Colour selectable columns

Abstract

A pixel driver arranged to drive a display device (e.g. phase modulator, LCOS SLM) to display a first and second colour pixel patterns on an array of pixels at the same time. The driver comprises n da

Description

FIELD The present disclosure relates to a display device, pixel driver and a pixel data stream. The present disclosure also relates to a method of receiving pixel values of a plurality of different colour pixel data sets in a common data stream. The present disclosure also relates to a method of spatially separated colour 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 computer-generated 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 micromirrors, 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 displays, "HUD", and head-mounted displays, "HMD", including near-eye devices, for example. SUMMARY Aspects of the present disclosure are defined in the appended independent claims. A first aspect of the present disclosure is a pixel driver arranged to drive a display device to display a first colour pixel pattern and second colour pixel pattern on an array of pixels thereof at the same time. The pixel driver comprises: n data channels and pixel voltage selector. The n data channels are respectively arranged to receive n consecutive pixel values of a row of pixel values of pixel data at substantially the same time. The pixel voltage selector connects a first data channels of the n data channels to a first colour gamma buffer and thereby retrieve a first analogue voltage in accordance with a first colour calibration of the pixels of the display device. The pixel voltage selector also connects a second data channel of the n data channels to a second colour gamma buffer and thereby retrieve a second analogue voltage in accordance with a second colour calibration of the pixels of the display device. Optionally, every nth pixel value of the pixel data corresponds to the same colour but different channels correspond to different gamma buffers or colours. Alternatively or additionally, it may be said that every nth pixel value of the pixel data use the same colour calibration. The association between data channels and gamma buffers is dynamic or programmable. The pixel voltage selector may further connect a third data channel of the n data channels to a third colour gamma buffer in order to retrieve a third analogue voltage in accordance with a third colour calibration. The first colour may be red, the second colour may be blue and the third colour may be green. The first, second and third data channels may be immediately consecutive data channels corresponding to immediately consecutive pixel values of the pixel data. The first, second and third data channels may not be immediately consecutive data channels and the pixel voltage selector may connect the n data channels to the different colour gamma buffers in the following repeating sequence: first colour, second colour, third colour, first colour, second colour, third colour, first colour, second colour, third colour. The first, second and channels data lines may not be immediately consecutive data channels and the pixel voltage selector may connect the n data channels to the different colour gamma buffers in the following repeating sequence: first colour, first colour, first colour, second colour, second colour, second colour, third colour, third colour, third colour. The first, second and third data channels may not be immediately consecutive data channels and the pixel voltage selector may connect the n data channels to the different colour gamma buffers in the following repeating sequence: third colour, first colour, first colour, second colour, first colour, third colour, second colour, second colour, third colour. The first, second and third data channels may not be immediately consecutive data channels and the pixel voltage selector may connect the n data channels to the different colour gamma buffers in the following repeating sequence: third colour, third colour, third colour, third colour, third colour, third colour, third colour, third colour, third colour. The pixel driver may further comprise a colour selector arranged to dynamically determine the single colour gamma buffer connected to each of the n data channels. The colour selector may comprise I2C or a register that assigns a single colour gamma buffer to each data channel. The colour selector may assign a single colour gamma buffer to each data channel as pixel values are received on the n data channels. The value, n, may be in the range 6 to 20 such as 8 to 12. Each pixel value of the pixel data may comprise at least 6-bits. Each pixel value may be a 6-bit or 8-bit binary number. 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; Figure 4 shows a possible assignment to columns in accordance with embodiments; Figure 5 shows how pixels in data could map to pixels; Figure 6 shows an example of demultiplexing gamma based on pixel value at the array; and Figure 7 shows colour selection per column 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 codependent 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 lightmodulating 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, Ia(x, y) and Ib(x, y), in the planes A and B respectively, are known and Ia(x, y) and Ib(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, U-Mx, y) and U-Mx, 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 Ia(x, y) and Ib(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: W] = ^'{exp(zy„[M,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; t] 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 pre-calculated, 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 computer-generated 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 phasemodulating 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 selection per column Colour selection in SLM typically means to change the assignments of each incoming grey level to the values on the pixel. These assignments are usually tuned for specific wavelengths and materials. Typically, in the analogue LCOS it would be 3 separate gamma buffers, one for each of RGB. These are typically global assignments (i.e. every pixel in the array at grey-level GL24 would get the same voltage from the currently selected gamma buffer, for example green). Due to the global nature of these assignments, it is harder to sub-divide the array for different colours. The data is written in line by line to the array. The values (e.g. voltage) at each pixel on the currently writing line are latched in from the column driver on edge of the array. These column drivers are connected to the gamma buffers (via a MUX) and take their values from there. The gamma buffer selection (DEMUX) is typically done at the start of the frame through a colour selection message in the high speed video stream, or sometimes the message is sent at the start of each line. See Figures 5 and 6. In overview, the present disclosure comprises, instead of using a message in the video signal to select which gamma to demux, the gamma is pre-selected for each column during device setup (e.g. via I2C messages), and is fixed until reprogrammed. Some different embodiments are shown in Figure 4. Specifically, Figure 4 shows some possible assignments to columns by way of example only of the present disclosure. In embodiment A, each column corresponds to a different colour. In embodiment B, groups of colours are shown. Embodiment C comprises an arbitrary arrangement of colours. Embodiment D comprises all a single colour. Figure 5 shows an example of how pixels in data could map to pixels. The example shown comprises a 10 lane MIPI. As shown in Figure 6, one approach is to demix the gamma based on pixel value at the array. Muxed gamma is selected by a colour message at the start of the frame from one of three gamma buffers, R, G or B. In more detail, Figure 5 shows how a row of pixel values may be received ten pixel values at a time. A first group of ten pixel values - Pixel(OO) to Pixel(09) - are received, in parallel, at a first time. A second group of ten pixel values - Pixel(lO) to Pixel(19) - are received, in parallel, at a second time immediately following the first time, and so on. Accordingly, a first row of pixel values of an array of pixel values (e.g. representing an image or hologram) is received in groups of ten: pixels 1-10 then pixels 11-20 etc. The array of pixel values may comprise rows of several thousand pixel values. Each pixel value of the batch of 10 pixel values is assigned a pixel driver voltage in accordance with a gamma calibration stored in a gamma buffer. The person skilled in the art will be familiar with the concept of a gamma buffer and how a pixellated liquid crystal device may be calibrated (e.g. for each wavelength such as red, green and blue) in order to convert a pixel data value (e.g. 6-bit binary number representing a grey level) into a cell voltage. In examples, a pixel driver is assigned a single colour gamma buffer at the start of a video frame to indicate which single colour gamma buffer should be consulted for the entire frame. The present disclosure deviates from this practice and allows a plurality of different colour pixel data sets to be delivered together and, optionally, assigned to different gamma buffers. This allows a plurality of different colour pixel patterns to be displayed at the same time on the array of pixels of the display device. This is approach is synergistic with holography wherein an image is reconstructed by diffraction so each "image point" does not need a red, green and blue value. In accordance with the present disclosure, the different colour hologram pixels can be distributed across the array pixels in accordance with any regular or irregular scheme. The different colour pixels can be jumbled up or arranged in a repeating pattern. These approaches are possible with holography and are not generally suitable for conventional display. Figure 7 shows colour selection per column following Embodiment C of Figure 4. In some embodiments, COLOUR SELECT is an I2C or similar register that assigns values to the columns of array that allow the gamma buffer to be selected quickly as video data is inserted into the row. In some embodiments, all gamma buffers are available simultaneously and are selected on-the-fly based on the assignment to each column. Some embodiments comprise registers that represent the colour selection of each column of pixels, for example a 2bit value representing which gamma to demux. The assignment to that would be done during device setup (e.g. over I2C), and could be represented by a register. As image data is assigned to the pixel, before the gamma can be demuxed the colour select value is checked which directs the pixel value to the correct gamma demux. This allows access to the relevant gamma buffer during the write time of the pixel in the row. See Figure 7. Advantageously, the present disclosure provides programmable regions of colour, each with unique gammas at full bit depth (1:1 with the incoming video bit depth), tuned for that colour. As shown in Figure 4, the present disclosure further provides arbitrary and reassignable regions of colour. The present disclosure also allows column-wise colour assignments without adding messages into the video stream. Additional features In some embodiments, when the improved spatial light modulator is comprised within an optical arrangement that includes a light source, the light source is a laser such as a laser diode. The methods and processes described herein may be embodied on a computer-readable medium. The term "computer-readable medium" includes a medium arranged to store data temporarily or permanently such as random-access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" shall also be taken to include any medium, or combination of multiple media, that is capable of storing instructions for execution by a machine such that the instructions, when executed by one or more processors, cause the machine to perform any one or more of the methodologies described herein, in whole or in part. The term "computer-readable medium" also encompasses cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example form of a solidstate memory chip, an optical disc, a magnetic disc, or any suitable combination thereof. In some example embodiments, the instructions for execution may be communicated by a carrier medium. Examples of such a carrier medium include a transient medium (e.g., a propagating signal that communicates instructions). It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. The present disclosure covers all modifications and variations within the scope of the appended claims and their equivalents.

Claims

1. A pixel driver arranged to drive a display device to display a first colour pixel pattern and second colour pixel pattern on an array of pixels thereof, wherein the pixel driver comprises:n data channels arranged to receive n consecutive pixel values of a row of pixel values of pixel data; anda pixel voltage selector that connects a first data channel of the n data channels to a first colour gamma buffer in order to retrieve a first voltage in accordance with a first colour calibration and a second data channel of the n data channels to a second colour gamma buffer to retrieve a second voltage in accordance with a second colour calibration.

2. A pixel driver as claimed in any preceding claim wherein the pixel voltage selector further connects a third data channel of the n channels to a third colour gamma buffer in order to retrieve a third voltage in accordance with a third colour calibration.

3. A pixel driver as claimed in claim 2 wherein the first colour is red, the second colour is blue and the third colour is green.

4. A pixel driver as claimed in claim 2 or 3 wherein the first, second and third data channels are immediately consecutive channels corresponding to immediately consecutive pixel values of the pixel data.

5. A pixel driver as claimed in claim 2 or 3 wherein the first, second and third data channels are not immediately consecutive data channels and the pixel voltage selector connects the n data lines to the different colour gamma buffers in the following repeating sequence: first colour, second colour, third colour, first colour, second colour, third colour, first colour, second colour, third colour.

6. A pixel driver as claimed in claim 2 or 3 wherein the first, second and third data channels are not immediately consecutive data channels and the pixel voltage selector connects the n data channels to the different colour gamma buffers in the followingrepeating sequence: first colour, first colour, first colour, second colour, second colour, second colour, third colour, third colour, third colour.

7. A pixel driver as claimed in claim 2 or 3 wherein the first, second and third data channels are not immediately consecutive data channels and the pixel voltage selector connects the n data channels to the different colour gamma buffers in the following repeating sequence: third colour, first colour, first colour, second colour, first colour, third colour, second colour, second colour, third colour.

8. A pixel driver as claimed in claim 2 or 3 wherein the first, second and third data channels are not immediately consecutive data channels and the pixel voltage selector connects the n data channels to the different colour gamma buffers in the following repeating sequence: third colour, third colour, third colour, third colour, third colour, third colour, third colour, third colour, third colour.

9. A pixel driver as claimed in any preceding claim further comprising a colour selector arranged to dynamically determine the single colour gamma buffer connected to each of the n data channels.

10. A pixel driver as claimed in claim 9 wherein the colour selector comprises I2C or a register that assigns a single colour gamma buffer to each data channel.

11. A pixel driver as claimed in claim 10 where the colour selector assigns a single colour gamma buffer to each data channel as pixel values are received on the n data channels.

12. A pixel driver as claimed in any preceding claim wherein n is in the range 6 to 20 such as 8 to 12.

13. A pixel driver as claimed in any preceding claim wherein each pixel value of the pixel data is a 6-bit binary number.

14. A pixel driver as claimed in any preceding claim wherein the display device is a phase modulator.

15. A pixel driver as claimed in any preceding claim wherein the display device is a liquid crystal on silicon spatial light modulator.

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