Method, computer-readable medium, display device, and video source

Abstract

Repeatable element of video signal, for display on a display device 514, having predetermined duration and comprising plurality of subframes. Displaying video data, on a display device, comprising: di

Description

Technical Field The present invention relates to the transmission and display of video data. Background The rate at which the video data is generated or received may be different than the rate at which the display refreshes. Examples include when video data is generated for display in real-time, such as in response to user input or changes in an external environment, and when video data is received at a variable rate, such as streaming video subject to varying network capacity and latency. The difference in the two rates can result in poor image quality. Double-buffering, VSync and Triple-buffering have been proposed as possible solutions. With double buffering, two screen buffers are maintained for display. A buffer is designated as “active” when it has been completely filled or rendered by the graphics software. The contents of the active buffer are output to the screen and the other buffer (previously being displayed) is filled or rendered. Should the filling or rendering be slower than the display refresh rate, there is no change to the active buffer and the existing active buffer is repeated for display. However, switching from one buffer to another mid-frame can result in visible artefacts like screen tearing. To avoid screen tearing, Vertical synchronization or “VSync” may be used. With VSync, a buffer switch only occurs at the end of a frame period, before the next frame is output, and only if the other buffer has been completely filled or rendered. This avoids screen tearing but does not allow frames to be rendered any faster than the display refresh rate. As with double buffering a frame is repeated if the next frame has not been completely rendered at the end of a frame display period. Double buffering with VSync may introduce additional latency in the case where the buffer which is not being displayed can be filled or rendered faster than the display refresh rate. Triple buffering has been proposed to reduce the latency. With triple buffering two non-active buffers are alternated between being filled by the received or rendered data, a more recent frame may then be available when the active buffer for display is switched. It would be desirable to improve the image quality when repetition of frames is necessary. Summary According to a first aspect, there is provided a method of displaying video data on a display device. The method comprises: displaying a first plurality of subframes sequentially over a predetermined duration, each of the first plurality of subframes comprising at least a portion of video data for display, wherein at least two of the first plurality of subframes are displayed through different shutters in a first optical path to a first eye of a viewer; and repeating the display of the first plurality of subframes sequentially over the predetermined duration when a second plurality of subframes is not available for display after the predetermined duration. Different shutters in the optical path may be used for several reasons, such as to expand an eyebox, to expand a visible field of view or to block zero orders in different positions for different light sources used in the display for particular subframes. The subframes combine via the viewer’s persistence of vision to form a frame. However, it has been found that such a system suffers from noticeable flickering when used with conventional techniques to align frame rate, such as double-buffering or triple-buffering with VSync. The inventors have found that flickering is caused when the number of shutters activated (opened or changed to transmissive) in a given period of time suddenly changes. Such a sudden change can be caused by a frame repetition to align frame rates. In the present disclosure, flickering is reduced by repeating a plurality of subframes through different shutters over the predetermined period. The repeating the display may be achieved in any suitable way, for example, the first plurality of subframes may be retransmitted by a video source, alternatively the first plurality of subframes may be buffered in the display and a signal is received from a video source to repeat the display of the plurality of subframes. The predetermined duration may be greater than a frame period of the display device. Repeating a set of subframes over a predetermined duration which is greater than the frame period of the display device has been found to reduce flickering. Being able to repeat a first plurality of subframes within a predetermined duration greater than the frame period of a chosen display device allows the system to tolerate a sporadic repetition of subframes caused by a mismatch between the rate at which subframes are produced, and the rate at which subframes are displayed. An integer number of frames of the display device in the predetermined duration is not essential, although it may be useful for a simpler operation. The number of different shutters activated over the predetermined duration may be the same for both the first plurality of subframes and the second plurality of subframes. A shutter is activated when it is opened or otherwise changed into a transmissive state to allow light to pass. The inventors have realised that the flickering may be because a viewer will notice if the number of distinct shutters activated (which may or may not be the same as the number of distinct subframes displayed) changes over a predetermined duration this may lead to visible flickering. In the case where a consistent number (i.e. the same number) of distinct shutters needs to be activated within the predetermined duration, it is useful to allow for cases where subframes of an image may need to be repeated, for example when the next set of subframes is not yet available. Activating a consistent number of shutters over each predetermined duration also reduces flickering. At least some of the first plurality of subframes may represent respective colour channels. The colour channels may be part of a progressive scan video image or colour a computer-generated hologram (CGH) image. Example colour channels include red, green and blue but are not limited to those colours and may also include white light channels. Colour sequential displays using different shutters for each light source can benefit from the reduction in flickering. Additionally, or alternatively, at least some of the first plurality of subframes may represent respective components of a CGH image. CGH images may be displayed as multiple subframes through different shutters in an optical path to expand an eyebox. Subframes of a CGH image may be single or multiple colour channels. Examples of CGH subframes including multiple colour channels in combination with different shutters in an optical path are described in PCT / GB2024 / 050782 filed on 22 March 2024 and published as WO2024 / 194654, incorporated herein by reference for all purposes. The predetermined duration may be less than or equal to 40ms, less than or equal to 20ms, or less than or equal to 16.7ms. These correspond to update rates of the plurality of subframes of 25 Hz, 50Hz and 60 Hz respectively. It has been found that these are effective to reduce flicker. A smaller predetermined duration may be advantageous to reduce latency. 16.7 ms (corresponding to 60 Hz) has been found to represent a good balance between reducing latency and reducing visible display flicker. The predetermined duration may be based on the brightness of the display. It has been found that shorter predetermined durations can be used to reduce flickering as the display brightness increases. The predetermined duration may be inversely proportional to the display brightness, for example, although a linear relationship is not essential. In any of the examples above, the first plurality of subframes may further comprise metadata associating a shutter with each of the subframes. This allows arbitrary allocation of shutter to the subframes. The metadata may be located between subframes, such as in a blanking interval between subframes, in some examples. Rather than metadata, other examples may use predefined display modes containing sets of subframes with predefined shutter activations. The first plurality of subframes may comprise at least two subframes which are for viewing in an optical path through different shutters in a second optical path to a second eye of a viewer. This allows stereoscopic data for viewing by left and right eyes of a viewer to be transmitted and repeated if necessary with reduced perceived flicker. The method described for the first aspect is for displaying data, according to a second aspect a method of transmitting such data is provided. In the second aspect, a method of transmitting video data by a video source comprises: buffering a first plurality of subframes for display sequentially over a predetermined duration, each of the first plurality of subframes comprising at least a portion of video data for display, wherein at least two of the first plurality of subframes are for viewing through different shutters in a first optical path to a first eye of a viewer; transmitting the first plurality of subframes to a display device for display sequentially over the predetermined duration; buffering a second plurality of subframes for display sequentially over the predetermined duration, each of the second plurality of subframes comprising at least a portion of video data for display, wherein at least two of the second plurality of subframes are for viewing through different shutters in the first optical path to the first eye of a viewer; and when the buffering the second plurality of subframes is not complete after the predetermined duration, causing display of the first plurality of subframes to be repeated by the display device. This method enables the benefits of reduced flickering in the displayed output should the plurality of subframes need to be repeated. The source of the subframes for the buffering may include rendering the subframes by the video source, such as in response to user input, or simply receiving them, such as from a streaming source subject to varying network capacity and / or latency. The subframes may be transmitted in any suitable way, such as over a wired or wireless communication link. Some examples may packetize the data before sending, for use with packet-based link protocols, other examples may serialize the data before sending, for use with serial protocols, such as TMDS or similar. In both cases data may also be transmitted in parallel, for example via different parallel wired links or parallel packets for different one of the plurality of subframes. The transmitting may be direct or indirect, for example the first plurality of subframes may be relayed or retransmitted on the way to the display device. Where the transmitting is indirect, the method may be carried out by an intermediate and / or additional video processing system. Causing the display of the subframes to be repeated may also be done in any suitable way, such as by retransmitting the first plurality of subframes to the display or transmitting a signal to repeat the display of the first plurality of subframes. The predetermined duration may be greater than a frame period of the display device. The number of different shutters for activation over the predetermined duration may be the same for both the first plurality of subframes and the second plurality of subframes. At least some of the first plurality of subframes may represent respective colour channels. Additionally, or alternatively, at least some of the first plurality of subframes may represent respective components of a CGH image. The predetermined duration may be less or equal to than 40ms, less than or equal to 20 ms, or less than or equal to 16.7ms. The predetermined duration may additionally, or alternatively, be based on the brightness of the display. Brightness may be communicated from the display (also referred to as a video sink) to the video source or defined by the source when transmitting the data. The first plurality of subframes may further comprise metadata associating a shutter with each of the subframes. The metadata may be associated in any suitable way. In one example, the transmitting the first plurality of subframes comprises transmitting the plurality of subframes sequentially with a blanking period therebetween, each blanking period comprising metadata of a shutter to be used during display of the immediately following subframe. In other examples the transmitting the metadata associating a shutter with each of the subframes may be transmitted as one block within a blanking period before the transmitting of the first plurality of subframes. In further examples, the shutter metadata may be embedded in the video data transmitted by the video source. The first plurality of subframes may comprise at least two subframes which are for viewing in an optical path through different shutters in a second optical path to a second eye of a viewer. According to a third aspect, there is provided a computer-readable medium comprising computer-readable instructions that when executed by a processing system of a video system comprising at least two shutters in an optical path to a first eye of a viewer, instruct the video system to carry out the method according to the first aspect, with or without any of the additional features also described. The computer-readable medium may be non-transitory. According to a fourth aspect, there is provided a video system comprising: a display device; at least two shutters in an optical path to an eye of a viewer; a memory; a communications interface for receiving display data; and a processing system. The processing system is configured to carry out the method of according to the first aspect, with or without any of the additional features also described. According to a fifth aspect, there is provided a computer-readable medium comprising computer-readable instructions that when executed by a processing system of a video source, instruct the video source to carry out the method according to the second aspect, with or without any of the additional features also described. The computer-readable medium may be non-transitory. According to a sixth aspect, there is provided a video source comprising: a memory for buffering video data; a communications link for transmitting display data; and a processing system. The processing system is configured to carry out the method of according to the second aspect, with or without any of the additional features also described. According to a seventh aspect, there is provided a repeatable element of a video data signal for display by a display device. The repeatable element has a predetermined duration and comprises: a plurality of subframes for sequential display over a predetermined duration, each subframe comprising at least a portion of video data for display. At least two of the plurality of subframes are for viewing through different shutters in a first optical path to a first eye of a viewer for display. The predetermined duration may be greater than a frame period of the display device. Data indicative of a shutter to open during display of a particular subframe may be encoded in a blanking period before the particular subframe. Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings. Brief Description of the Drawings Figures 1A and IB show a diagrammatic representation of an example repeatable element of a video signal; Figure 2 shows a diagrammatic representation of an example shutter; Figure 3 shows a flow chart of an example video data display method; Figure 4 shows a flow chart of an example video data transmission method; Figure 5 shows an example system comprising a video source and display device for implementing the method of Figures 3 and 4. Detailed Description Figures 1A and IB show diagrammatic representations of example repeatable elements of a video data signal according to this disclosure. The repeatable element includes a plurality of subframes and has a predetermined duration over which the plurality of subframes are displayed sequentially. Time is shown as increasing vertically downwards by arrow 101 so that Figures 1A and IB, so that Figures 1A and IB show a diagrammatic representation of serial transmission line-by-line. The subframes each comprise a portion of a video frame and are displayed sequentially at a rate faster than the video frame rate. They are combined at a viewer’s eye with persistence of vision to produce the full image from the combination of two or more subframes. Each subframe is for display through a corresponding shutter, which is discussed in more detail below. Metadata of a corresponding shutter for each subframe, or which shutter to switch to a transmissive state may be encoded into the blanking interval immediately preceding the subframe. This allows arbitrary selection of the shutters. Other examples may encode the data of shutter activations differently, for example all in a first transmitted blanking period, via different predefined display modes defining patterns of shutter activations, or embedded into the subframes for display. In the example of Figure 1A, there are three subframes 102, 104, 106. The subframes are separated by blanking intervals 103, 105, with an additional blanking interval 101, also referred to as a blanking period preceding the first subframe. The repeatable unit has a predetermined duration 122 over which the three subframes are displayed sequentially. Taking an example of 16.7ms as the predetermined duration, the three subframes may each be displayed for around 5.6ms. In the example of Figure IB, there are six subframes 108, 110, 112, 114, 116, 118. The subframes are separated by blanking intervals 109, 111, 113, 115, 117 with an additional blanking interval 107 preceding the first subframe. The repeatable unit has a predetermined duration 124 over which the six subframes are displayed sequentially. Taking an example of 16.7ms as the predetermined duration, the six subframes may each be displayed for around 2.8ms. Figure 2 is a diagrammatic representation of a shutter system 200 which is positioned in an optical path from the display to an eye of a viewer. In this example, the shutter comprises three shutters 202, 204, 206. Each of the subframes in the repeatable unit is associated with a particular one of the shutters 202, 204, 206, which is controlled to be transmissive and allow light to pass when the associated subframe is displayed. Figure 2 is simplified and other examples may have different shutter shapes and different numbers of shutters, depending on the design of the display system. In this example, the shutters 202, 204, 206 are made from liquid crystal which can be controlled to be substantially blocking or substantially transmissive. Other example may use different shutter constructions, such as mechanical or micromechanical systems. By providing several shutters in an optical path to the same eye, it is possible to block zero orders at different positions and / or an eyebox can be increased. Increasing the eyebox is especially useful with computer generated hologram (CGH) displays which tend to have a small eyebox. (The eyebox is the area over which a viewer’s pupil can view the display.) By controlling the appropriate shutter to be in the transmissive state, the desired modulated light is transmitted and the remainder is blocked. Techniques such as double-buffering and triple-buffering with VSync allow frames to be repeated when the frames are rendered slower than the frame rate of the display device. These have been found not to be as effective when applied to display data with subframes displayed through different shutters and can result in visible flicker. To avoid this, the repeatable unit includes a plurality of subframes which are repeated as a group should the next repeatable unit (plurality of subframes) not be available. The plurality of subframes are spread over different shutters in the optical path to the same eye, so the repetition also uses different shutters over the duration of the repetition. In this example the repeatable unit includes subframes from more than one frame period of the display device. The example of Figure 1A may be used in systems with two subframes per video frame, so that the repeatable unit includes subframes from two different video frames. The example of Figure IB may be used in systems with two, three, four or five subframes per video frames, so that repeatable units include subframes from two or three different video frames. While the examples of Figure 1A and IB included three and six subframes respectively per overall video frame, any other number of subframes can be used. For example the number of subframes may be as many as ten, fifteen or more depending on the capabilities of the display hardware, overall video frame rate and the underlying rate at which the subframes are displayed. The subframes may be for the same or different colour channels, providing that at least two are associated with different shutters. For example, the subframes may all be for the same colour channel for a monochrome image, or separate red, green and blue for colour images. The principles described in PCT / GB2024 / 050782 filed on 22 March 2024 and published as WO2024 / 194654, incorporated herein by reference for all purposes, can also be used to allow simultaneous display of full colour CGH subframes. The example of Figure IB may include subframes from an integer number of video frames. For example at three subframes per video frame, the repeatable unit of Figure IB would then comprise subframes making up two frames when displayed, each frame having three subframes. The number of frames in a repeatable unit is not limited to an integer, although integer multiples of frames may be simpler to implement. To give an example, consider a repeatable unit consisting of four subframes in a display system which has three subframes per frame. A first repeatable unit might contain three subframes from a first frame and one subframe from a second frame, a second repeatable unit would then contain two subframes from the second frame and two subframes from a third frame, a third repeatable unit would then contain one subframe from the third frame and three subframes from a fourth frame, and so on. The repeatable element comprises subframes for sequential display over a predetermined duration. By repeating the subframes, and their associated different shutter activations, over the predetermined duration flickering is reduced. The predetermined duration itself required to reduce flicker may depend on the image brightness, brighter images can tolerate repeatable units with shorter predetermined durations. A predetermined duration of 16.7ms (corresponding to 60 Hz repetition rate) has been found to provide good results across a range of brightnesses. Turning now to Figure 3, a flow chart of a method of displaying video data is depicted. The method repeats a repeatable element comprising a plurality of subframes to reduce flickering of an image when it is necessary to repeat frames, such as due to slower rendering. At block 302 a set of subframes are displayed sequentially over the predetermined duration. This may include, for an example using CGH subframes, forming each subframe sequentially on a modulator, illuminating the modulator and activating an associated shutter in the optical path so that light from the modulator can be viewed through the associated shutter. Other display methods can also be used, such as controlling emissive displays rather than illuminating a modulator. Once display of the subframes is complete and the predetermined duration has elapsed, if a next set of subframes is not available then the repeatable element comprising a plurality of subframes is repeated. This is shown diagrammatically as a logical test at block 304. A logical test may be omitted in some examples, for example the video source may simply retransmit the previous repeatable element if the next set is not available, or the video source may transmit a command to repeat the display of the existing plurality of subframes rather than retransmitting all the data. When the next set of subframes is available, the method progresses to block 306 where the next set of subframes is displayed sequentially over the predetermined period. Once that predetermined period has elapsed and all the subframes have displayed, those subframes are repeated if the next set is not available. This is again represented by a logical test at block 308, but other examples may not implement decision logic in the display device as discussed above. The method then continues with further displaying and possible repetition steps as represented by the dashed boundary 310. The blocks within the dashed boundary are repeated as required to display video data with repetition of a plurality of subframes when the next set is not available. Figure 4 depicts a flow chart of transmitting video data. The method starts at 402 by buffering a first set of subframes for display, such as in a first buffer or in a first area of memory designated to store the subframes. The subframes may be rendered by the video source, or received, for example received from an external streaming service. Once buffering is complete (represented by decision block 404), the buffered frames are transmitted at block 406. For example, a flag or variable may be set that the buffered subframes in the first buffer or first area of memory are ready for transmission. The transmission may be serial, such as using TMDS (Transition-minimized differential signaling) or another serial transmission protocol. The transmission may also be packet-based, such as by packetizing the data and transmitting over a packetbased protocol. The duration of the transmission may be substantially the same as the duration of the predetermined period, but other examples may use faster transmission. Faster transmission is possible when the display device also includes memory to buffer the subframes for display and may allow the communication interface to be used for other data in addition to with the transmission of video data. During the transmission at block 406, a second set of subframes are buffered into a different buffer at 408. The buffer may be a second buffer or second designated area of memory. The buffer fills with subframes at the rate they are generated or received. As indicated by the logical test at 410, this continues until the second buffer is full, after which frame generation awaits the transmission of the current repeatable element to finish at block. Completion of transmission can be determined in any suitable way, such as by a VSync signal being generated or communication between the process filling the buffer and the process transmitting the subframes. Once the subframes are being transmitted from the second buffer, the buffering returns to filling the first buffer at 402. Meanwhile when the transmission of the first set of subframes is complete, the same set of subframes is repeated if the next set is not available (represented by logical test 414). If the next set is available, it is transmitted at 416. After that, the subframes are either repeated if the next set is not available (represented by logical test 418) or the process moves on to the transmission from the first buffer at block 406 which has been updated with new subframes during the transmission of the subframes from the second buffer. The method of Figure 4 is similar to double-buffering but the buffering is at the level of a plurality of subframes rather than individual subframes. Other examples are not limited to double-buffering, for example triple-buffering may also be used again with a plurality of subframes. Moving on to Figure 5, a diagrammatic representation of an example system comprising a display device 514 and a video source 502 is shown. The video source 502 comprises a memory 504, a processing system 506 and a communications interface 508. The processing system 506 buffers sets of subframes for transmission in the memory 504, such as by using the method of Figure 4. For example, the method of Figure 4 may be used with corresponding first buffer 510 and second buffer 512. The plurality of subframes is transmitted to the display device514 via the communications interface 508. The display device 514 includes memory 516, a processing system 518, a display 520 comprising shutters 522 in the optical path to a viewer’s eye 524 and a communications interface 526. In use, the processing system 518 may use the method of Figure 3 to display subframes received over the communication interface 526. The display includes controlling the shutters 522 so that an appropriate shutter is transmissive or allows light to pass depending on the subframe currently being displayed. The display device may use the examples described in WO2022 / 074373, incorporated by reference herein in its entirety, to synchronize the shutter activation with the display of the subframe. Some examples may omit the memory 516, for example when repetition of the plurality of subframes is by retransmission of the plurality of subframes from the video source 502. The above examples are to be understood as illustrative examples of the invention. Further examples of the invention are envisaged. For example, while the examples all consider data for transmission to a single eye, further examples may include subframes in the repeatable element that relate to first and second eyes, such as left and right eyes. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

1. A method of displaying video data on a display device, the method comprising: displaying a first plurality of subframes sequentially over a predetermined duration, each of the first plurality of subframes comprising at least a portion of video data for display, wherein at least two of the first plurality of subframes are displayed through different shutters in a first optical path to a first eye of a viewer; andrepeating the display of the first plurality of subframes sequentially over the predetermined duration when a second plurality of subframes is not available for display after the predetermined duration, wherein the predetermined duration is greater than a frame period of the display device.

2. The method of claim 1, wherein the number of different shutters activated over the predetermined duration is the same for both the first plurality of subframes and the second plurality of subframes.

3. The method of claim 1 or 2, wherein at least some of the first plurality of subframes represent respective colour channels.

4. The method of any of claims 1 to 3, wherein at least some of the first plurality of subframes represent respective components of a CGH image.

5. The method of any of claims 1 to 4, wherein the predetermined duration is less than or equal to 40ms.

6. The method of claim 5, wherein the predetermined duration is less than or equal to 16.7ms.

7. The method of any of claims 1 to 6, wherein the predetermined duration is based on the brightness of the display.

8. The method of any of claims 1 to 7, wherein the first plurality of subframes further comprises metadata associating a shutter with each of the subframes.

9. The method of any of claims 1 to 8, wherein the first plurality of subframes comprises at least two subframes which are for viewing in an optical path through different shutters in a second optical path to a second eye of a viewer.

10. A method of transmitting video data by a video source, the method comprising: buffering a first plurality of subframes for display sequentially over a predetermined duration, each of the first plurality of subframes comprising at least a portion of video data for display, wherein at least two of the first plurality of subframes are for viewing through different shutters in a first optical path to a first eye of a viewer;transmitting the first plurality of subframes to a display device for display sequentially over the predetermined duration;buffering a second plurality of subframes for display sequentially over the predetermined duration, each of the second plurality of subframes comprising at least a portion of video data for display, wherein at least two of the second plurality of subframes are for viewing through different shutters in the first optical path to the first eye of a viewer; andwhen the buffering the second plurality of subframes is not complete after the predetermined duration, causing display of the first plurality of subframes to be repeated by the display device;wherein the predetermined duration is greater than a frame period of the display device.

11. The method of claim 10, wherein the number of different shutters for activation over the predetermined duration is the same for both the first plurality of subframes and the second plurality of subframes.

12. The method of claim 10 or 11, wherein at least some of the first plurality of subframes represent respective colour channels of a progressive scan video image.

13. The method of claim 10 or 11, wherein at least some of the first plurality of subframes represent respective components of a CGH image.

14. The method of any of claims 10 to 13, wherein the predetermined duration is less than or equal to 40ms.

15. The method of claim 14, wherein the predetermined duration is less than or equal to 16.7ms.

16. The method of any of claims 10 to 15, wherein the predetermined duration is based on the brightness of the display.

17. The method of any of claims 10 to 16, wherein the first plurality of subframes further comprises metadata associating a shutter with each of the subframes.

18. The method of claim 17, wherein the transmitting the first plurality of subframescomprises transmitting the plurality of subframes sequentially with a blanking period therebetween, each blanking period comprising metadata of a shutter to be used during display of the immediately following subframe.

19. The method of any of claims 10 to 18, wherein the first plurality of subframes comprises at least two subframes which are for viewing in an optical path through different shutters in a second optical path to a second eye of a viewer.

20. A computer readable medium comprising computer-readable instructions that when executed by a processing system of a video system comprising at least two shutters in an optical path to a first eye of a viewer, instruct the video system to carry out the method of any of claims 1 to 9.

21. A video system comprising:a display deviceat least two shutters in an optical path to an eye of a viewer;29 09 25a memory;a communications interface for receiving display data; anda processing system configured to carry out the method of any of claims 1 to 8.5 22. A computer readable medium comprising computer-readable instructions thatwhen executed by a processing system of a video source, instruct the video source to carry out the method of any of claims 10 to 19.

23. A video source comprising:10 a memory for buffering video data;a communications interface for transmitting display data; anda processing system configured to carry out the method of any of claims 10 to 19.15 24. A repeatable element of a video data signal for display by a display device, therepeatable element having a predetermined duration and comprising a plurality of subframes for sequential display over a predetermined duration, each subframe comprising at least a portion of video data for display, wherein at least two of the plurality of subframes are for viewing through different shutters in a first optical path20 to a first eye of a viewer for display, and the predetermined duration is greater than a frame period of the display device.

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