GB2446852A

GB2446852A - Providing holographic data to mobile devices for display

Info

Publication number: GB2446852A
Application number: GB0702659A
Authority: GB
Inventors: Edward Buckley
Original assignee: Light Blue Optics Ltd
Current assignee: Light Blue Optics Ltd
Priority date: 2007-02-12
Filing date: 2007-02-12
Publication date: 2008-08-27
Anticipated expiration: 2027-02-12
Also published as: WO2008099211A3; GB2446852B; WO2008099211A2; GB0702659D0

Abstract

A two-dimensional image display is provided for a mobile device (fig. 1) by encoding the 2D image as a hologram, sending hologram data for the hologram to the mobile device and displaying the hologram data at the mobile device to display the two-dimensional image. The mobile displays the 2D image via a projection to allow a larger display than possible using the screen of the mobile only. Also disclosed is digitally watermarking hologram data by obtaining a set of pseudo-random values being deterministically specified by seed data. Further disclosed is distributing video to a plurality of users (fig. 8) comprising generating hologram data for the video and distributing the data, wherein different ones of the users receive different versions of the hologram data which when replayed reproduce substantially identical video. Also disclosed is generating encrypted digital hologram data by performing a holographic transform of image data and operating on the digital hologram data using a 2D key. Corresponding decryption is also disclosed.

Description

Data Communication in Processing Systems

FiELD OF THE INVENTION

This invention generally relates to systems, methods and computer program code for communicating and processing images, in paTti cul ar video using holographic techniques.

BACKGROUND TO THE INVENTION

Many small, portable consumer electronic devices incorporate a graphical image display, generally a LCD (Liquid Crystal Display) screen. These include digital cameras, mobile phones, personal digital assi stants/organisers, portable music devices such as the iPOD (trade mark), portable video devices, laptop computers and the like. In many cases it would be advantageous to be able to provide a larger and/or projected image but to date this has not been possible, primarily because of the size of the optical system needed for such a display.

We have previous described, for example in WO 2005/059660, a method for image projection and display using appropriately calculated computer generated holograms displayed upon dynamically addressable liquid crystal (LC) spatial light modulators (SLMs). However applications for mobile devices were not mentioned in W02005/059660 since there are difficulties in implementation in such devices, even with the substantially improved efficiency of this method, because of the computational load.

Broadly speaking, in technique of W02005/059660 an image is displayed by displaying a plurality of holograms each of which spatially overlaps in the replay field and each of which, when viewed individually, would appear relatively noisy because noise is added by phase quantisation by the holographic transform of the image data. However when viewed in rapid succession the replay field images average together in the eye of a viewer to give the impression of a reduced (low) noise image. TIie noise in successive temporal subframes may either be pseudo-random (substantially independent) or the noise in a subfrarne may be dependent on the noise in one or more earlier subframcs with the aim of at least partially cancelling this out, or a combination of both may be employed. More details of such OSPR-type procedures are described later.

Figure 1 shows an example a consumer electronic device 10 incorporating a holographic image projection module 12 to project a displayed image 14. Displayed image 14 comprises a plurality of holographically generated sub-images each of the same spatial extent as displayed image 1 4, and displayed rapidly in succession so as to give the appearance of the displayed image. Each holographic sub-frame is generated using an OSPR-type procedure.

Figure 2a shows an example optical system for the holographic projection module of Figure 1. Referring to Figure 2a, a laser diode 20 (for example, at 532nm), provides substantially collimated light 22 via a mirror 23 to a spatial light modulator (SLM) 24 such as a pixcilated liquid crystal modulator. (As illustrated, the SLM is a reflective SLM but a transrnissive SLM may also be employed). The SLM 24 phase modulates light 22 with a hologram and the phase modulated light is preferably provided to a demagnifying optical system 26. In the illustrated embodiment, optical system 26 comprises a pair of lenses (L3, L4) 28, 30 with respective focal lengths f3, f4, f4<f3, spaced apart at distance f3+f4. Optical system 26 increases the size of the projected holographic image (replay field R) by diverging the light forming the displayed image; it effectively reduces the pixel size of the modulator, thus increasing the diffraction angle. Lenses L1 and L2 form a beam-expansion pair which expands the beam from the light source so that it covers the whole surface of the modulator; depending on the relative size of the beam 22 and SLM 24 this may be omitted. A spatial filter may be included to filter out unwanted parts of the displayed image, for example a zero order undiffracted spot or a repeated first order (conjugate) image, which may appear as an upside down version of the displayed image, depending upon how the hologram for displaying the image is generated.

An example of a suitable binary phase SLM is the SXGA (1280x 1024) reflective binary ihase modulating ferroelcctric liquid crystal SLM made by CRL Opto (Forth Dimension Displays Limited, of Scotland, UK). A ferroelectric liquid crystal SLM is advantageous because of its fast switching time; binary phase devices are convenient but devices with three or more quantized phases may also be employed (use of more than binary phase enables the conjugate image to be sLippressed, see WO 2005/059660).

A singlc optical arrangement can be used for beam expansion prior to modulation, and for demagni Iical.ion of the modulated light. Thus the lens pair I l and L2 and the lens pair L3 and L4 may comprise at least part of a common optical system, used in reverse, in conjunction with a reflective SLM, for light incident on and reflected from the SLM.

Figure 2h illustrates such a lens sharing arrangement, in which a polariser is included to suppress interference between light travelling in different directions, that is into and out of the SLM. Figure 2c shows, schematically, a preferred practical configuration of such a system, in which the laser diode (LD) does not obscure a central portion of the replay field. En the arrangement o Figure 2c a polarising beam splitter 32 is used to direct the output, modulated light at 90 degrees on the image plane, and also to provide the function of the polariser in Figure 2b.

Figure 2d shows a further arrangement in which just L2/L3 is shared between the collimation and dernagnification stages. In this example a waveplate 34 is also employed to rotate the polarisation of the incident beam for the beamsplitter. In the optical arrangements of each of Figures 2a-2d an intermediate image is formed between lenses L3 and L4 of the demagnification optics, at which the replay field (which is reproduced there) may be spatially filtered. Thus the arrangement of Figure 2d includes an aperture 36 in the intermediate image plane of the demagnifying optics to block off the zero order (undi Ifracted light), the conjugate image, and higher diffraction orders.

A colour holographic projection system may be constructed by employing an optical system as described above to create three optical channels, red, blue and green superiniposed to generate a colour image. in practice this is difficult because the different colour images must he aligned on the screen and a better approach is to create a combined red, green and blue beam and provide this to a common SLM and demagnifying optics. In this case, however, the different colour images are of different sizes; techniques to address this arc described in our co-pending UK patent application no. GBO61 0784.1 filcd 2 June 2006, hereby incorporated by reference.

Referring again to Figure 2a, a digital signal processor 1 00 has an input 1 02 to receive image data from the consumer electronic device defining the image to be displayed.

The DSP 100 implements an OS PR-type procedure to generate phase hologram data for a plurality of holographic sub-frames which is provided from an output 1 04 of the DSP to the SLM 24, optionally via a driver integrated circuit if needed. The DSP 100 drives SLM 24 to project a plurality of phase hologram sub-frames which combine to give the impression of displayed image 14 in the replay field (RPF). The DSP 100 may comprise dedicated hardware and/or Flash or other read-only memory storing processor control code to implement the hologram generation procedure.

One difficulty with the display of images holographically is that the determination of a computer-generated hologram from an image is computationally very intensive. A technique for spatial sub-division to reduce the computational burden is described in US2004/0233487. OSPR-type techniques, which generate an image using a plurality of temporal sub-frames, substantially reduce the amount of computation required for a high quality holographic image display and the temporal averaging reduces the level of perceived noise. However the computational burden can still be significant.

We have previously described, in WO 2006/134398, hardware for implementing an OSPR-type procedure but, nonetheless, particularly in mobile devices, there is a continued desire for inter alia, lower power consumption. Despite this, holographic image display techniques potentially offer significant advantages, for example a display size which is not limited to the size of the mobile device and a focus-free displayed image (that is, the focus of a displayed image does not substantially depend on the distance from the mobile device to the display screen).

There is therefore a need for improved techniques.

SUMMARY OF TIlE INVENTION

According to a first aspect olthe invention there is therefore provided a method of providing a two-dimensional image display for a mobile devicc, the method comprising: encoding said two-dimensional image as a hologram; sending hologram data for said hologram to said mobile dcvicc; and displaying said hologram data at said mobile device to project said two-dimensional image display.

In embodiments the mobile device comprises a buffer and a spatial light modulator (SLM), the hologram data received at the mobile device being stored in the buffer and used to drive the SLM whilst the SLM is illuminated, for example using one or more lasers, in order to project a two-dimensional image (or video frame) encoded by the hologram data.

In this way embodiments of the method can be used to provide a mobile device with a two-dimensional image display with the advantages of a holographic image display such as an image larger than the physical size of the device which is in focus at substantially all distances from the device, without the burden of computing the hologram itself at the device.

The mobile device preferably comprises a wireless mobile device such as a mobile phone, organiser, portable computer, portable TV or video player or the like. Preferably the inlage data comprises video image data, the hologram data being streamed to the mobile device.

In embodiments of the method the size of the hologram displayed on the SLM is not varied with the image content. Thus in a typical embodiment, an image of pixel dimensions X and Y is transformed to a hologram of pixel dimensions U and V for display on a suitably sized SLM. However since the whole of the hologram display area on the SLM contributes energy to the image displayed by the mobile device, an image which is mostly black will have more energy in the bright parts of the image than an image which has only a small area of black. Preferably, therefore, the hologram data includes display control data for controlling the display of the hologram in order to control a perceived brightness of the displayed image to reduce variations in the brightness of the image due to differing image content, more particularly (as described later) due to differing image coverage". Thus preferably the display control data is used to reduce the brightness of an image with a lower coverage of the display area as compared to an image with a higher coverage. Thus in embodiments the display control data may comprise a value preferably approximated by a relatively low number of bits, for example less than 5, 4,3 or 2 bits, which is inversely dependent upon the coverage of a displayed image.

The display control data may be employed to control an illumination level of the SLM (for example by modulating the intensity of an illuminating laser) and/or a display time of the hologram data, for example by controlling an illumination time of the SLM (for example by modulating a laser illuminating SLM) or by controlling a time for which the hologram data is displayed on the SLM, or by controlling a separate shutter in the optical path from the illumination source to the image display.

According to an aspect of the invention a method of providing a two-dimensional image display for a mobile device as described above, when the hologram data includes display control data, need not he restricted to a two-dimensional image display, nor need it be restricted to a display for a mobile device.

Embodiments of a method as described above are not restricted to use with any particular method of encoding the two-dimensional image as a hologram. However counter-intuitively -since it potentially involves sending an increased quantity of hologram data -the image encoding encodes the two-dimensional image into a plurality of holograms each comprising a hologram for a subframe of the image, more particularly a temporal suhframe of the image. The hologram data displaying may then comprise displaying hologram data for each of the subfrarnes in order to project the image display. In embodiments the temporal averaging of noisy hologram data can facilitate the use of an error correction protocol with reduced capability of detecting/correcting errors. (The sending of the hologram data may employ any conventional data transmission scheme and such schemes, in general, incorporate some form of error dctcctionlcorrcction, often to a variable degree according to requirements of a particular application; examples of such schemes are well known to those skilled in the art.

In embodiments of the method the encoding of the image as a hologram may also include watermarking the hologram data generated by the encoding, for example by generating one or a set of holograms using a deterministic but pseudo-random noise sequence to generate hologram data which is characteristic of the sequence, more particularly of a seed determining the sequence.

In embodiments the hologram data may also be encrypted, in preferred embodiments using a (two-dimensional) phase key.

in another aspect the invention provides a method of displaying video using a mobile device the method comprising: encoding said video as a time-sequence of holograms; streaming data for said time-sequence of holograms to said mobile device; and displaying said streamed data on a spatial light modulator (SLM) at said mobile device to project a display of said video.

Again counter-intuitively in some preferred embodiments the time-sequence of holograms comprises multiple holograms for a single frame of the video, thus simplifying the computationally intensive process of encoding a video stream as a time-sequence of holograms at the sending end and also, potentially, increasing the overall robustness of the method to data transmission errors.

In a first related aspect the invention further provides a system for providing a two-dimensional image display for a mobile device, the system comprising: means for encoding said two-dimensional image as a hologram; means for sending hologram data for said hologram to said mobile device, and means for displaying said hologram data at said mobile device to project said two-dimensional image display.

In a further related aspect the invention also provides a system for displaying video using a mobile device, the system comprising: means for encoding said video as a time-sequence of holograms; means for streaming data for said time-sequence of holograms to said mobile device; and means for displaying said streamed data on a spatial light modulator (SLM) at said mobile device to project a display olsaid video.

in a still further aspect of the invention there is provided a carrier carrying processor control code to implement a method and/or system according to an aspect or embodiment of the invention as described above.

Other aspects of the invention provide code for an image server, and separately for a mobile device, according to aspects and embodiments of the invention described below.

Thus in a further aspect there is provided an image server for providing data for an image display to a mobile device, the server comprising: an input to receive image data for an image to display; an encoder coupled to said input to encode said image data as at least one hologram and to provide hologram data for said at least one hologram; a data transmission system coupled to an output of said encoder to send said hologram data to said mobile device.

In preferred embodiments the image comprises a two-dimensional image, for example a frame of video image data. In embodiments a single said image or frame is encoded as a plurality of holograms, preferably a plurality of temporal subframes of the image. In embodiments the encoder includes a system to generate display control data for controlling one or both of a display time of the at least one hologram and an illumination level of the at least one hologram to compensate for image content related changes in brightness of the image display, more particularly as described above.

In a still further related aspect the invention provides a mobile device for holographically displaying an image, the mobile device comprising: a data receiver to receive hologram data for at least one hologram encoding said image; a spatial light modulator (SLM) to display said hologram; a light source to illuminate said hologram displayed on said SLM to project said image for viewing; and a buffer coupled to said data receiver to store said received hologram data for display on said SLM.

Again the image may comprise a video frame, and may he encoded as a plurality of holograms. in embodiments the mobile device is configured to receive display control data and to control a display time and/or illumination level of the hologram, for example by modulating a laser illuminating the hologram.

Watermarking hologram data The invention also provides a method of digitally watermarking hologram data, the method comprising: inputting image data defining at least one image for conversion to a hologram; obtaining a set of pseudo-random values, said set of pseudo-random values being detenninistically specified by seed data; and generating hologram data for at least one hologram which, when displayed, gives the appearance of said image; wherein said generating of said hologram data uses said set olpseudo-random values; and wherein different said sets of pseudo-random values specified by different said seed data generate hologram data for different said holograms which, when displayed, give substantially the same said appearance of said image; whereby said hologram data is digitally watermarked such that two images with substantially the same appearance may be distinguished using said hologram data.

In embodiments the seed data may comprise any data which deterministically specifies a set of pseudo-random values, in particular to enable their regeneration for tracking/verification purposes. In preferred embodiments the seed data comprises data for initialising a pseudo-random number generator but, potentially, the seed data might also comprise information identifying a form of pseudo-random number generator, for example a set of taps to use on a shift register with feedback using a predetermined initial state.

In embodiments the set of pseudo-random values comprises a set of row and column pixel values in either a spatial domain or a spatial frequency domain of the image or hologram respectively. Thus, for example, in an OSPR-type hologram generation procedure spatial domain pseudo- random values are preferably employed whereas in, say, a DBS (Direct Binary Search) hologram generation procedure the pseudo-random data may be used to initialise the hologram to a known starting condition from which the MMSE (Minimum Mean Square Error) search is then performed, again in a deterministic fashion. As the skilled person will be aware, other hologram generation procedLires such as the Gerchberg-Saxton procedure and the simulated annealing procedure also rely on a pseudo-random noise input which may be chosen in a deterministic maimer to watermark the hologram data thus generated.

In embodiments which employ an OSPR-type procedure in which a plurality of (temporal) subframcs is generated for each image frame preferably a different set of pseudo-random values is generated for each subframe, although this iiccd not require re-initialising the pseudo-random sequence. Thus some embodiments may employ the same seed data for a plurality of subfranics and/or for a plurality of image frames, (whether or not an OSPR-type procedure is being employed) for example of a video stream. Other embodiments may employ a new seed for each successive image and/or each successive image suhframe. In general a choice as to the frequency of re-initialising the seed data can be made dependent upon available storage, desired degree of security and similar considerations.

in embodiments of the method the set of pseudo-random values or, more preferably, simply the seed data is stored so that it may be used to regenerate the hologram data for comparison with the hologram data of a particular user or group of users to determine whether or not the hologram data matches. In this way a user or a group of users may be verified or the source of hologram data maybe identified by comparing hologram data fi-om different users with regenerated hologram data until a match is found.

The watermarked hologram data can be distributed to a plurality of users by generating a number of different hologram data streams one for each user or group of users. The hologram data in the different streams may be unique but nonetheless regenerates substantially the same displayed image or image sequence (video).

An image distribution system of the type described above may also include a system for detecting an unlawful user, for example by providing a computer program (which may be a plug-in or add-on to an existing program) which can connect to an image server to verify authenticity and/or permissions to provide DRM (Digital Rights Management) function.

Thus in a related aspect the invention provides a method of distributing video to a plurality of users, the method comprising generating hologram data for said video and distributing said hologram data, said hologram data being configured to reproduce said video when displayed on an SLM, and wherein different ones of said users receive different versions of said hologram data encoding different holograms which, when replayed, reproduce substantially the same perceived video.

In a still further aspect the invention provides a computer system for digitally watermarking hologram data, the system comprising: an input to receive image data defining at least one image for conversion to a hologram; a pseudo-random number generator to provide a set of pseudo-random values, said set of pseudo-random values being deterministically specified by seed data; an image coder to generate hologram data for at least one hologram which, when displayed, gives the appearance of said image; and a data store to store one or both of said seed data and said set of pseudo-random values; wherein different said sets of pseudo-random values specified by different said seed data generate hologram data for different said holograms which, when displayed, give substantially the same said appearance of said image; whereby said hologram data is digitally watermarked such that two images with substantially the same appearance may be distinguished using said hologram data.

The invention further provides processor control code to implement the above-described systems and methods, in particular on a data carrier such as a disk, CD-or DVD-ROM, programmed memory such as read- only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. Code (and/or data) to implement embodiments of the invention may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code, code for setting up or controlling an ASiC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array), or code for a hardware description language such as Verilog (Trade Mark) or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate such code and/or data may be distributed between a plurality of coupled components in communication with one another.

Encryption In an aspect of the invention there is provided a method of generating encrypted digital hologram data, the method comprising: inputting image data defining at least one image for conversion to a hologram; performing a holographic transform on said image data to generate digital hologram data, said digital hologram data comprising two-dimensional data defining pixel values for two-dimensional display on a spatial light modulator to replay an image approximating said at least one image; and operating on said digital hologram data using a two-dimensional key comprising a set of values for modifying said l)ixel values to generate said encrypted digital hologram data.

In embodiments of the method the digital hologram data comprises data for a plurality of holographic subframes which, in combination, replay an image approximating the desired image. Operating with the two-dimensional key may then comprise either operating on each of these holograms (subframes) separately or operating the set of holograms (sub frames) in combination. For example each hologram may be stored in a buffer as one or more bit frames and corresponding locations in the bit claims may then be taken to represent different bits of a binary value comprising at least one bit associated with corresponding pixel positions in each holographic subframe. The key may then operate on this binary value rather than on each subfrarne separately. The results of this operation can be different to those of separate operations on each subframe as can be seen, for example, by considering the case of adding a value which could result in a carry from subfranie to the next, thus mixing the data between sub frames.

The two-dimensional key preferably has a value for each pixel of the two-dimensional digital hologram data. This may be an arbitrary value which may be combined with the hologram data in any of a variety of ways including, but not limited to, addition, subtraction, multiplication, division, convolution and the like. However in some particularly preferred embodiments this value comprises a phase value which is used to modify a phase value of a corresponding pixel in the hologram data (or the combined data for multiple holographic subframes). For example in some preferred embodiments the key comprises a phase key defining a phase value for each pixel of the hologram and this phase value is added (or subtracted) to encrypt the digital hologram data. This is useful because it is particularly computationally simple and convenient to implement.

The two-dimensional key may comprise, for example, a set of pseudo-random val Lies although, in embodiments, rows and/or columns of the two-dimensional key may be repeated or replicated within the key and/or the two-dimensional key may have data values repeated in some other pattern. Thus in some embodiments an entire two- dimensional key niay be constructed in a deterministic fashion from a single pseudo-random value since simply adding the same pseudo-random value to each pixel will effectively scramble the digital hologram data so that the desired image is not recreated.

The number of pseudo-random values used to generate a key may be selected according to a trade off between a desired degree of security and an amount of data to be transferred by a different channel to the encrypted hologram data. In embodiments the encrypted digital hologram data is sent over a first channel and data for the two-dimensional key is sent over a second, different channel (which may be the same physical channel as a first channel, for example differently encrypted or used at a different time). For example the encrypted digital hologram data may be sent over a point-to-point multicast or broadcast data distribution system or a network such as the internet, a wide or local or personal al-ca network or a communications network such as digital mobile phone network or cable TV network. Likewise data for the two-dimensional key may be sentover a different chaimel of a similar network or in some other way, for example by post. In embodiments the data for the key may be sent to a person and the encrypted digital hologram data to a machine; in this case it is preferable that the key is relatively short and, therefore, a single pseudo-random number be employed to define data for a complete key, for example functioning as a seed for a pseudo-random number generator or in some other way as suggested above.

In a related aspect the invention provides a computer system to generate encrypted digital hologram data, the system comprising: an input to receive image data defining at least one image for conversion to a hologram; a codcr to perform performing a holographic transform on said image data to generate digital hologi'arn data, said digital hologram data comprising two-dimensional data defining pixel values for two-dimensional display on a spatial light modulator to replay an image approximating said at least one image; and a hologram data encrypter to operate on said digital hologram data using a two-dimensional key comprising a set of values for modifying said pixel values to generate said encrypted digital hologram data.

The invention further provides a method of decrypting encrypted digital hologram data, the method comprising: inputting encrypted digital hologram data, said encrypted digital hologram data comprising two-dimensional encrypted data defining encrypted pixel values of digital hologram data, said digital hologram data comprising two-dimensional data defining pixel values for two-dimensional display on a spatial light modulator to replay an image; inputting data for a two-dimensional key, said two-dimensional key comprising a set of values for modifying said encrypted pixel values; and operating on said encrypted digital hologram data using said two-dimensional key to generate decrypted said digital hologram data.

Similar comments to those made above with respect to encryption apply in a complementary fashion to decryption and, in particular, simi] ar types of key and encrypted digital hologram datalkey data distribution systems may be employed.

In a further complementary aspect the invention provides a system to decrypt encrypted digital hologram data, the system comprising: an input to receive encrypted digital hologram data, said encrypted digital hologram data comprising two-dimensional encrypted data defining encrypted pixel values of digital hologram data, said digital hologram data comprising two-dimensional data defining pixel values for two-dimensional display on a spatial light modulator to replay an image; an input to receive data for a two-dimensional key, said two-dimensional key comprising a set of values for modifying said encrypted pixel values; and a hologram data decrypter to operate on said encrypted digital hologram data using said two-dimensional key to generate decrypted said digital hologram data.

In a further aspect the invention provides a data signal carrying encrypted digital hologram data, encrypted using a method of the type described above optionally in association with data for a two-dimensional key (on a different communications channel).

The invention further provides processor control code to implement the above-described systems and methods, in particular on a data carrier such as a disk, CD-or DVD-ROM, programmed memory Such as read- only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. Code (and/or data) to implement embodiments of the invention may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code, code for setting up or controlling an ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array), or code for a hardware description language such as Verilog (Trade Marlc) or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate such code and/or data may he distributed between a plurality of coupled components in communication with one another.

Features of the above-described different aspects and embodiments of the invention may be combined in any permutation.

The hologram data in embodiments of the aspects of the invention described above may comprise data for binary or multilevel or continuous phase modulation by an SLM or for binary or multilevel or continuous amplitude modulation by an SLM or for a combination of quantised modulation of one type (phase) and multilevel or substantially continuous modulation of another type (amplitude) in particular as described in our co-pending UK patent application number 0625364.5 filed on 20 December 2006, hereby incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which: Figure 1 shows an example of a consumer electronic device incorporating a holographic projection module; Figures 2a to 2d show, respectively, an example of an optical system for the holographic projection module of figure 1, and lens sharing arrangements used with a reflective SLM; FigLire 3 shows a block diagram of a hologram data calculation system; Figure 4 shows a block diagram of a hologram data calculation system, and operations performed within the hardware block of the hologram data calculation system; Figure 5 shows the energy spectra of a sample image before and after multiplication by a random phase matrix; Figure 6 shows an example of a hologram data calculation system with parallel quantisers for the simultaneous generation of two sub-frames from real and imaginary components of complex holographic sub-frame data respectively; Figures 7a and 7h show, respectively, first and second block diagrams of embodiments of a holographic video server according to an aspect of the invention; Figure 8 shows example content distribution systems for the servers of Figure 7; Figure 9 shows an embodiment of a mobile device for use with a holographic image distribution system according to an embodiment of the invention; Figures lOa to lOc show, respectively, a flow diagram of a holographic image display procedure for the mobile device of Figure 9, a watermarking verification procedure for implementation by the mobile device of Figure 9, and a watermarking verification procedure for implementation on a holographic server of the type shown in Figure 7; Figures 11 a to 11 c show, respectively, a block diagram of a holographic video server according to an embodiment of the invention incorporating encryption, a flow diagram of a procedure for displaying encrypted holographic video on a mobile device, and an illustration of an encryption procedure performed by the video server of Figure ha.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It is convenient to illustrate operation of embodiments of the invention with reference to an OSPR-type hologram generation procedure. However embodiments of the invention are not restricted to such a hologram generation procedure and may be employed with other types of hologram generation procedure including, but not limited to: a Gerchberg-Saxton procedure (R. W. Gerchberg and W. 0. Saxton, "A practical algorithm for the determination of phase from image and diffraction plane pictures" Optik 35, 237-246 (1972)) or a variant thereof, Direct Binary Search (M. A. Seldowitz, J. P. Allebach and D. W. Sweeney, "Synthesis of digital holograms by direct binary search" AppI. Opt. 26, 2788-2798 (1987)), siimilated annealing (see, for example, M. P. Dames, R. J. Dowling, P. McKee, and D. Wood, "Efficient optical elements to generate intensity weighted spot arrays: design and fabrication," AppI. Opt. 30, 2685-2691 (1991)), or a POCS (Projection Onto Constrained Sets) procedure (see, for example, C. -H. Wu, C. -L. Chen, and M. A. Fiddy, "Iterative procedure for improved computer-generated-hologram reconstruction," Appi. Opt. 32, 5135-(1993)).

We will describe example OSPR-type procedures to aid in understanding the operation of embodiments of the invention. Further details can be found in GB0518912.1 (PCTIGB2006/050291) and GB0601481.5 (PCT/GB2007/050037), both hereby incorporated by reference.

OSPR

Broadly speaking in our preferred method the SLM is modulated with holographic data approximating a hologram of the image to be displayed. However this holographic data is chosen in a special way, the displayed image being made up of a plurality of temporal sub-frames, each generated by modulating the SLM with a respective sub-frame hologram. These sub-fl-ames are displayed successively and sufficiently fast that in thc eye of a (human) observer the sub-frames (each of which have the spatial extent of the displayed image) al-c integrated together to create the desired image for display.

Each of the sub-frame holograms may itself be relatively noisy, for example as a result of quantising the holographic data into two (binary) or more phases, but temporal averaging amongst the sub-frames reduces the perceived level of noise. Embodiments of such a system can provide visually high quality displays even though each sub-frame, were it to be viewed separately, would appear relatively noisy.

The procedure is a method of generating, for each still or video frame I = J,, sets of N binary-phase holograms hW... h. In embodiments such sets of holograms form replay fields that exhibit mutually independent additive noise. An example is shown below: Let = iexp where is uniformly distributed between 0 and 2,t for I < N/2 and 1 < x, v 1 2. Let = [G] where F Lepresents the two-dimensional inverse Fourier transform operator, for I N/2 3. Let = [g? } for I n N/2 4. Let!nW N/2) {,::) } for 1 < ,i < N/2 ) f -i < -f (n) 5, Let h,, = where Q = median I. I if;n. > Q(H1 and I <ij N Step 1 forms N targets (i equal to the amplitude of the supplied intensity target J, but with independent identically-distributed (Li.t.), unifomily-random phase. Step 2 computes the N con-esponding full complex Fourier transform holograms g. Steps 3 and 4 compute the real part and imaginary part of the holograms, respectively.

Binarisation of each of the real and imaginary parts of the holograms is then performed in step 5: thresholding around the median of m ensures equal numbers of -1 and I points are present in the holograms, achieving DC balance (by definition) and also minimal reconstruction error. The median value of in may be assumed to be zero with minimal effect on perceived image quality.

Figure 3a (from our W02006/l 34398, incorporated by reference) shows a block diagram of a hologram data calculation system configured to implement this procedure.

l'he input to the system is preferably image data from a source such as a computer, although other sources arc equally applicable. The input data is temporarily stored in one or more input buffer, with control signals for this process being supplied from one or more controller units within the system. The input (and output) buffers preferably comprise dual-port memory such that data may be written into the buffer and read out from the buffer simultaneously. The control signals comprise timing, initialisation and flow-control information and preferably ensure that one or more holographic sub-frames are produced and output, for example to an SLM, per video frame period.

The output from the input comprises an image frame, labelled 1, and this becomes the input to a hardware block (although in other embodiments some or all of the processing may be performed in software). The hardware block performs a series of operations on each of the aforementioned image frames, I, and for each one produces one or more holographic sub-frames, h, which are sent to one or more output buffer. The sub-frames are supplied from the output buffer to a display device, such as a SLM, optionally via a driver chip.

Figure 3b shows details of the hardware block of Figure 3a; this comprises a set of elements designed to generate one or more holographic sub-frames for each image frame that is supplied to the block. Preferably one image frame, l,, is supplied one or more times per video frame period as an input. Each image frame, I,,, is then used to produce one or more holographic sub-frames by means of a set of operations comprising one or more of: a phase modulation stage, a space-frequency transformation stage and a quantisation stage. In embodiments, a set of N sub-frames, where N is greater than or equal to one, is generated per frame period by means of using either one sequential set of the aforementioned operations, or a several sets of such operations acting in parallel on different sub-frames, or a mixture of these two approaches.

The purpose of the phase-modulation block is to redistribute the energy of the input frame in the spatial-frequency domain, such that improvements in final image quality are obtained after performing later operations. Figure 5 shows au example of how the energy of a sample image is distributed before and after a phase-modulation stage in which a pseudo-random phase distribution is used. It can be seen that modulating an image by such a phase distribution has the effect of redistributing the energy more evenly throughout the spatial-frequency domain. The skilled person will appreciate that there are many ways in which pseudo-random binary-phase modulation data may be generated (for example, a shift register with feedback).

The quantisation block takes complex hologram data, which is produced as the output of the preceding space-frequency transform block, and maps it to a restricted set of values, which correspond to actual modulation levels that can be achieved on a target SLM (the different quantised phase retardation levels may need not have a regular distribution). The number of quantisation levels may be set at two, for example for an SLM producing phase retardations of 0 or 7C at each pixel.

in some preferred embodiments the quantiser is configured to separately quantise real and imaginary components of the holographic sub-frame data to generate a pair of holographic sub-frames, each with two (or more) phase-retardation levels, for the output buffer. Figure 6 shows an example o:f such a system. It can be shown that for discretely pixellated fields, the real and imaginary components of the complex holographic sub-frame data are uncorrelated, which is why it is valid to treat the real and imaginary components independently and produce two uncorrelated holographic sub-frames.

In the OSPR approach we have described above subframe holograms are generated independently and thus exhibit independent noise. In control terms, this is an opeii-loop system. However one might expect that better results could be obtained if, instead, the generation process for each subframe took into account the noise generated by the previous subfrarncs in order to cancel it out, effectively "feeding back" the perceived image formed after, say, n OSPR frames to stage n+] of the algorithm. In control terms, this is a closed-loop system.

One example of this approach comprises an adaptive OSPR algorithm which uses feedback as follows: each stage ii of the algorithm calculates the noise resulting from the previously-generated holograms H1 to I-lb, and factors this noise into the generation of the hologram H,, to cancel it out. As a result, it can he shown that noise variance falls as 1/N2. An example procedure takes as input a target image T, and a parameter N specifying the desired number of hologram subframes to produce, and outputs a set of N holograms H1 to HN which, when displayed sequentially at an appropriate rate, form as a far-field image a visual representation of T which is perceived as high quality. More details can be found in GBO5 18912.1 and GB0601481.5 (ibid), hereby incorporated by reference in their entirety.

Image data transmission Referring to Figure 7a, this shows a first example of a video server 700 to provide video image data for display on a mobile device, the video image data being encoded as a plurality of holograms. The server accepts a video data stream 702 as an input to a hologram coder 704, for example of the type shown in Figure 3 implemented in hardware and/or implemented in software using a controller 704a comprising a processor, working memory and program memory storing a program to implement a procedure such as that shown in Figure 4. Processor control code and the program memory may be provided on a removable storage medium 704b.

The hologram coder 704 has an input 706 from a pseudo-random noise generator 708; this pseudo-random noise is also stored in a data store 710. The pseudo-random noise on input 706 may comprise phase modulation data or exp(j,) to provide an input to an OSPR-type procedure of the type shown in Figure 4. However the skilled person will be aware that substantially all known procedures for generating a hologram from image data employ noise as an input and thus the pseudo-random noise generator 708 may instead generate noise for one of these other procedures if such an other procedure is used. The hologram coder provides a hologram data stream output 711, preferably including display control data as described further below.

Tn some preferred embodiments the holographic video image server 700 also provides digital watermarking of the holograms which when displayed replay the video image sequence. In preferred embodiments watermarking of a hologram, more particularly of a hologram data stream, is performed substantially without loss of any image quality by taking advantage of inherent redundancy in the noise input to the hologram coder.

Thus, more specifically, an individual hologram data stream is watermarked by the noise used to crcatc the hologram data stream (since the procedure for generating hologram data from an image is itself deterministic). Thus in a naïve embodiment the noise input 706 is stored in data store 710 and distinguishable noise is used for each user or group of users. In one implementation of an OSPR-type encoding process a separate set of noise data is employed for each subframe; in other implementations an adaptive OSPR procedure may be used olthe type described above, although preferably there is still a noise input to each subfrarne in such a procedure. Successive image frames of the video sequence preferably, but not necessarily, use different noise data.

Thus, as illustrated in Figure 7a, the output 711 of hologram coder 704 is a set of n data streams, each having been generated from a different set of pseudo-random noise.

It will be appreciated that the arrangement shown in Figure 7a in which the noise from pseudo-random noise generator 708 is stored in data store 710 stores a substantial quantity of noise data. Figure 7b shows an alternative holographic video image server 750 (in which like elements to those of Figure 7a are indicated by like reference numerals), in this example the data store 710 storing seed data from a seed generator 712 rather than the pseudo-random noise sequence itself. In one embodiment the pseudo-random noise generator 708 may comprise a shift register with exor feedback and the seed generator 712 may specify a starting point in the pseudo-random sequence.

The pseudo-random sequence may be made arbitrarily long by adding stages to the shift register, and hence an arbitrarily high number of different pseudo-random sequences may be generated since, for the purposes of the hologram coding, two different starting points of the same sequence effectively provide two different sequences. Many different procedures may be implemented to provide pseudo-random noise generator 708 apart from the aforementioned shift register with feedback, for example a linear congruential generator, a Mersennc twister or a Blurn Blum Schub generator. (Similar pseudo-random noise generators may also be used to generate a phase key for the encryption method described later).

Figure 7b also shows the system incorporating a video image server 714 coupled to a movie database 716 to provide a stream of video images to the hologram coder for encryption. In embodiments the hologram coder also provides data store 710 data identifying a video stream, for example a movie and a frame of the video, and these data arc stored together with the seed data for the watermarking noise and data linking the seed for the waterniarking noise with a user or group of users. This enables hologram data from a user to be verified at a server as described later.

Figure 8 shows some example distribution systems for the hologram data streams of Figures 7a and 7h. Broadly speaking any convenient distribution may be employed including, but not limited to: a cable distribution system (coax or fibre), a wireless distribution system, a multicast distribution system, the Internet, other types of computer network. in the example of Figure 8 users 800a -n receive holographic data streams on laptop computers via the internet 802 whilst users 804a -n receive holographic data streams on mobile devices via a wireless transmission system 806 such as a DVB (Digital Video Broadcast) transmission system. Some embodiments of the user devices we describe later employ a backhaul communications link. This again may be provided by any convenient means including, but not limited to, a fixed or mobile phone connection, and a wired or wireless computer network communications link.

Referring now to Figure 9, this shows an embodiment of a mobile device 900 to receive and display the hologram data provided by a system of the type shown in Figure 7. The optical arrangement of Figure 9 is similar to that shown in Figure 2a and like elements are indicated by like reference numerals.

The mobile device 900 has a first communications interface 902, for example to a communications system of the type illustrated in Figure 8, on which is received a hologram data stream. Optionally this interface also provides backhaul communications. The received data is provided to a 24-bit memory block 904 which buffers the data for later display. For example in embodiments each bit plane of memory block 904 may store a hologram data for one temporal subfranie of data from an OSPR-type procedure. This hologram data is provided to SLM 24 via an (optional) SLM display driver 906. Reception and display of the hologram data is controlled by a microprocessor 908 coupled to communications interface 902, to memory block 904, and to program memory 910, for example Flash RAIv1 or RUM.

The received hologram data preferably also includes display control data for modulating the intensity of laser 12 and the control of modulation input 912. The illustrated system provides a monochrome display but may be extended to colour as previously described.

Figure lOa shows a procedure which may be implemented by processor control code in program memory 910 to control mobile device 900. Thus at step 1010 the procedure inputs hologram data h (see Figure 4) including display control data. The display control data provides input to a laser intensity control process 1012 and the hologram display data is buffered and output to the SLM subfrarne-by-subframe (1014).

BriIitness control Preferably a holographic video transmission system of the type we describe includes a display control system to modulate the hologram display dependent on to the content of the displayed image, more particularly the coverage or average energy of the image (or a similar parameter).

For a holographic imaging system one can define a parameterp which is a measure of the total noise energy or background noise level independent of the image displayed.

For example, for OSPRp = 0.6321, implying that about 30% of the light energy goes into noise as compared with the image. One can further define a parameter c which defines the coverage of an image, that is the energy in the desired image as a proportion of the maximum available energy. The average contrast ratio in a holographically replayed irnagc is given by the expression 1+ p, which for OSPR (p = 0.6321) c(l-p) results in the theoretical contrast ratios given below in Table 1, for various different lest images.

Test image Coverage c Coiitrast ratio R:1 Two-level test (AJC text) 0.0672 26.6 Low-coverae video frame OX)758 23.7 -(iiierbiead man) ______________________ _____________________________ Quarter-white block 0,1582 11.9 Medium-coverage video 0.2058 9.3 frame_(2_1_Century_Fox) ______________________ _____________________________ High-coverage video frame 0.3 187 6,4 (car at gates) _________________ _______________________ All-white video image 0.6328 3.7 Table I -. Test images and associated coverages, and contrast ratios obtained with OSPR.

In general, because different images have different coverages, they will form reproductions with different relative brightness (which are vary with -a--). For an image we define C (I} the coverage c{1} as follows: (1)2 phase levels > 2 M2 2 (1)2 binary phase (due M to conjugate image) For a phase hologram with M x M pixels the normalization factor of M2 arises because each pixel, being phase only, has an amplitude (and therefore energy) of I so the total replay field energy is It'!2 regardless of image content. However because in general only an approximate value for c{1} is needed, if amplitude modulation is employed a similar or the same expression may also be employed.

In order to compensate for this one can employ one of several methods, which all give equivalent results: * Display each subframe of image J for a time proportional to p * Display each subframc of subfranie of image] for the same length of time t, but illuminatc the subframe for a time t' =t proportional to p * Display cach subframe of subframc of image I for the same length of time 1, but modulate the illumination power proportional to p Thus display control data to compensate br variations in brightness of a replayed image due to image content may comprise an approximate value for c{I} or a value dependent on c{J}, for example comprising say 2 data bits, determined as described above. Again, because in embodiments only a very approximate value for c is employed a simplified expression may alternatively be used, for example without the squaring. At the display end these may be used to control a shutter for the SLM-illuminating laser and/or to modulate the laser to control the illumination intensity and/or time.

Watermark verification Referring now to Figure lOb, this shows a procedure for implementation on mobile device 900 to provide data to a holographic video server for verifying a watennark.

Thus at step 1020 the procedure reads data for one or more subframes from buffer 904, this data including an identifier for the corresponding video stream and frame and, optionally, sub frame. This data is preferably stored in memory block 904 in association with the hologram data for display, having been transmitted by the holographic video server as described above. This data is then sent back to the video server via the communications backhaul, accompanied by an identifier for the user or user group (step 1022).

Figure lOc shows one example of a procedure which may be implemented at a holographic video server 700, 750 to verify the watermark. Thus at step 1030 the data sent by the mobile device 900 is received and, at step 1032, image data for the identified video frame is retrieved for example from movie database 716 and seed data for the identilied user is retrieved from data store 710. The identified frame is then re-encoded (1034) using the pseudo-random noise sequence determined by the seed to regenerate hologram data for the one or more subfranies. This is then compared (1036) with the data received from the user and if this is identical (1038) the user is authorised, otherwise a potential DR M (Digital Rights Management) problem may be flagged.

Encryption Referring now to Figure 11 a this shows an embodiment of an encrypting video image video image server 770 in which like elements to those of Figures 7a and 7b arc indicated by like reference numerals. In the server of Figure 11 a the hologram data for a user or group of

users (n) is encrypted by operating on this data in an encryption block 772 in combination with a two-dimensional phase key X The encryption block 772 may be implemented in hardware and/or software and may comprise, for example, a 24-bit memory block similar to memory block 904 of Figure 9 coupled to hardware and/or software for performing an encryption operation. In preferred embodiments each pixel of h1 has a corresponding pixel in X, and the encryption operation may comprise a pixel-by-pixel operation of any type (providing it is reversible), for example addition, subtraction, multiplication or division. Alternatively a global operation involving h, and X,, may be performed, for example convolution of one by the other (in which case h1%, and X, may be of different sizes).

In a still further example X may operate on a group of holographic image subframes in combination. For example where such a set of subframes is stored in, say, 24-bit memory X, may he added or subtracted to the 24-bit number defincd by each memory location in which each bit defining the number is taken from a different holographic image subfiame. This type of operation has the effect of mixing data between subframcs, for example because a carry operation can move data from one plane (hit) to another plane (bit). in general the operation pcrformcd by the encryption block 772 can be represented by the equation below where, in general, h, can represent one or more subframes): + = h This is illustrated graphically in Figure lic, which shows an example SLM pattern (hologram data) and a set of random phases of a phase key X4 which, when phase values of corresponding pixels are added, scrambles the hologram data and which may also result in an undisplayable result, depending on the capacities of the SLM. It will be appreciated that this technique may be applied to amplitude, phase, or both.

Referring again to Figure 1 la, the holographic video image server 770 includes a phase key generator 774 which, in embodiments, comprises a second pseudo-random noise generator in combination with a second seed generator 776. Preferably data store 710 stores the seed of a phase key as well as the seed of a noise input to the hologram coding procedure. A seed may be generated in any convenient way, for example by incrementing a counter, since the security is provided by the pseudo-random noise generator. As illustrated, the phase key X is preferably transmitted to a user via a separate channel (which may be the same channel at a different time and/or differently encrypted).

Referring again to Figure 9, the phase key X, is received at a second communications interface 914 and stored in non-volatile memory 916 providing an input to microprocessor 908 for decryption of encrypted received hologram data.

Figure 1 lb shows a procedure which may be implemented on microprocessor 908 of Figure 9 to perform such decryption. At step 1100 the procedure inputs encrypted hologram data (including display control data) on a first channel and, at step 1102, inputs the phase key on a second channel. The procedure then performs a decryption operation (step 1104) which, in general, is simply the inverse of the encryption operation, for example: 1(n) UV 1t4V The decrypted hologram data is then buffered and output to the SLM (1106) as before and, likewise, a laser intensity control process (1108) operates on received display control data as previously described.

Applications for the above described system include, but are not limited to, the following: mobile phone; PDA; laptop; digital camera; digital video camera; games console; in-ear cinema; navigation systems (in-car or personal e.g. wristwatch GPS); head-up and helmet-mounted displays for automobiles and aviation; watch; personal media player (e.g. MP3 player, personal video player); dashboard mounted display; laser light show box; personal video projector (a "video iPod (RTM)" concept); advertising and signage systems; computer (including desktop); remote control unit; an architectural fixture incorporating a holographic image display system; more generally any device where it is desirable to share pictures and/or for more than one person at once to view an image.

No doubt many effective alternatives will occur to the skilled person. The techniques we described arc not limited to OSPR-type procedures for generating hologram data, although such procedures have some significant advantages. The techniques we describe may be employed to colour images/video as well as to monochrome images/video. The techniques are not limited to any particular type of spatial light modulator such as binary phase or continuous amplitude modulator.

It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Claims

CLAIMS: 1. A method of providing a two-dimensional image display for a

mobile device, the method comprising: encoding said two-dimensional image as a hologram; sending hologram data for said hologram to said mobile device; and displaying said hologram data at said mobile device to project said two-dimensional image display.
2. A method as claimed in claim 1 wherein said image data comprises video image data.
3. A method as claimed in claim 2 wherein said sending of said hologram data comprises streaming said data to said mobile device.
4. A method as claimed in claim 1, 2 or 3 wherein said mobile device comprises a buffer and a spatial light modulator (SLM), and wherein said displaying comprises receiving said hologram data, storing said hologram data in a said buffer, illuminating said SLM, and driving said illuminated SLM using said stored hologram data to project said two-dimensional image display.
5. A method as claimed in claim 4 wherein said hologram data includes control data, and wherein said displaying comprises controlling one or both of a display time of said hologram data and an illumination level of said SLM responsive to said display control data to compensate for image content-related changes in brightness of said image display.
6. A method as claimed in claim 5 wherein said illuminating of said SLM uses one or more lasers, and wherein said controlling comprises modulating a said laser.
7. A method as claimed in any preceding claim wherein said encoding comprises encoding a said two-dimensional image into a plurality of said holograms, each hologram of said plurality of said holograms comprising a hologram for a sub-frame of said image, wherein said sending comprises sending said hologram data for said holograms for subframcs of said image, and wherein said displaying comprises displaying said hologram data for said holograms for subframes of said image to project said two-diinensi onal image display.
8. A method as claimed in claim 7 wherein said subframes comprise temporal suhframes which average, over time, to give an impression of said two-dimensional image display to an observer.
9. A method as claimed in any preceding claim used for generating said two-dimensional image at a plurality oldifferent said mobile devices, and wherein said sending comprises sending hologram data encoding different holograms to different ones of said mobile devices, said different holograms generating substantially the same said two dimensional image.
10. A method as claimed in any one of claims I to 9 wherein said hologram data comprises data defining pixel values for display in two dimensions on a spatial light modulator, the method further comprising encrypting said hologram data using a two-dimensional key prior to said sending, said two-dimensional key modifying said pixel values for display in said two dimensions; and decrypting said hologram data prior to said displaying.
11. A method of displaying video using a mobile device the method comprising: encoding said video as a time-sequence of holograms; streaming data for said time-sequence of holograms to said mobile device; and displaying said streamed data on a spatial light modulator (SLM) at said mobile device to project a display of said video.
12. A method as claimed in claim 11 wherein said time-sequence of holograms comprises multiple holograms for a single frame of said video.
13. A system for providing a two-dimensional image display for a mobile device, the system comprising: means for encoding said two-dimensional image as a hologram; means for sending hologram data for said hologram to said mobile device, and means for displaying said hologram data at said mobile device to project said two-dimensional image display.
14. A system for displaying video using a mobile device, the system comprising: means for encoding said video as a time-sequence of holograms; means for streaming data for said time-sequence of holograms to said mobile device; and means for displaying said streamed data on a spatial light modulator (SLM) at said mobile device to project a display of said video.
1 5. An image server for providing data for an image display to a mobile device, the server comprising: an input to receive image data for an image to display; an encoder coupled to said input to encode said image data as at least one hologram and to provide hologram data for said at least one hologram; a data transmission system coupled to an output of said encoder to send said hologram data to said mobile device.
16. An image server as claimed in claim 15 wherein said image comprises a two-dimensional image, and wherein said hologram data comprises data for at least one hologram which replays to give the impression of said two-dimensional image.
17. An image server as claimed in claim 15 or 16 wherein said encoder is configured to encode data for a single said image as a plurality of said holograms.
18. An image server as claimed in claim 15, 16, or 17 wherein said image data comprises video image data, wherein said encoder is configured to encode said video image data as a time-sequence of holograms, and wherein said data transmission system is configured to stream said hologram data to said mobile device.
19. An image server as claimed in any one of claims 15 to 18 wherein said encoder includes a system to generate display control data for controlling one or both of a display time of said at least one hologram aiid an illumination level of said at least one hologram to compensate for image content-related changes in brightness of said image display, and wherein said data transmission system is configured to send said display control data to said mobile device is association with said hologram data.
20. A mobile device br holographically displaying an image, the mobile device comprising: a data receiver to receive hologram data br at leasi. one hologram encoding said image; a spatial light modulator (SLM) to display said hologram; a light source to illuminate said hologram displayed on said SLM to project said image for viewing; and a buffer coupled to said data receiver to store said received hologram data for display on said SLM.
21. A mobile device as claimed in claim 20 wherein said hologram data comprises data for a plurality of holograms encoding a single said image; and wherein said buffer is configured to store said hologram data for said plurality of holograms.
22. A mobile device as claimed in claim 20 or 21 for holographically displaying video, wherein said image comprises a video frame.
23. A mobile device as claimed in any one of claims 20 to 22 configured to receive display control data in association with said hologram data, and wherein said mobile device includes a system to control one or both of a display time of said at least one hologram and an illumination level of said at least one hologram to compensate for image content-related changes in brightness of said displayed image.
24. A method of digitally watermarking hologram data, the method comprising: inputting image data defining at least one image for conversion to a hologram; obtaining a set of pseudo-random values, said set of pseudo-random values being deterministically specified by seed data; and generating hologram data for at least one hologram which, when displayed, gives the appearance of said image; wherein said generating of said hologram data uses said set of pseudo-random values; and wherein different said sets of pseudo-random values specified by different said Seed data generate hologram data for different said holograms which, when displayed, give substantially the same said appearance o said image; whereby said hologram data is digitally watermarked such that two images with substantially the same appearance may he distinguished using said hologram data.
25. A method as claimed in claim 24 wherein said hologram data comprises data for a plurality of rows and columns of pixel values for displaying on an SLM, and wherein a said set of pseudo-random values comprises a complementary set olpseudo-random row and column pixel values in one or both of a spatial domain of said image data and a spatial frequency domain of said hologram.
26. A method as claimed in claim 25 wherein said generating of said hologram data comprises phase modulating data derived from said image data in said spatial domain using a spatial domain said set of pseudo-random values.
27. A method as claimed in claim 24, 25 or 26 wherein said image comprises a video frame of a sequence of video frames forming a video stream, and wherein said generating of said hologram data uses different said seed data for different said video frames of said video stream.
28. A method as claimed in claim 27 wherein said generating of said hologram data comprises generating hologram data for a plurality of temporal subframes for a said image, the method further comprising employing a plurality of values of said seed data for said generating of said hologram data for said plurality of said subframes.
29. A method of verifying a user of hologram data digitally watermarked according to the method of any one of claims 24 to 28, the method coniprising storing one or both olsaid seed data and said set of pseudo-random values, regenerating said hologram data using said stored data, and comparing hologram data from said user with said regenerated hologram data to verify said user.
30. A method of distributing an image to a plurality of users, the method comprising digitally watermarking hologram data for said image using the method of any one of claims 24 to 28 to generate a plurality of different versions of said hologram data with different said seed data, and sending said different versions of said hologram data to different ones of said users.
31. A method of distributingvideo to a plurality of users, the method comprising generating hologram data for said video and distributing said hologram data, said hologram data being configured to reproduce said video when displayed on an SLM, and wherein different ones of said users receive different versions of said hologram data encoding different holograms which, when replayed, reproduce substantially the same perceived video.
32. A method of distributing video to a plurality of users as claimed in claim 31 wherein a frame of said video is encoded into hologram data for a plurality of holographic sub frames, and wherein different ones of said users receive hologram data defining different said holographic subframes.
33. A caffier carrying processor control code to, when running, implement the method of any one of claims 24 to 32.
34. A computer system for digitally watermarking hologram data, the system comprising: an input to receive image data defining at least one image for conversion to a hologram; a pseudo-random number generator to provide a set of pseudo-random values, said set of pseudo-random values being deterministically specified by seed data; an image coder to generate hologram data for at least one hologram which, when displayed, gives the appearance olsaid image; and a data store to store one or both of said seed data and said set of pseudo-random values; wherein different said sets of pseudo-random values specified by different said seed data generate hologram data for different said holograms which, when displayed, give substantially the same said appearance of said image; whereby said hologram data is digitally watermarked such that two images with substantially the same appearance may be distinguished using said hologram data.
35. A method of generating encrypted digital hologram data, the method comprising: inputting image data defining at least one image for conversion to a hologram; performing a holographic transform on said image data to generate digital hologram data, said digital hologram data comprising two-dimensional data defining pixel values for two-dimensional display on a spatial light modulator to replay an image approximating said at least one image; and operating on said digital hologram data using a two-dimensional key comprising a set of values for modifying said pixel values to generate said encrypted digital hologram data.
36. A method as claimed in claim 35 wherein said digital hologram data comprises data for a plurality of holograms which, in combination, replay an image approximating said at least one image.
37. A method as claimed in claim 36 wherein said operating comprises operating separately with a said two-dimensional key on each of said plurality of holograms.
38. A method as claimed in claim 36 wherein said operating comprises operating with a said two-dimensional key on said plurality of holograms in combination.
39. A method as claimed in claim 36, 37 or 38 wherein said plurality of holograms comprise holograms for temporal subframes of said image which, when replayed, average in an observer's eye to give an impression of said at least one image.
40. A method as claimed in any oiie of claims 35 to 39 wherein said two-dimensional key comprises a set of phase values for modifying phase values of said pixels of said digital hologram data.
41. A method as claimed in claim 40 wherein said two-dimensional key has a set of phase values comprising one phase value for each of said pixel values of said digital hologram data, and wherein said operating comprises operating on a said pixel value of said digital hologram data using a corresponding phase value of said two-dimensional key.
42. A method as claimed in claim 41 wherein said operating comprises one of adding and subtracting.
43. A method of sending image data to a recipient, the method comprising: generating encrypted digital hologram data from said image data using the method claimed in any one of claims 35 to 42; sending said encrypted digital hologram data to said recipient Over a lirsi channel; and sending data for said two-dimensional key to said recipient over a second, different channel.
44. A carrier carrying processor control code to, when running, implement the method of any one of claims 35 to 43.
45. A computer system to generate encrypted digital hologram data, the system comprising: an input to receive image data defining at least one image for conversion to a hologram; a coder to perform performing a holographic transform on said image data to generate digital hologram data, said digital hologram data comprising two-dimensional data defining pixel values for two-dimensional display on a spatial light modulator to replay an image approximating said at least one image; and a hologram data encrypter to opel-ate on said digital hologram data using a two-dimensional key comprising a set of values for modifying said pixel values to generate said encrypted digital hologram data.
46, A method of decrypting encrypted digital hologram data, the method comprising: inputting encrypted digital hologram data, said encrypted digital hologram data comprising two-dimensional encrypted data defining encrypted pixel values of digital hologram data, said digital hologram data comprising two-dimensional data defining pixel values for two-dimensional display on a spatial light modulator to replay an image; inputting data for a two-dimensional Icey, said two-dimensional key comprising a set of values for modifying said encrypted pixel values; and operating on said encrypted digital hologram data using said two-dimensional key to generate decrypted said digital hologram data.
47. A method as claimed in claim 46 wherein said encrypted digital hologram data comprises data for a plurality of holograms which, in combination, replay an image approximating said image.
48. A method as claimed in claim 47 wherein said operating comprises operating separately with a two-dimensional key oii encrypted data for each of said plurality of holograms.
49. A method as claimed in claim 47 wherein said operating comprises operating with a said two-dimensional key on encrypted data for said plurality of holograms in combination.
50. A method as claimed in claim 47, 48 or 49 wherein said plurality of holograms comprises holograms for temporal suhframes of said image which, when replayed, average in an observer's eye to give an impression of said image.
51. A method as claimed in any one of claims 46 to 50 wherein said two-dimensional key comprises a set of phase values for modifying phase values olsaid pixels of said encrypted digital hologram data to generate decrypted said digital hologram data.
52. A method as claimed in claim 51 wherein said two-dimensional key has a set of phase values comprising one phase value for each of said pixel values of said encrypted digital hologram data, and wherein said operating comprises operating on a said pixel value of said encrypted digital hologram data using a corresponding phase value of said two-dimensional key.
53. A method as claimed in claim 52 wherein said operating comprises one of adding and subtracting.
54. A method of receiving an image, the method comprising: receiving encrypted digital hologram data over a first channel; receiving data for a two-dimensional key over a second, different channel; and decrypting said encrypted digital hologram data using the method of any one of claims 46 to 53 to generate digital hologram data to replay said image.
55. A carrier carrying processor control code to, when running, implement the method of any one of claims 46 to 54.
56. A system to decrypt encrypted digital hologram data, the system comprising: an input to receive encrypted digital hologram data, said encrypted digital hologram data comprising two-dimensional encrypted data defining encrypted pixel values of digital hologram data, said digital hologram data comprising two-dimensional data defining pixel values for two-dimensional display on a spatial light modulator to replay Lfl image; an input to receive data for a two-dimensional key, said two-dimensional key comprising a set of values for modifying said encrypted pixel values; and a hologram data decrypter to operate on said encrypted digital hologram data using said two-dimensional key to generate decrypted said digital hologram data.
57. A mobile device including the system of claim 56.
58. A data signal bearing digital hologram data encrypted according to the method of any one of claims 35 to 42.