (PROC) configured to obtain a quantization parameter; - the first quantization step and the second quantization step are determined according to the quantization parameter; When the coding mode of the current block corresponds to the second coding mode, the processor performs, for at least one pixel of the current block: - obtaining a prediction of said pixel from another pixel, said another pixel belonging to said current block of said image; - obtaining a prediction residual associated with said pixel from said prediction of said pixel; - coding the quantized prediction residual; the device is further configured to code the current block by

13. 10. A computer program comprising instructions for carrying out the method for decoding according to any one of claims 1 to 2 or 5 to 10 or the method for coding according to any one of claims 3 to 7 or 9 when said program is executed by a processor.

Images