Colour multiplexing

Abstract

A method of colour display comprises receiving and spatially multiplexing a first data stream and second data stream wherein the first data stream comprises a plurality of first colour (e.g. red) pixe

Description

FIELD The present disclosure relates to a method of colour display and a display system. More generally, the present disclosure relates to a method of processing a plurality of different data streams and a processor therefor. Some embodiments relate to a method of driving a spatial light modulator, a drive scheme for a spatial light modulator and a display driver. Some embodiments relate to a holographic projector and a head-up display. BACKGROUND AND INTRODUCTION Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well-known interference techniques to form a holographic recording, or "hologram", comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object. Computer-generated holography may numerically simulate the interference process. A computergenerated hologram may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. A Fourier hologram may be considered a Fourier domain / plane representation of the object or a frequency domain / plane representation of the object. A computer-generated hologram may also be calculated by coherent ray tracing or a point cloud technique, for example. A computer-generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and / or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micro-mirrors, for example. A spatial light modulator typically comprises a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The spatial light modulator may be reflective meaning that modulated light is output in reflection. The spatial light modulator may equally be transmissive meaning that modulated light is output in transmission. A holographic projector may be provided using the system described herein. Such projectors have found application in head-up display, "HUD". SUMMARY Aspects of the present disclosure are defined in the appended independent claims. A first aspect of the present disclosure is a method of colour display. The method comprises receiving and spatially multiplexing a first data stream and second data stream. The first data stream comprises a plurality of first colour (e.g. red) pixel values and the second data stream comprises a plurality of second colour (e.g. green) pixel values. The process of spatially multiplexing the first data stream and second data stream forms an output data stream. The output data stream comprises a plurality of output pixel values. Each output pixel value is representative of a first colour pixel value or a second colour pixel value. The step of multiplexing comprises representing first colour pixel values using a first range of allowable output pixel values and representing second colour pixel values using a second range of allowable output pixel values. Broadly, the first aspect provides the technical advancement of more efficient colour display. This advancement is achieved by, firstly, providing a scheme in which a plurality of different pixel values of different pixel patterns (e.g. pixel values for different colour images or holograms) are combined into a single data set. This approach allows a single display device to be used for the plurality of different colour images or holograms. The advancement of the first aspect is further achieved by reducing the demand for memory and / or processing by enabling a single, common gamma calibration to be employed for all three colours. For example, only one gamma calibration (e.g. function or look-up table) needs to be stored in memory and processing logic to select a gamma calibration from a plurality of different gamma calibrations is not required because there is only one gamma calibration. The inventors have found that efficient colour display can be provided with this scheme without an observable loss of image quality. The first range of allowable output pixel values and second range of allowable output pixel values may be non-overlapping such that colour pixel values are uniquely identified. The number of grey levels (or different pixel values) of the first range of output pixel values may be different to a number of grey levels (or different pixel values) of the second range of output pixel values. These embodiments enable grey level resolution to depend on e.g. colour. For example, the eye is more sensitive to green light than red light. It may be beneficial to provide more grey levels for green than red. The first range of (allowable) output pixel values may be defined (or definable) by an n-bit number and the second range of (allowable) output pixel values may be defined (or definable) by an m-bit number, wherein n is not equal to m. These embodiments can better optimise the use of computation resources such as memory. The first colour pixel values and / or second colour pixel values may be defined by an x-bit number, wherein x is not equal to m and / or n. The method may comprise converting each output pixel value to a pixel voltage using a common gamma calibration (e.g. function, conversion or table). The common gamma calibration comprises at least one (mathematical) discontinuity such as two discontinuities. The method may further comprise illuminating each display pixel of a display device in accordance with the corresponding output pixel value. The method may comprise illuminating a first display pixel having an output pixel value within the first range of allowable output pixel values with first colour light and illuminating a second display pixel having an output pixel value of allowable output pixel values within the second range with second colour light. The method may further comprise receiving a third data stream comprising a plurality of third colour (e.g. blue) pixel values; and spatially multiplexing the third data stream with the first data stream and second data stream to form the output data stream, wherein third colour pixel values are represented using a third range of (allowable) output pixel values (that does not overlap the first or second range of output pixel values). A second aspect of the present disclosure is a display system comprising a display driver. The display driver is arranged to receive and spatially multiplex a first data stream and a second data stream. The first data stream comprises a plurality of first colour (e.g. red) pixel values. The second data stream comprises a plurality of second colour (e.g. green) pixel values. The process of spatially multiplexing the first data stream and second data stream forms an output data stream comprising a plurality of output pixel values. Each output pixel value is representative of a first colour pixel value or a second colour pixel value. First colour pixel values are represented by a first range of allowable output pixel values. Second colour pixel values are represented by a second range of allowable output pixel values. The term "gamma calibration" is used herein to refer to a mathematical function or relationship that converts a grey level to a drive value such as a liquid crystal cell voltage that achieves a required level of light modulation such as phase delay in the range o to 2tt. The gamma calibration is therefore a calibration function. The grey level may be the pixel value of an image pixel or the pixel value of a hologram pixel determined by a hologram engine, for example, from the image. The gamma calibration or gamma function is strongly dependent on wavelength. It is conventional in the art of display to have one gamma calibration or function per colour (wavelength). Each gamma calibration consumes memory resource. The display device may be a phase modulator. The display device may be a liquid crystal on silicon spatial light modulator. The term "hologram" is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, regarding the object. The term "holographic reconstruction" is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The system disclosed herein is described as a "holographic projector" because the holographic reconstruction is a real image and spatially-separated from the hologram. The term "replay field" is used to refer to the 2D area within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in the form of a plurality diffracted orders wherein each diffracted order is a replica of the zeroth-order replay field. The zeroth-order replay field generally corresponds to the preferred or primary replay field because it is the brightest replay field. Unless explicitly stated otherwise, the term "replay field" should be taken as referring to the zeroth-order replay field. The term "replay plane" is used to refer to the plane in space containing all the replay fields. The terms "image", "replay image" and "image region" refer to areas of the replay field illuminated by light of the holographic reconstruction. In some embodiments, the "image" may comprise discrete spots which may be referred to as "image spots" or, for convenience only, "image pixels". The terms "encoding", "writing" or "addressing" are used to describe the process of providing the plurality of pixels of the SLM with a respective plurality of control values which respectively determine the modulation level of each pixel. It may be said that the pixels of the SLM are configured to "display" a light modulation distribution in response to receiving the plurality of control values. Thus, the SLM may be said to "display" a hologram and the hologram may be considered an array of light modulation values or levels. It has been found that a holographic reconstruction of acceptable quality can be formed from a "hologram" containing only phase information related to the Fourier transform of the original object. Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram but the present disclosure is equally applicable to amplitude-only holography. The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information related to the Fourier transform of the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram which contains both amplitude and phase information related to the original object. Such a hologram may be referred to as a fully-complex hologram because the value (grey level) assigned to each pixel of the hologram has an amplitude and phase component. The value (grey level) assigned to each pixel may be represented as a complex number having both amplitude and phase components. In some embodiments, a fully-complex computer-generated hologram is calculated. Reference may be made to the phase value, phase component, phase information or, simply, phase of pixels of the computer-generated hologram or the spatial light modulator as shorthand for "phase-delay". That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2n) which represents the amount of phase retardation provided by that pixel. For example, a pixel of the spatial light modulator described as having a phase value of n / 2 will retard the phase of received light by n / 2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term "grey level" may be used to refer to the plurality of available modulation levels. For example, the term "grey level" may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term "grey level" may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator. The hologram therefore comprises an array of grey levels - that is, an array of light modulation values such as an array of phase-delay values or complex modulation values. The hologram is also considered a diffractive pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light having a wavelength comparable to, generally less than, the pixel pitch of the spatial light modulator. Reference is made herein to combining the hologram with other diffractive patterns such as diffractive patterns functioning as a lens or grating. For example, a diffractive pattern functioning as a grating may be combined with a hologram to translate the replay field on the replay plane or a diffractive pattern functioning as a lens may be combined with a hologram to focus the holographic reconstruction on a replay plane in the near field. Although different embodiments and groups of embodiments may be disclosed separately in the detailed description which follows, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged. BRIEF DESCRIPTION OF THE DRAWINGS Specific embodiments are described by way of example only with reference to the following figures: Figure 1 is a schematic showing a reflective SLM producing a holographic reconstruction on a screen; Figure 2A illustrates a first iteration of an example Gerchberg-Saxton type algorithm; Figure 2B illustrates the second and subsequent iterations of the example Gerchberg-Saxton type algorithm; Figure 2C illustrates alternative second and subsequent iterations of the example Gerchberg-Saxton type algorithm; Figure 3 is a schematic of a reflective LCOS SLM; Figures 4A to 4C show a scheme comprising three different gamma calibrations for three different colours; Figure 5 shows an embodiment of the present disclosure; and Figure 6 shows a single, common gamma calibration in accordance with embodiments. The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION OF EMBODIMENTS The present invention is not restricted to the embodiments described in the following but extends to the full scope of the appended claims. That is, the present invention may be embodied in different forms and should not be construed as limited to the described embodiments, which are set out for the purpose of illustration. Terms of a singular form may include plural forms unless specified otherwise. A structure described as being formed at an upper portion / lower portion of another structure or on / under the other structure should be construed as including a case where the structures contact each other and, moreover, a case where a third structure is disposed there between. In describing a time relationship - for example, when the temporal order of events is described as "after", "subsequent", "next", "before" or suchlike - the present disclosure should be taken to include continuous and non-continuous events unless otherwise specified. For example, the description should be taken to include a case which is not continuous unless wording such as "just", "immediate" or "direct" is used. Although the terms "first", "second", etc. may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the appended claims. Features of different embodiments may be partially or overall coupled to or combined with each other, and may be variously inter-operated with each other. Some embodiments may be carried out independently from each other, or may be carried out together in co-dependent relationship. Optical configuration Figure 1 shows an embodiment in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object for reconstruction. It may therefore be said that the hologram is a Fourier domain or frequency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon, "LCOS", device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a replay field, for example, a light receiving surface such as a screen or diffuser. A light source 110, for example a laser or laser diode, is disposed to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a generally planar wavefront of light to be incident on the SLM. In Figure 1, the direction of the wavefront is off-normal (e.g. two or three degrees away from being truly orthogonal to the plane of the transparent layer). However, in other embodiments, the generally planar wavefront is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. In the embodiment shown in Figure 1, the arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a light-modulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to optics including a Fourier transform lens 120, having its focus at a screen 125. More specifically, the Fourier transform lens 120 receives a beam of modulated light from the SLM 140 and performs a frequency-space transformation to produce a holographic reconstruction at the screen 125. Notably, in this type of holography, each pixel of the hologram contributes to the whole reconstruction. There is not a one-to-one correlation between specific points (or image pixels) on the replay field and specific light-modulating elements (or hologram pixels). In other words, modulated light exiting the light-modulating layer is distributed across the replay field. In these embodiments, the position of the holographic reconstruction in space is determined by the dioptric (focusing) power of the Fourier transform lens. In the embodiment shown in Figure 1, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and the Fourier transform is performed optically. Any lens can act as a Fourier transform lens but the performance of the lens will limit the accuracy of the Fourier transform it performs. The skilled person understands how to use a lens to perform an optical Fourier transform. Hologram calculation In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or Fourier-based hologram, in which an image is reconstructed in the far field by utilising the Fourier transforming properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field in the replay plane back to the lens plane. Computer-generated Fourier holograms may be calculated using Fourier transforms. A Fourier transform hologram may be calculated using an algorithm such as the Gerchberg-Saxton algorithm. Furthermore, the Gerchberg-Saxton algorithm may be used to calculate a hologram in the Fourier domain (i.e. a Fourier transform hologram) from amplitude-only information in the spatial domain (such as a photograph). The phase information related to the object is effectively "retrieved" from the amplitude-only information in the spatial domain. In some embodiments, a computer-generated hologram is calculated from amplitude-only information using the Gerchberg-Saxton algorithm or a variation thereof. The Gerchberg Saxton algorithm considers the situation when intensity cross-sections of a light beam, lA(x, y) and lB(x, y), in the planes A and B respectively, are known and lA(x, y) and lB(x, y) are related by a single Fourier transform. With the given intensity cross-sections, an approximation to the phase distribution in the planes A and B, tpA(x, y) and tpB(x, y) respectively, is found. The Gerchberg-Saxton algorithm finds solutions to this problem by following an iterative process. More specifically, the Gerchberg-Saxton algorithm iteratively applies spatial and spectral constraints while repeatedly transferring a data set (amplitude and phase), representative of lA(x, y) and lB(x, y), between the spatial domain and the Fourier (spectral or frequency) domain. The corresponding computer-generated hologram in the spectral domain is obtained through at least one iteration of the algorithm. The algorithm is convergent and arranged to produce a hologram representing an input image. The hologram may be an amplitude-only hologram, a phase-only hologram or a fully complex hologram. In some embodiments, a phase-only hologram is calculated using an algorithm based on the Gerchberg-Saxton algorithm such as described in British patent 2,498,170 or 2,501,112 which are hereby incorporated in their entirety by reference. However, embodiments disclosed herein describe calculating a phase-only hologram by way of example only. In these embodiments, the Gerchberg-Saxton algorithm retrieves the phase information tp [u, v] of the Fourier transform of the data set which gives rise to a known amplitude information T[x, y], wherein the amplitude information T[x, y] is representative of a target image (e.g. a photograph). Since the magnitude and phase are intrinsically combined in the Fourier transform, the transformed magnitude and phase contain useful information about the accuracy of the calculated data set. Thus, the algorithm may be used iteratively with feedback on both the amplitude and the phase information. However, in these embodiments, only the phase information tp[u, v] is used as the hologram to form a holographic representative of the target image at an image plane. The hologram is a data set (e.g. 2D array) of phase values. In other embodiments, an algorithm based on the Gerchberg-Saxton algorithm is used to calculate a fully-complex hologram. A fully-complex hologram is a hologram having a magnitude component and a phase component. The hologram is a data set (e.g. 2D array) comprising an array of complex data values wherein each complex data value comprises a magnitude component and a phase component. In some embodiments, the algorithm processes complex data and the Fourier transforms are complex Fourier transforms. Complex data may be considered as comprising (i) a real component and an imaginary component or (ii) a magnitude component and a phase component. In some embodiments, the two components of the complex data are processed differently at various stages of the algorithm. Figure 2A illustrates the first iteration of an algorithm in accordance with some embodiments for calculating a phase-only hologram. The input to the algorithm is an input image 210 comprising a 2D array of pixels or data values, wherein each pixel or data value is a magnitude, or amplitude, value. That is, each pixel or data value of the input image 210 does not have a phase component. The input image 210 may therefore be considered a magnitude-only or amplitude-only or intensity-only distribution. An example of such an input image 210 is a photograph or one frame of video comprising a temporal sequence of frames. The first iteration of the algorithm starts with a data forming step 202A comprising assigning a random phase value to each pixel of the input image, using a random phase distribution (or random phase seed) 230, to form a starting complex data set wherein each data element of the set comprising magnitude and phase. It may be said that the starting complex data set is representative of the input image in the spatial domain. First processing block 250 receives the starting complex data set and performs a complex Fourier transform to form a Fourier transformed complex data set. Second processing block 253 receives the Fourier transformed complex data set and outputs a hologram 280A. In some embodiments, the hologram 280A is a phase-only hologram. In these embodiments, second processing block 253 quantises each phase value and sets each amplitude value to unity in order to form hologram 280A. Each phase value is quantised in accordance with the phase-levels which may be represented on the pixels of the spatial light modulator which will be used to "display" the phase-only hologram. For example, if each pixel of the spatial light modulator provides 256 different phase levels, each phase value of the hologram is quantised into one phase level of the 256 possible phase levels. Hologram 280A is a phase-only Fourier hologram which is representative of an input image. In other embodiments, the hologram 280A is a fully complex hologram comprising an array of complex data values (each including an amplitude component and a phase component) derived from the received Fourier transformed complex data set. In some embodiments, second processing block 253 constrains each complex data value to one of a plurality of allowable complex modulation levels to form hologram 280A. The step of constraining may include setting each complex data value to the nearest allowable complex modulation level in the complex plane. It may be said that hologram 280A is representative of the input image in the spectral or Fourier or frequency domain. In some embodiments, the algorithm stops at this point. However, in other embodiments, the algorithm continues as represented by the dotted arrow in Figure 2A. In other words, the steps which follow the dotted arrow in Figure 2A are optional (i.e. not essential to all embodiments). Third processing block 256 receives the modified complex data set from the second processing block 253 and performs an inverse Fourier transform to form an inverse Fourier transformed complex data set. It may be said that the inverse Fourier transformed complex data set is representative of the input image in the spatial domain. Fourth processing block 259 receives the inverse Fourier transformed complex data set and extracts the distribution of magnitude values 211A and the distribution of phase values 213A. Optionally, the fourth processing block 259 assesses the distribution of magnitude values 211A. Specifically, the fourth processing block 259 may compare the distribution of magnitude values 211A of the inverse Fourier transformed complex data set with the input image 510 which is itself, of course, a distribution of magnitude values. If the difference between the distribution of magnitude values 211A and the input image 210 is sufficiently small, the fourth processing block 259 may determine that the hologram 280A is acceptable. That is, if the difference between the distribution of magnitude values 211A and the input image 210 is sufficiently small, the fourth processing block 259 may determine that the hologram 280A is a sufficiently-accurate representative of the input image 210. In some embodiments, the distribution of phase values 213A of the inverse Fourier transformed complex data set is ignored for the purpose of the comparison. It will be appreciated that any number of different methods for comparing the distribution of magnitude values 211A and the input image 210 may be employed and the present disclosure is not limited to any particular method. In some embodiments, a mean square difference is calculated and if the mean square difference is less than a threshold value, the hologram 280A is deemed acceptable. If the fourth processing block 259 determines that the hologram 280A is not acceptable, a further iteration of the algorithm may be performed. However, this comparison step is not essential and in other embodiments, the number of iterations of the algorithm performed is predetermined or preset or user-defined. Figure 2B represents a second iteration of the algorithm and any further iterations of the algorithm. The distribution of phase values 213A of the preceding iteration is fed-back through the processing blocks of the algorithm. The distribution of magnitude values 211A is rejected in favour of the distribution of magnitude values of the input image 210. In the first iteration, the data forming step 202A formed the first complex data set by combining distribution of magnitude values of the input image 210 with a random phase distribution 230. However, in the second and subsequent iterations, the data forming step 202B comprises forming a complex data set by combining (i) the distribution of phase values 213A from the previous iteration of the algorithm with (ii) the distribution of magnitude values of the input image 210. The complex data set formed by the data forming step 202B of Figure 2B is then processed in the same way described with reference to Figure 2A to form second iteration hologram 280B. The explanation of the process is not therefore repeated here. The algorithm may stop when the second iteration hologram 280B has been calculated. However, any number of further iterations of the algorithm may be performed. It will be understood that the third processing block 256 is only required if the fourth processing block 259 is required or a further iteration is required. The output hologram 280B generally gets better with each iteration. However, in practice, a point is usually reached at which no measurable improvement is observed or the positive benefit of performing a further iteration is out-weighted by the negative effect of additional processing time. Hence, the algorithm is described as iterative and convergent. Figure 2C represents an alternative embodiment of the second and subsequent iterations. The distribution of phase values 213A of the preceding iteration is fed-back through the processing blocks of the algorithm. The distribution of magnitude values 211A is rejected in favour of an alternative distribution of magnitude values. In this alternative embodiment, the alternative distribution of magnitude values is derived from the distribution of magnitude values 211 of the previous iteration. Specifically, processing block 258 subtracts the distribution of magnitude values of the input image 210 from the distribution of magnitude values 211 of the previous iteration, scales that difference by a gain factor a and subtracts the scaled difference from the input image 210. This is expressed mathematically by the following equations, wherein the subscript text and numbers indicate the iteration number: = ^'{exp( / »,v])} = ZF{t? •exp( / ZFB[x,j])} ri = T[x, j] - a(\R„ [x, j]| - T[x, j]) where: F' is the inverse Fourier transform; F is the forward Fourier transform; R[x, y] is the complex data set output by the third processing block 256; T[x, y] is the input or target image; Z is the phase component; tp is the phase-only hologram 280B; r] is the new distribution of magnitude values 211B; and a is the gain factor. The gain factor a may be fixed or variable. In some embodiments, the gain factor a is determined based on the size and rate of the incoming target image data. In some embodiments, the gain factor a is dependent on the iteration number. In some embodiments, the gain factor a is solely function of the iteration number. The embodiment of Figure 2C is the same as that of Figure 2A and Figure 2B in all other respects. It may be said that the phase-only hologram tp(u, v) comprises a phase distribution in the frequency or Fourier domain. In some embodiments, the Fourier transform is performed using the spatial light modulator. Specifically, the hologram data is combined with second data providing optical power. That is, the data written to the spatial light modulation comprises hologram data representing the object and lens data representative of a lens. When displayed on a spatial light modulator and illuminated with light, the lens data emulates a physical lens - that is, it brings light to a focus in the same way as the corresponding physical optic. The lens data therefore provides optical, or focusing, power. In these embodiments, the physical Fourier transform lens 120 of Figure 1 may be omitted. It is known how to calculate data representative of a lens. The data representative of a lens may be referred to as a software lens. For example, a phase-only lens may be formed by calculating the phase delay caused by each point of the lens owing to its refractive index and spatially-variant optical path length. For example, the optical path length at the centre of a convex lens is greater than the optical path length at the edges of the lens. An amplitude-only lens may be formed by a Fresnel zone plate. It is also known in the art of computer-generated holography how to combine data representative of a lens with a hologram so that a Fourier transform of the hologram can be performed without the need for a physical Fourier lens. In some embodiments, lensing data is combined with the hologram by simple addition such as simple vector addition. In some embodiments, a physical lens is used in conjunction with a software lens to perform the Fourier transform. Alternatively, in other embodiments, the Fourier transform lens is omitted altogether such that the holographic reconstruction takes place in the far-field. In further embodiments, the hologram may be combined in the same way with grating data - that is, data arranged to perform the function of a grating such as image steering. Again, it is known in the field how to calculate such data. For example, a phase-only grating may be formed by modelling the phase delay caused by each point on the surface of a blazed grating. An amplitude-only grating may be simply superimposed with an amplitude-only hologram to provide angular steering of the holographic reconstruction. The second data providing lensing and / or steering may be referred to as a light processing function or light processing pattern to distinguish from the hologram data which may be referred to as an image forming function or image forming pattern. In some embodiments, the Fourier transform is performed jointly by a physical Fourier transform lens and a software lens. That is, some optical power which contributes to the Fourier transform is provided by a software lens and the rest of the optical power which contributes to the Fourier transform is provided by a physical optic or optics. In some embodiments, there is provided a real-time engine arranged to receive image data and calculate holograms in real-time using the algorithm. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the holograms are precalculated, stored in computer memory and recalled as needed for display on a SLM. That is, in some embodiments, there is provided a repository of predetermined holograms. Embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms which may be calculated by a similar method. The present disclosure is also applicable to holograms calculated by other techniques such as those based on point cloud methods. Light modulation A spatial light modulator may be used to display the diffractive pattern including the computergenerated hologram. If the hologram is a phase-only hologram, a spatial light modulator which modulates phase is required. If the hologram is a fully-complex hologram, a spatial light modulator which modulates phase and amplitude may be used or a first spatial light modulator which modulates phase and a second spatial light modulator which modulates amplitude may be used. In some embodiments, the light-modulating elements (i.e. the pixels) of the spatial light modulator are cells containing liquid crystal. That is, in some embodiments, the spatial light modulator is a liquid crystal device in which the optically-active component is the liquid crystal. Each liquid crystal cell is configured to selectively-provide a plurality of light modulation levels. That is, each liquid crystal cell is configured at any one time to operate at one light modulation level selected from a plurality of possible light modulation levels. Each liquid crystal cell is dynamically-reconfigurable to a different light modulation level from the plurality of light modulation levels. In some embodiments, the spatial light modulator is a reflective liquid crystal on silicon (LCOS) spatial light modulator but the present disclosure is not restricted to this type of spatial light modulator. A LCOS device provides a dense array of light modulating elements, or pixels, within a small aperture (e.g. a few centimetres in width). The pixels are typically approximately 10 microns or less which results in a diffraction angle of a few degrees meaning that the optical system can be compact. It is easier to adequately illuminate the small aperture of a LCOS SLM than it is the larger aperture of other liquid crystal devices. An LCOS device is typically reflective which means that the circuitry which drives the pixels of a LCOS SLM can be buried under the reflective surface. The results in a higher aperture ratio. In other words, the pixels are closely packed meaning there is very little dead space between the pixels. This is advantageous because it reduces the optical noise in the replay field. A LCOS SLM uses a silicon backplane which has the advantage that the pixels are optically flat. This is particularly important for a phase modulating device. A suitable LCOS SLM is described below, by way of example only, with reference to Figure 3. An LCOS device is formed using a single crystal silicon substrate 302. It has a 2D array of square planar aluminium electrodes 301, spaced apart by a gap 301a, arranged on the upper surface of the substrate. Each of the electrodes 301 can be addressed via circuitry 302a buried in the substrate 302. Each of the electrodes forms a respective planar mirror. An alignment layer 303 is disposed on the array of electrodes, and a liquid crystal layer 304 is disposed on the alignment layer 303. A second alignment layer 305 is disposed on the planar transparent layer 306, e.g. of glass. A single transparent electrode 307 e.g. of ITO is disposed between the transparent layer 306 and the second alignment layer 305. Each of the square electrodes 301 defines, together with the overlying region of the transparent electrode 307 and the intervening liquid crystal material, a controllable phase-modulating element 308, often referred to as a pixel. The effective pixel area, or fill factor, is the percentage of the total pixel which is optically active, taking into account the space between pixels 301a. By control of the voltage applied to each electrode 301 with respect to the transparent electrode 307, the properties of the liquid crystal material of the respective phase modulating element may be varied, thereby to provide a variable delay to light incident thereon. The effect is to provide phase-only modulation to the wavefront, i.e. no amplitude effect occurs. The described LCOS SLM outputs spatially modulated light in reflection. Reflective LCOS SLMs have the advantage that the signal lines, gate lines and transistors are below the mirrored surface, which results in high fill factors (typically greater than 90%) and high resolutions. Another advantage of using a reflective LCOS spatial light modulator is that the liquid crystal layer can be half the thickness than would be necessary if a transmissive device were used. This greatly improves the switching speed of the liquid crystal (a key advantage for the projection of moving video images). However, the teachings of the present disclosure may equally be implemented using a transmissive LCOS SLM. Colour display Various methods of colour display have been previously disclosed. British patent 2,547,929B discloses an approach that uses a plurality of display devices. Specifically, a different display device is used for each single colour channel. For example, a first single colour channel (e.g. red) may utilise a first display device, a second single colour channel (e.g. green) may utilise a second display device and, optionally, a third single colour channel (e.g. blue) may utilise a third display device. The three single colours may be optically combined to form a full colour image. Frame sequential colour is an approach in which different colour images (or different holograms thereof) are displayed one after the other in quick succession (e.g. within the integration of the human eye) on a common display device to give the perception of a full colour image. British patent 2,496,108B discloses an approach known as spatially separated colours in which a common display device may be used to form red, green and blue images in order to provide full colour display. In this approach, different areas of the display device are used for the different colours. That is, a first pixel area of the display device is used to display a first hologram giving rise to e.g. red image content, a second pixel area of the display device is used to display a second hologram giving rise to e.g. green image content and a third pixel area of the display device is used to display a third hologram giving rise to e.g. blue image content. The first, second and third holograms may be displayed and illuminated at the same time such that the red, green and blue images are formed at substantially the same time. British patent publication 2,616,305A discloses a technique in which a single hologram is determined for the different colours. That is, one hologram forms a red, green and blue holographic reconstruction even if the red, green and blue images are different. This approach utilises the phenomenon of phase-wrapping, in which different voltages with the same range can provide the same level of phase modulation, and a best-fit approach. A single display device can therefore be used to provide a full colour image. There is disclosed herein an alternative way of providing full colour display using one display device. In overview, red, green and blue pixel values are combined - e.g. spatially interlaced - to form a single set of pixel values each representative of either a red, green or blue pixel value. The approach involves splitting the available grey level range of the display by colour such that a first sub-range of grey levels is assigned to a first colour, a second sub-range of grey levels is assigned to a second colour and, optionally, a third sub-range of grey levels is assigned to a third colour. The sub-ranges of the plurality of sub-ranges are non-overlapping. Figures 4A to 4C represent a conventional approach which may be used in the scheme requiring a plurality of display devices or the scheme known as frame sequence colour. Specifically, Figure 4A shows a first gamma calibration which may be used to convert a first pixel grey level (e.g. a red pixel value) 401 to a first voltage for a liquid crystal cell. The first gamma calibration is determined at a first wavelength. More specifically, the first gamma calibration is determined to provide a required range of light modulation levels, such as 0 to 2tt in a phase modulation scheme, for light of a first wavelength such as 630 + / - 10 nm (red). The first grey level may be an 8-bit number in the range 0 to 255. The voltages may be in the range 0 to 5V. Likewise, Figure 4B shows a second gamma calibration which may be used to convert a second pixel grey level (e.g. a green pixel value) 402 to a second voltage for a liquid crystal cell. The second gamma calibration is determined at a second wavelength. More specifically, the second gamma calibration is determined to provide a required range of light modulation levels, such as 0 to 2tt in a phase modulation scheme, for light of a second wavelength such as 540 + / -10 nm (green). The grey level may be an 8-bit number in the range 0 to 255. The voltages may be in the range 0 to 5V. Likewise, Figure 4C shows a third gamma calibration which may be used to convert a third pixel grey level (e.g. a blue pixel value) 403 to a third voltage for a liquid crystal cell. The third gamma calibration is determined at a third wavelength. More specifically, the third gamma calibration is determined to provide a required range of light modulation levels, such as 0 to 2tt in a phase modulation scheme, for light of a third wavelength such as 450 + / - 10 nm (blue). The grey level may be an 8-bit number in the range 0 to 255. The voltages may be in the range 0 to 5V. The first, second and third gamma calibrations are different. For example, the voltage required to provide a light modulation level of e.g. x (such as rt in a phase modulation scheme) may be different for each of the first, second and third wavelengths. Accordingly, a step of selecting one of the first, second and third gamma calibrations may be required before the cell voltage is applied or the appropriate light source is activated. For the avoidance of doubt, three gamma calibrations are required. In some examples, this requires at least three memory buffers. Figure 5 illustrates a first embodiment of the present disclosure. In this embodiment, the first grey level 401, second grey level 402 and third grey level 403 are converted to a single grey level which is referred to herein as an "output pixel value" 500. In this embodiment, the first grey level 401 is assigned to a first range of the allowable output pixel values. The allowable pixel values may be 0 to 255. The first range of output pixel values may be 0 to 63. The first grey level may be an 8-bit number. The first range of output pixel values may be definable by, or may necessitate, a 6-bit number. The second grey level 402 is assigned to a second range of the allowable output pixel values. The second range of output pixel values may be 64 to 191. The second range of output pixel values may be definable by, or may necessitate, a 7-bit number. The third grey level 403 is assigned to a third range of the allowable output pixel values. The third range of output pixel values may be 192 to 255. The third range of output pixel values may be definable by a 6-bit number. The size of the first, second and thirds ranges may be equal or non-equal. That is, the number of grey levels of each range may be the same or different. In some embodiments, a grey level resolution or a number of grey levels of at least one of the colours is reduced by the process. In some embodiments, the number of grey levels used for each colour is reduced by a factor of approximately three. The output pixel value 500 is converted into a voltage using a common gamma calibration for all three colours as shown in Figure 6. The common gamma calibration may comprise at least one discontinuity associated with a transition between the adjacent sub-ranges. The reader will appreciate that the colour display method of this disclosure is therefore dynamically reconfigurable. For example, in one time slot an example pixel may be used for red and in a second time slot, the same example pixel may be used for e.g. green. The person skilled in the art will appreciate how the illumination scheme will need to be coordinated with the colour pattern of the display pixels. Any number of different schemes may be used for coordinating the display and light sources and the present disclosure is not limited to any particular approach.

Claims

1. A method of colour display, the method comprising:receiving a first data stream comprising a plurality of first colour pixel values and a second5 data stream comprising a plurality of second colour pixel values; andspatially multiplexing the first data stream and second data stream to form an output data stream comprising a plurality of output pixel values each representative of a first colour pixel value or a second colour pixel value, wherein the step of multiplexing comprises representing first colour pixel values using a first range of output pixel values and representing second colour pixel values10 using a second range of output pixel values, wherein the first range of output pixel values and second range of output pixel values are non-overlapping such that the first colour pixel values and the second colour pixel values are uniquely identified in the plurality of output pixel values.

2. A method of colour display as claimed in any preceding claim wherein a number of grey15 levels of the first range of output pixel values is different to a number of grey levels of the second range of output pixel values.

3. A method of colour display as claimed in any preceding claim wherein the first range of output pixel values is defined by an n-bit number and the second range of output pixel values is20 defined by an m-bit number, wherein n is not equal to m.

4. A method of colour display as claimed in any preceding claim wherein the first colour pixel values and / or second colour pixel values are defined by an x-bit number, wherein x is not equal to m and / or n.

255. A method of colour display as claimed in any preceding claim wherein the method comprising converting each output pixel value to a pixel voltage using a common gamma calibration or function.30 6. A method of colour display as claimed in any preceding claim wherein the method furthercomprises illuminating each display pixel of a display device in accordance with the corresponding output pixel value.

7. A method of colour display as claimed in any preceding claim wherein the method comprises illuminating a first display pixel having an output pixel value within the first range of allowable output pixel values with first colour light and illuminating a second display pixel having an output pixel value of allowable output pixel values within the second range with second colour light.

58. A method of colour display as claimed in any preceding claim wherein the method further comprises receiving a third data stream comprising a plurality of third colour pixel values; and spatially multiplexing the third data stream with the first data stream and second data stream to form the output data stream, wherein third colour pixel values are represented using a third range 10 of output pixel values.

9. A method of colour display as claimed in claim 8, wherein the third range of output pixel values does not overlap the first range of output pixel values or the second range of output pixel values.1510. A display system comprising a display driver arranged to: receive a first data stream comprising a plurality of first colour pixel values and a second data stream comprising a plurality of second colour pixel values; and spatially multiplex the first data stream and second data stream to form an output data stream comprising a plurality of output pixel values each representative of a20 first colour pixel value or a second colour pixel value, wherein spatially multiplexing comprises representing first colour pixel values using a first range of output pixel values and representing second colour pixel values using a second range of output pixel values, wherein the first range ofoutput pixel values and second range of output pixel values are non-overlapping such that the first colour pixel values and the second colour pixel values are uniquely identified in the plurality of25 output pixel values.

11. A display system as claimed in claim 10 wherein the display driver is further arranged to convert each output pixel value to a pixel voltage using a common gamma calibration.30 12. A display system as claimed in claim 11 further comprising a display device, wherein thesystem is arranged to illuminate each display pixel of the display device in accordance with the corresponding output pixel value.

13. A display system as claimed in claim 12 further comprising a first colour light source and a35 second colour light source, wherein the system is arranged to illuminate a first display pixel having an output pixel value within the first range of allowable output pixel values with first colour lightfrom the first colour light source and illuminating a second display pixel having an output pixel value of allowable output pixel values within the second range with second colour light from the second colour light source.13 1224

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