Updating a depth buffer

Abstract

There is described a method of updating a depth buffer of a two-dimensional image of a scene, the method comprising: identifying a surface in the scene; identifying a depth of the surface; and in dependence on the surface being non-opaque, updating a value of a depth buffer based on the identified depth.

Description

[0001]Updating a depth buffer Field of the Disclosure The present disclosure relates to methods, systems, and apparatuses for updating a depth buffer, in particular methods, systems, and apparatuses for updating a buffer associated with the rendering of a three-dimensional representation of a scene. Background to the Disclosure Three-dimensional representations of environments are used in many contexts, including for the generation of virtual reality videos, in which depth information for a plurality of points of the representation is used to generate different images for a left eye and a right eye of a user. Typically, substantial processing power is required to determine such a three-dimensional representation, and the file size of files associated with these representations is typically large so that substantial amounts of storage are needed to keep the files and substantial amounts of bandwidth are required to transfer the files. Summary of the Disclosure According to the present disclosure, there is described a method of updating a depth buffer of a two- dimensional image of a scene, the method comprising: identifying a surface in the scene; identifying a depth of the surface; and in dependence on the surface being non-opaque, updating a value of a depth buffer based on the identified depth. Preferably, the method comprises: identifying a current value of the depth buffer; and updating the value of the depth buffer in dependence on the depth of the surface being different to, preferably less than, the current value of the depth buffer. Preferably, the depth buffer is a second depth buffer and wherein the image is further associated with a first depth buffer. Preferably, the first depth buffer is associated with opaque surfaces. Preferably, the second depth buffer is associated with non-opaque surfaces. Preferably, the method comprises: identifying a current value of the first depth buffer; and comparing the depth of the surface to a value of the first depth buffer. Preferably, the method comprises: updating the value of the first depth buffer based on the identified depth in dependence on said identified depth being different to, preferably less than, the current value of the first depth buffer. Preferably, the method comprises updating a value of a pixel of the two-dimensional image in dependence on the depth of the surface being different to, preferably less than, the current value of the first depth buffer. Preferably, the method comprises updating the value of the first depth buffer based on the surface being opaque. Preferably, the method comprises, in dependence on the depth of the surface being less than the value of the first depth buffer, comparing the depth of the surface to a value of the second depth buffer. Preferably, the method comprises updating the value of the second depth buffer based on the identified depth in dependence on said identified depth being different to, preferably less than, the current value of the second depth buffer. Preferably, the method comprises updating a value of a pixel of the two- dimensional image in dependence on the depth of the surface being different to, preferably less than, a current value of the second depth buffer. Preferably, the method comprises updating the value of the second depth buffer based on the surface being non-opaque. Preferably, the method comprises: applying an offset to the value of the second depth buffer to determine a comparison value; and comparing the depth of the surface to the comparison value. Preferably, the method comprises updating the depth buffer and / or the second depth buffer in dependence on the comparison. Preferably, the method comprises updating the value of the depth buffer and / or the second depth buffer based on a combination of the depth of the surface and an offset value. Preferably, the method comprises updating the value of the depth buffer and / or the second depth buffer based on a combination of the depth of the surface and a predetermined offset value. Preferably, a value of the offset is dependent on a distance of the surface from a viewing zone of the three- dimensional representation. Preferably, the two-dimensional image is associated with a three-dimensional representation of a scene, and wherein identifying the depth of the surface comprises identifying a distance of a point of the three- dimensional representation from a viewing location associated with the two-dimensional image. Preferably, the viewing location is associated with a viewing zone of the three-dimensional representation. Preferably, the method comprises determining a transparency value of the surface. Preferably, the method comprises determining the transparency value by determining a transparency value of a point of a three- dimensional representation that defines the surface. Preferably, determining the transparency value comprises comparing an alpha value of the surface to an alpha threshold. Preferably, the surface is deemed to be opaque if the alpha value exceeds the alpha threshold. Preferably, the surface is deemed to be non-opaque if the alpha value is below the alpha threshold. Preferably, the alpha threshold is dependent on a distance of the surface from a viewing zone of the three- dimensional representation. Preferably, the method comprises initialising a first depth buffer and a second depth buffer. Preferably, the method comprises initialising the first depth buffer and the second depth buffer with maximum depth values. Preferably, the method comprises determining and / or updating a value for a pixel of the two-dimensional image based on the depth of the surface. Preferably, the depth buffer comprises a matrix with a number of rows and columns that is equal to a number of rows and columns of pixels of the two-dimensional image. Preferably, the method comprises: identifying a plurality of surfaces of the scene; and ordering the surfaces in dependence on a distance of each surface from a viewing location and / or a viewing zone of the scene. Preferably, the method comprises: identifying a plurality of surfaces of the scene; identifying a first set of opaque surfaces; identifying a second set of non-opaque surfaces; for each surface in the first set of surfaces, comparing each of the surfaces to a present value of a depth buffer; and thereafter, for each surface in the second set of surfaces, comparing each of the surfaces to a present value of a depth buffer. Preferably, the method comprises outputting the depth buffer and / or the two-dimensional image. Preferably, the method comprises rendering the two-dimensional image based on the depth buffer. Preferably, the method comprises forming a bitstream comprising the depth buffer and / or the two- dimensional image. Preferably, the three-dimensional representation is associated with a viewing zone, the viewing zone comprising a subset of the scene and / or the viewing zone enabling a user to move through a subset of the scene. Preferably, the user is able to move within the viewing zone with six degrees of freedom (6DoF). Preferably, the viewing zone has a volume of less than 50% of the volume of the scene, less than 20% of the volume of the scene, and / or less than 10% of the volume of the scene. Preferably, the viewing zone has, or is associated with, a volume, preferably a real-world volume, of less than five cubic metres (5m3), less than one cubic metre (1m3), less than one-tenth of a cubic metre (0.1m3) and / or less than one- hundredth of a cubic metre (0.01m3). Preferably, the three-dimensional representation comprises a point cloud. Preferably, the method comprises storing the three-dimensional representation and / or outputting the three- dimensional representation. Preferably, the method comprises outputting the three-dimensional representation to a further computer device. Preferably, the method comprises generating an image and / or a video based on the three-dimensional representation. Preferably, the method comprises forming one or more two-dimensional representations of the scene based on the three-dimensional representation. Preferably, the method comprises comprising forming a two- dimensional representation for each eye of a viewer. Preferably, the point is associated with one or more of: a location; an attribute; a transparency; a colour; and a size. Preferably, the point is associated with an attribute for a right eye and an attribute for a left eye. Preferably, the scene comprises one or more of: an extended reality (XR) scene; a virtual reality (VR) scene; an augmented reality (AR) scene; and a mixed reality (MR) scene. Preferably, the method comprises forming a bitstream that includes the point. According to another aspect of the present disclosure, there is described a method of updating a value of a pixel of a two-dimensional image of a scene, the method comprising: identifying a surface in the scene; identifying a depth of the surface; comparing the depth of the surface to a value of a first depth buffer, wherein the first depth buffer is associated with opaque surfaces; in dependence on the depth of the surface being less than the value of the first depth buffer, comparing the depth of the surface to a value of a second depth buffer, wherein the second depth buffer is associated with non-opaque surfaces; and in dependence on the depth of the surface being different to a current value of the second depth buffer, updating a value of a pixel of the two-dimensional image. According to another aspect of the present disclosure, there is described a method of rendering a two- dimensional image based on a plurality of depth buffers, the method comprising: for one or more pixels of the image: determining a depth of a surface; determining a value in a first depth buffer; determining a value in a second depth buffer; and determining a value for the pixel in dependence on: a comparison between the depth of the surface and the value in the first depth buffer; and a comparison between the depth of the surface and the value in the second depth buffer; preferably wherein the second depth buffer is associated with non-opaque surfaces and the method comprises: determining the value for the pixel based on the depth of the surface being beneath the value in the first depth buffer; and / or determining the value for the pixel based on the depth of the surface being substantially different to the value in the second depth buffer. According to another aspect of the present disclosure, there is described a system for carrying out the aforesaid method, the system comprising one or more of: a processor; a communication interface; and a display. According to another aspect of the present disclosure, there is described an apparatus for updating a depth buffer of a two-dimensional image of a scene, the method apparatus comprising: means for (e.g. a processor for) identifying a surface in the scene; means for (e.g. a processor for) identifying a depth of the surface; and means for (e.g. a processor for) in dependence on the surface being non-opaque, updating a value of a depth buffer based on the depth of the surface. According to another aspect of the present disclosure, there is described a bitstream comprising a plurality of depth buffers for a two-dimensional image, preferably comprising a first depth buffer associated with opaque surfaces and a second depth buffer associated with non-opaque surfaces. Preferably, the bitstream comprises one or more two-dimensional images associated with the depth buffers. According to another aspect of the present disclosure, there is described an apparatus (e.g. an encoder) for forming and / or encoding the aforesaid bitstream. According to another aspect of the present disclosure, there is described an apparatus (e.g. a decoder) for receiving and / or decoding the aforesaid bitstream. Any feature in one aspect of the disclosure may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, features implemented in hardware may be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly. Any apparatus feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure, such as a suitably programmed processor and associated memory. It should also be appreciated that particular combinations of the various features described and defined in any aspects of the disclosure can be implemented and / or supplied and / or used independently. The disclosure also provides a computer program and a computer program product comprising software code adapted, when executed on a data processing apparatus, to perform any of the methods described herein, including any or all of their component steps. The disclosure also provides a computer program and a computer program product comprising software code which, when executed on a data processing apparatus, comprises any of the apparatus features described herein. The disclosure also provides a computer program and a computer program product having an operating system which supports a computer program for carrying out any of the methods described herein and / or for embodying any of the apparatus features described herein. The disclosure also provides a computer readable medium having stored thereon the computer program as aforesaid. The disclosure also provides a signal carrying the computer program as aforesaid, and a method of transmitting such a signal. The disclosure extends to methods and / or apparatus substantially as herein described with reference to the accompanying drawings. The disclosure will now be described, by way of example, with reference to the accompanying drawings. Description of the Drawings Figure 1 shows a system for generating a sequence of images. Figure 2 shows a computer device on which components of the system of Figure 1 may be implemented. Figure 3 shows a method of determining a three-dimensional representation of a scene. Figures 4a and 4b show method of determining a point based on a plurality of sub-points. Figure 5 shows a scene comprising a viewing zone. Figures 6a and 6b show arrangements of capture devices for determining points of the three-dimensional representation. Figure 7 shows a point that can be captured by a plurality of capture devices. Figures 8a and 8b show grids formed by the different capture devices. Figure 9 describes a method of determining a location of a point of the three-dimensional representation. Figure 10 shows a method of determining an angle of a point from a capture device used to capture the point. Figures 11a and 11b shown a scene comprising a plurality of surfaces. Figure 12 shows a method of updating a depth buffer. Figure 13 shows a method of updating a depth buffer based on a transparency of a surface. Figures 14 and 15 show pixel brackets associated with a scene that comprises overlapping non-opaque points. Figures 16a and 16b show a method of updating a plurality of depth buffers based on a transparency of a surface. Description of the Preferred Embodiments Referring to Figure 1, there is shown a system for generating a sequence of images. This system can be used to generate, and then display, a representation of an environment, which may comprise a VR environment (or an XR environment). The system comprises an image generator 11, an encoder 12, a transmitter 13, a network 14, a receiver 15, a decoder 16 and a display device 17. These components may each be implemented on separate apparatuses. Equally, various combinations of these components may be implemented on a shared apparatus; for example, the image generator 11, the encoder 12, and the transmitter 13 may all be part of a single image data generation device. Similarly, the receiver 15, the decoder 16, and the display device 17 may all be a part of a single image rendering device. Typically, the system comprises at least one encoding computer device (e.g. a server of a content provider) and at least one rendering computer device (e.g. a VR headset). Referring to Figure 2, each of the components, and in particular the image generator 11, the encoder 12, the transmitter 13, the receiver 15, the decoder 16 and the display device 17 is typically implemented on a computer device 20, where, as described above, a plurality of these components may be implemented on a shared computer device. Each computer device comprises one or more of: a processor 21 for executing instructions (e.g. so as to perform one or more of the steps of the various methods described below), a communication interface 22 for facilitating communication between computer devices (e.g. an ethernet interface, a Bluetooth® interface, or a universal serial bus (UBS) interface, a memory 23 and / or storage 24 for storing information and instructions (e.g. a random access memory (RAM), a read only memory (ROM), a hard drive disk (HDD) a solid state drive (SSD), and / or a flash memory, and a user interface 25 (e.g. a display, a mouse, and / or a keyboard) for enabling a user to interact with the computer device. These components may be coupled to one another by a bus 25 of the computer device. The computer device 20 may comprise further (or fewer) components. In particular, the computer device (e.g. the display device 17) may comprise one or more sensors, such as an accelerometer, a GPS sensor, or a light sensor. These sensors typically enable the computer device to identify an environmental condition and / or an action of wearer of the display device. Turning back to Figure 1, the image generator 11 is configured to generate a sequence of image data (e.g. a sequence of image frames) to enable the display device 17 to use this image data to display a plurality of images. The image data may comprise one or more digital objects and the image data may be generated or encoded in any format. For example, the image data may comprise point cloud data, where each point has a 3D position and one or more attributes. These attributes may, for example, include, a surface colour, a transparency value, an object size and a surface normal direction. Each attribute may have a value chosen from a continuous range or may have a value chosen from a discrete set. The image data enables the later rendering of images. This image data may enable a direct rendering (e.g. the image data may directly represent an image). Equally, the image data may require further processing in order to enable rendering. For example, the image data may comprise three-dimensional point cloud data, where rendering a two-dimensional image using this data requires processing based on a viewpoint of this two-dimensional image. The image data may comprise depth map data, where one or more pixels or objects in the image is associated with a depth that is specified by the depth map data. The depth map data may be provided as a depth map layer, separate from an image layer. In some contexts, such as MPEG Immersive Video (MIV), the image layer may instead be described as a texture layer. Similarly, in some contexts, the depth map layer may instead be described as a geometry layer. The image data may include a predicted display window location. The predicted display window location may indicate a portion of an image that is likely to be displayed by the display device 17. The predicted display window location may be based on a viewing position (such as a virtual position and / or orientation of the user in a 3D environment) of the user, where this viewing position may be obtained from the display device. The predicted display window location may be defined using one or more coordinates. For example, the predicted display window location may be defined using the coordinates of a corner or center of a predicted display window, and may be defined using a size of the predicted display window. The predicted display window location may be encoded as part of metadata included with the frame. The image data for each image (e.g. each frame) may include further information, which may be provided as a part of an image, e.g. as part of the point cloud data, or as separate layers. In particular, the image data may include audio information or haptic feedback information indicating audio or haptics which can accompany displayed visual data. An audio layer or haptic layer may accompany each image, and may be omitted for images where no accompanying audio or haptics are required. Similarly, the image data may comprise interactivity information, where the image data may contain or indicate elements with which a user can interact. The interactivity information may, for example, define a behaviour of an element, where a user is able to interact with the element based on this behaviour. The behaviour typically defines a change in an element that occurs as a result of a user interaction where this change may comprise a change in the attributes of the element or in the rendering of the element. As an example, where an image contains a target element, the target element may be arranged to disappear when a user interacts with this element, or to provide feedback indicating that the user has interacted with the target. This interactivity data may be provided as part of, or separately to, the image data. The image data may indicate, or may be combinable with, a state of the virtual environment, a position of a user, or a viewing direction of the user. Here, the position and viewing direction may be physical properties of the user in the real-world, or position and viewing direction may also be purely virtual, for example being controlled using a handheld controller. The image generator 11 may, for example, obtain information from the display device 17 that indicates the position, viewing direction, or motion of the user. Equally, the image generator may generate image data such that it can later be combined with this position, viewing direction, or motion, where the image generator may generate a full scene which is only partially viewed by a user depending on the position of that user. In some cases, the generated image may be independent of user position and viewing direction. This type of image generation typically requires significant computer resources such as a powerful GPU, and may be implemented in a cloud service, or on a local but powerful computer. For example, a cloud service (such as a Cloud Rendering Service (CRN)) may reduce the cost per-user and thereby make the image frame generation more accessible to a wider range of users. Here “rendering” refers at least to an initial stage of rendering to generate an image. Further rendering may occur at the display device 17 based on the generated image to produce a final image which is displayed. The image generator 11 may, for example, comprise a rendering engine for initially rendering a virtual environment such as a game or a virtual meeting room. The encoder 12 is configured to encode frames to be transmitted to the display device 17. The encoder may be implemented using executable software or may be implemented on specific hardware such as an ASIC. In some embodiments, the image generator 11 may transmit raw, unencoded, data through the network 14. However, such transmission typically leads to a high file size and requires a high bandwidth so that it is typically desirable to encode the data prior to the transmission. The encoder 12 may encode the image data in a lossless manner or may encode the data a lossy manner. The encoder may apply inter-frame or intra-frame compression based on a currently-encoded frame and optionally one or more previously encoded frames. The encoder may be a multi-layer encoder, such as a low complexity enhancement video codec (LCEVC) enabled encoder. Where the generated frames comprise depth map data, the encoder 12 may perform layered encoding on each instance of image data (e.g. each frame) to generate an encoded frame comprising a base depth map layer and an enhancement depth map layer. Encoding a depth map in this way may improve compression. In some applications, such as HDR video, depth maps are desirably highly detailed with a bit depth of up to twelve or fourteen bits, which is a significant increase in the data to be transmitted. As a result, providing ways to improve compression of the depth map can make more realistic depth map-based displays viable when performing rendering or transmission of rendered data in real-time. Furthermore, this type of layered encoding makes it easy to drop (and then pick back up) one or more of the layers, which provides flexibility and tools for bandwidth management. Layered encoding is also helpful as the final decoder / user device (such as a user display device) can choose whether to process these extra layers. For example, in a non-layered approach, the best the end device (i.e. the receiver, decoder or display device associated with a user that will view the images) can do is determine that it does not have enough resources for a given quality (be it resolution, frame rate, inclusion of depth map) and then signal to the controller / renderer / encoder that it does not have enough resources. The controller then will send future images at a lower quality. In that alternative scenario, the end device still unfortunately has to process the higher quality data until the lower quality data arrives, if it can process the received images at all. In some of the described embodiments, this situation is improved upon because when / if the end device determines for example that it does not have the processing capabilities to handle the highest level of quality, then it can drop and / or choose not to process certain layers. The end device may also signal to the controller that it needs a lower level of quality, but in the meantime the end device can only process the number of layers that it can handle. Therefore, the end device can react to conditions much more quickly. In some cases, depth map data may be embedded in image data. In this case, the base depth map layer may be a base image layer with embedded depth map data, and the enhancement depth map layer may be an enhancement image layer with embedded depth map data. Alternatively, when the generated images comprise a depth map layer separate from an image layer and multi-layer encoding is applied, the encoded depth map layers may be separate from the encoded image layers. This has the advantage that the encoded depth map layers can be dropped under some conditions while still retaining image layers that can be displayed (albeit with a lower level of realism). For example, the encoded depth map layers can be dropped by a transmitter or encoder when available communication resources are reduced, or can be dropped by an end device which lacks the processing resources to handle the highest level of quality. Similarly, if some images comprise an audio base layer, a haptic feedback base layer, an audio enhancement layer or a haptic feedback enhancement layer, these can be processed or dropped flexibly. Again similarly, if some images comprise an interactivity data base layer or an interactivity enhancement layer these can be processed or dropped flexibly. For example, certain interactions may only be possible where a threshold bandwidth is available, where complex interactions (e.g. those enabling a conversation with a digital object) may be disabled before less complex interactions (e.g. changing a pixel colour) are disabled. Additionally or alternatively, where the image data comprises point cloud data, the encoder may apply a point cloud data encoding technique such as described in European patent application EP21386059.6, which is incorporated herein by reference. Such a point cloud encoder may act as a base encoder for a layered encoding technique such as LCEVC or VC-6. Notably LCEVC and VC-6 techniques encode and decode a layered signal, but are agnostic about the content type of data encoded in the signal. For example, the signal can include textures, video frames, geometry or depth data, meshes, point clouds, rendering attributes or physics engine attributes. The transmitter 13 may be any known type of transmitter for wired or wireless communications, including an Ethernet transmitter or a Bluetooth transmitter. The transmitter 13 may be configured to make decisions about how to transmit the image data, and / or may provide feedback to the encoder 12 or the image generator 11. For example, the transmitter may determine available communication resources (e.g. bandwidth) for transmitting image data, and may drop one or more layers from an encoded frame, or indicate to the image generator and / or encoder that image data should be generated and encoded with fewer layers, when insufficient bandwidth is available for transmission of all generated data. As specific examples, the transmitter may be configured to drop a depth map layer, an LCEVC enhancement layer, or a VC-6 enhancement layer from a frame when insufficient communication resources are available. The network 14 provides a channel for communication between the transmitter 13 and the receiver 15, and may be any known type of network such as a WAN or LAN or a wireless Wi-Fi or Bluetooth network. The network may further be a composite of several networks of different types. Many users only have access to a network with a bandwidth of 30MBps which can lead to latency jitter when streaming. The required bandwidth and the observed latency can be reduced by means of tactics such as forward-looking rendering and last-millisecond reprojection, which are enabled by improved compression. The receiver 15 may be any known type of receiver for wired or wireless communications, including an Ethernet transmitter or a Bluetooth transmitter. The decoder 16 is configured to receive and decode an encoded frame. The decoder may be implemented using executable software or may be implemented on specific hardware such as an ASIC. The display device 17 may for example be a television screen or a VR headset. The timing of the display may be linked to a configured frame rate, such that the display device may wait before displaying the image. The display device may be configured to perform warping, that is, to obtain a final display window location, adjust a warpable image to obtain a final image corresponding to a final viewing direction of the user, and display the final image. In this regard, the image data is typically arranged to provide a warpable image for which a portion of the image that is displayed at the display device 17 is dependent on a position or orientation of a viewer. The warpable image may then be rendered before a most up to date viewing direction of the user is known. The warpable image may be transmitted to the display device, or the warpable image may be transmitted to a rendering node which is near to the display device, and the display device or rendering node may perform time warping to generate a displayed image portion based on the warpable image and the most up to date viewing direction of the user. As mentioned above, a single device may provide a plurality of the described components. For example, a first rendering node may comprise the image generator 11, encoder 12 and transmitter 13. Additional similar rendering nodes may be included in the system, and may work together to generate the sequence of frames. In one case, multiple rendering nodes may each provide separate image data to an image data assembling node; for example, each rendering node may provide a part of a sequence of frames to a frame assembling node. For example, the receiver 15, decoder 16 or display device 17 may be configured to assemble parts of image data from multiple sources to generate a sequence of images for display on the display device. Alternatively, the image data assembling node may be separate from the receiver 15, decoder 16 and display device 17. Additionally or alternatively, multiple rendering nodes may be chained. In other words, successive rendering nodes may add to a sequence of image data as it passes from rendering node to rendering node, and eventually a complete sequence of image data is then provided to the receiver 15. Furthermore, each rendering node may obtain components of a render from multiple upstream rendering nodes and / or distribute components of a render to multiple downstream rendering nodes. A chain of rendering nodes may be useful for performing different rendering tasks that require different quantities of processing resources, or different frame rates. For example, a company may provide distributed processing in the form of a centralised hub which has abundant processing resources but is distant from users, and peripheral locations which have more scarce processing resources but are closer to users. Expensive but fairly static rendering features such as background lighting or environmental impact on sound may be generated at the central hub (for example using ray tracing), while features that require fewer resources but faster responses or higher frame rates may be generated closer to the user. In other words, the more responsive a rendering feature needs to be, the lower latency it needs between the rendering node which generates the feature and the user display and, in a chain of rendering nodes, the node which generates each rendering feature can be chosen based on a required maximum latency of that feature. On the other hand, if it is expensive to generate a rendering feature, then it may be preferable to generate the feature less frequency and with a higher maximum latency. For example, a static, high-quality background feature may be generated early in the chain of rendering nodes and a dynamic, but potentially lower-quality, foreground feature may be generated later in the chain of rendering nodes, closer to the user device. Here, environmental impact on sound means, for example, a set of surfaces may be constructed where each surface has different sound reflection and absorption properties depending upon material and shape. The frame rates may be matched by creating multiple frames with features generated at the lower frame rate, and combining them with the frames with features generated at the higher frame rate. In a non- limiting embodiment, a preliminary rendering generates volumetric object data including motion vectors at a first (lowest) frame rate, then produces 2D rendered frames plus depth information for a specific user at a second (higher) frame rate, then transmits video plus depth data to the user device, which produces final frames for display via space warping (depth-based reprojections) at a third (highest) frame rate. One or more of these steps may be performed in combination with the other described embodiments. The viewing position of the user may change as additional rendering tasks are performed at different rendering nodes in the chain. Each or any rendering node may obtain an updated viewing position before performing its respective rendering task. Additionally, the system may simultaneously generate multiple sequences of image data for different respective users or different respective display devices. For example, in the context of a VR or AR experience, each user or display device may view a different 3D environment, or may view different parts of a same 3D environment. When using a chain of rendering nodes, each node may serve multiple users or just one user. For example, a starting rendering node (e.g. at a centralised hub) may serve a large group of users. For example, the group of users may be viewing nearby parts of a same 3D environment. In this case, the starting node may render a wide zone of view (“field of view”) which is relevant for all users in the large group. The starting node may send this wide field of view to a first middle rendering node which renders additional aspects of the 3D environment. These additional aspects may for example be aspects which require less processing power to render, or may be aspects which are specific to individual users of the group. Additionally, the middle rendering node may render features in a smaller field of view than the starting node – this smaller field of view may be relevant to each user rather than the group of users. The first middle rendering node may additionally only serve a smaller number of users (e.g. half of the large group of users), with the remaining users being served by a second middle rendering node which also receives the wide field of view from the starting node. The middle rendering node(s) may then send sequences of second partially or fully rendered frames to an end device for each user. The end device may perform further processes such as warping or focal distance adjustments, optionally using depth map data. Preferably, each rendering node encodes the partially or fully rendered frames before transmitting them on to a next rendering node or to the receiver 15. This means that the required communication resources can be reduced when the rendering nodes are separated by one or more networks, or more generally are implemented in a distributed system such as a cloud. However, each rendering node in a chain is encoding a different partially or fully rendered frame, with different data. Therefore, it may be advantageous for different rendering nodes to use different rendering formats and / or encoding formats. For example, the output from a first rendering node may be point cloud data which logically describes a 3D scene. This point cloud data can be encoded using the techniques of EP21386059.6. A second rendering node may then operate on the point cloud data to generate image data that is more readily displayed by a generic display device, without requiring the display device to model the 3D environment. This image data may be encoded using video coding techniques. The chaining of rendering nodes may be extended to arbitrary tree structures, where a rendering node obtains partially rendered frames from more than one preceding rendering node, and generates further partially or fully rendered frames based on the multiple obtained sequences of partially rendered frames. For example, a content rendering network (CRN) comprising numerous rendering nodes may be used to serve a volumetric event to a large number of same-time users, such as users participating in a shared virtual environment. Rendering the same event for each user is far more expensive in terms of computation time and power consumption than rendering the volumetric effect once and performing the rendering equivalent of multicasting the volumetric effect for multiple users. For example, each user may have a second rendering node (such as a VR headset), and the network may comprise a central first rendering node. The first rendering node may render the volumetric event, and distribute partially rendered frames depicting the volumetric event to the different second rendering nodes. The second rendering node for each user may then integrate the partially rendered frames depicting the volumetric event into a view of the virtual environment which is currently being shown to each user, based on parameters such as the user’s virtual position. The receiver 15, decoder 16 and display device 17 may be consolidated into a single device, or may be separated into two or more devices. For example, some VR headset systems comprise a base unit and a headset unit which communicate with each other. The receiver 15 and decoder 16 may be incorporated into such a base unit. In some embodiments, the network 14 may be omitted. For example, a home display system may comprise a base unit configured as an image source, and a portable display unit comprising the display device 17. In the event that the decoder 16 or the display device 17 does not or cannot handle one or more layers, the receiver 15 or another transmitter associated with the decoder or display device may send a corresponding layer drop indication back through the network 14. The layer drop indication may be received by each rendering node. A rendering node which generates partially or fully rendered frames for that specific decoder or display device may cease generating the dropped layer. On the other hand, a rendering node which generates partially or fully rendered frames for multiple end devices may disregard a layer drop indication received from one end device (as the dropped layer is still needed for other devices). Alternatively, rendering nodes which serve multiple end devices may record received layer drop indications, and may cease generating the dropped layer only when all end devices served by the rendering node indicate that the layer is to be dropped. In preferred examples, the encoders or decoders are part of a tier-based hierarchical coding scheme or format. Hierarchical coding enables frames to be communicated with higher resolution and / or higher frame rate than is possible in single-tier coding schemes. In hierarchical coding, one or more enhancement layers is communicated with base data, where the enhancement layers can be used to up-sample the base data at the decoder, for example providing up-sampling in a spatial or temporal dimension. When combined with equivalent down-sampling of the original frames and generation of the enhancement layer at an encoder, hierarchical coding can overall provide lossless compression of data, with higher resolution and / or higher frame rate for a given transmission bit rate. Examples of a tier-based hierarchical coding scheme include LCEVC: MPEG-5 Part 2 LCEVC (“Low Complexity Enhancement Video Coding”) and VC-6: SMPTE VC-6 ST-2117, the former being described in PCT / GB2020 / 050695, published as WO 2020 / 188273, (and the associated standard document) and the latter being described in PCT / GB2018 / 053552, published as WO 2019 / 111010, (and the associated standard document), all of which are incorporated by reference herein. However, the concepts illustrated herein need not be limited to these specific hierarchical coding schemes. A further example is described in WO2018 / 046940, which is incorporated by reference herein. In this example, a set of residuals are encoded relative to the residuals stored in a temporal buffer. LCEVC (Low-Complexity Enhancement Video Coding) is a standardised coding method set out in standard specification documents including the Text of ISO / IEC 23094-2 Ed 1 Low Complexity Enhancement Video Coding published in November 2021, which is incorporated by reference herein. The system describes above is suitable for generating and presenting a representation of a scene, where this scene displays media content to a user. The scene typically comprises an environment, where the user is able to move (e.g. to move their head or to turn their head) to look around the environment and / or to move around the environment. For example, the scene may be a scene of a room in a building, where the user is able to move around the room (e.g. by moving in the real-world and / or by providing an input to a user interface) in order to inspect various parts of the room. Typically, the scene is a XR (e.g. a VR) scene, where the user is able to move about the scene in three degrees of freedom (3DoF) or six degrees of freedom (6DoF) so as to experience the scene. As has been described with reference to Figure 1, the image generator 11 may be arranged to determine point cloud data, where each point of the point cloud has a 3D position and one or more attributes. More generally, the image generator (or another component) is arranged to determine a three-dimensional representation of a scene, where this three-dimensional representation is thereafter used to generate two- dimensional images that are presented to a user at the display device 17. While the points are typically points of a point cloud, more generally the disclosure extends to any point that is associated with a location and a value. Therefore, the points may, more generally, be considered to be data (or datapoints), which data is associated with a location and a value, and the ‘points’ may comprise polygons, planes (regular or irregular), Gaussian splats, etc. Referring to Figure 3, there is described a method of determining (an attribute for) a point of such a three- dimensional representation. The method comprises determining the attribute using a capture device, such as a camera or a scanner. The scene may comprise a real scene, in which attribute values are captured using a camera, or a virtual scene (e.g. a three-dimensional model of a scene), in which attribute values are captured using a virtual scanner. Where this disclosure describes ‘determining a point’ it will be understood that this generally refers to determining a point that has a location and an attribute value, where determining the point comprises determining the attribute value and / or storing a point that comprises at least an attribute value and a location value (these values may be indirect values, e.g. where the location is identified relative to another point). Once a plurality of points have been captured, these points can be stored as a three-dimensional representation (e.g. a point cloud) so as to enable the reconstruction of the three-dimensional scene based on this representation. Typically, the scene comprises a simulated scene that exists only on a computer. Such a scene may, for example, be generated using software such as the Maya software produced by Autodesk®. The attributes determined using the methods described herein may then depend on virtual objects located within the scene as well as a virtual lighting arrangement used in the scene. In a first step 31, a computer device initiates a capture process for a capture device, the capture process being initiated with an initial azimuth angle (e.g. of 0°) and an initial elevation angle (e.g. of 0°). In a second step 32, the computer device causes a point to be captured using the capture device at the current azimuth angle and current elevation angle. Capturing a point typically comprises assigning an attribute value to the point, which attribute value may, for example, be a color of the point and / or a transparency value of the point. Typically, the point has one or more color values associated with each of a left eye and a right eye of a viewer. Capturing the point may also comprise determining a normal value associated with the point, e.g. a normal of a surface on which the point lies. Typically, capturing the point further comprises determining a location of the point, e.g. by determining a distance of the point from the camera. In practice, determining the point may comprise sending a ‘ray’ from the capture device and then stepping through a computer model to determine which surface of the computer model is impacted by the ray. The color, transparency, and normal of this surface are then recorded alongside the distance of the surface from the capture device. In a third step, 33, the computer device determines whether a point has been captured for the capture device at each azimuth of a range of azimuths and in a fourth step 34, if points have not been captured at each azimuth, then the azimuth angle is incremented and the method returns to the second step 32 and another point is captured. The azimuth angle may, for example, be incremented by between 0.01° and 1° and / or by between 0.025° and 0.1°. Typically, the range of azimuth angles is selected to be 360° (i.e. so that the capture device captures points surrounding the entirety of the capture device), but it will be appreciated that other ranges are possible. Once a point has been captured for each azimuth, in a fifth step 35, the computer device determines whether a point has been captured for the capture device at each elevation of a range of elevations and in a sixth step 16, if points have not been captured at each elevation, then the azimuth angle is reset to the initial value, elevation angle is incremented and the method returns to the second step 32 and another point is captured. The elevation angles may, for example, be incremented by between 0.01° and 1° and / or by between 0.025° and 0.1°. Typically, the range of elevation angles is selected to be 360° (i.e. so that the capture device captures points surrounding the entirety of the capture device), but it will be appreciated that other ranges are possible. In a seventh step 37, once points have been captured for each azimuth angle and each elevation angle, the scanning process ends. This method enables a capture device to capture points at a range of elevation and azimuth angles. This point data is typically stored in a matrix. The point data may then be used to provide a representation of the scene to a user, e.g. the three-dimensional representation formed by the point data may be processed to produce two-dimensional images for each eye of a user, with these images then being shown to a user via the display device 17 to provide a virtual reality experience to the viewer. By using the captured data, a video can be provided to a viewer that enables the viewer to move their head to look around the scene (while remaining at the location of the capture device). It will be appreciated that the capture pattern (or scanning pattern) described with reference to Figure 3 is purely exemplary and that numerous capture patterns are possible. In general, the capture process for each capture device comprises capturing one or more points at one or more azimuth angles and / or one or more elevation angles. The ‘points’ captured by the capture device are typically associated with a size, such as a height, a width, or a depth. That is, the points typically relate to two-dimensional planes / pixels and / or three-dimensional voxels. In this regard, there is necessarily some space between the locations of adjacent points (since if the points had no width, then an infinite number of points would be required to capture points at each angle). The size provides points that depict a non-negligible area of the three-dimensional space so that a plurality of points can be fit together to provide a depiction of the scene to a viewer. The width and height of each point is typically dependent on the distance of that point from the capture device, where more distant points have a larger width / height. The width and height of each point is typically determined so that when each point is displayed, there is no space between adjacent points (indeed, there may be some overlap between points to ensure that no gaps appear between points). This height / width of each point can be determined at the time of capturing the points, or can be determined or defined after the capture of the points. Typically, the points comprise a size value, which is stored as a part of the point data. For example, the points may be stored with a width value and / or a height value. Typically, the minimum width and the minimum height of a point are set by the angle increment of the azimuth angle and the elevation angle respectively. The size may be then specified in terms of this angle increment and / or in terms of this minimum width / minimum height (e.g. as being a multiple of the angle increment). In some embodiments, the size value is stored as an index, which index relates to a known list of sizes (e.g. if the size may be any of 1x1, 2x1, 1x2, 2x2, pixels this may be specified by using 3 bits and a list that relates each combination of bits to a size). The size may be stored based on an underscan value. In this regard, where an object is very near to the viewing zone it may be captured using an unnecessarily dense arrangement of points. Therefore, certain surfaces or areas of the representation may be associated with an underscan value, which underscan value defines a reduction in the number of points captured as compared to a representation without underscan. The size of the points may be defined so as to indicate this underscan value. In an exemplary embodiment, the underscan value is an integer value between 0 and 3 and the size is stored as a combination of point dimensions (e.g. a width in the range [0,2]) and a height in the range ([0,2]) and an underscan factor (e.g. an underscan factor in the range [0,3]). In some embodiments, the width and the height are dependent on the underscan factor. For example, when the underscan factor exceeds a threshold value, the possible height and width values may be limited. In a specific example, when the underscan factor is 3, the width and the height may be limited to the range [0,1]. The size may then be defined as size = underscan*9 + height*3 + width. Such a method provides efficient storage and indication of width, height, and underscan values. As shown in Figure 4a, typically, for each capture step (e.g. each azimuth angle and / or each elevation angle), a plurality of sub-points SP1, SP2, SP3, SP4, SP5 is determined. For example, where the azimuth angle increment is 0.1° then for an azimuth angle of 0°, sub-points may be determined at azimuth angles of -0.05°, -0.025°, 0, 0.025°, and 0.05° (and similar sub-points may be determined for a plurality of elevation angles). Attribute values of these sub-points may then be combined to obtain an attribute value for the point. For example, a maximum attribute value of the sub-points may be used as the value for the point, an average attribute value of the sub-points may be used as the value for the point, and / or a weighted average of the sub-points may be used as the value for the point. It will be appreciated that numerous other methods for combining the attribute values of the sub-points are possible. By determining the attribute of a point based on the attributes of sub-points, the accuracy of the capture process can be increased. While it would be possible to simply reduce the increment of the angle steps to provide a higher resolution scene, by considering sub-points but only storing attributes for points, a balance can be struck between accuracy and file size (since storing every sub-point would lead to a substantial increase in the amount of data that needs storing). With the example of Figure 4a, for each point of the three-dimensional representation that is captured by a capture device, this capture device may obtain attributes associated with each of the sub-points SP1, SP2, SP3, SP4, SP5, combine these attributes to obtain a point attribute, and then store a point with a distance that is an average (e.g. a weighted average) of the distances of the sub-points from the capture device, at the nominal angle of the point, with the point attribute. As shown in Figure 4b, where a plurality of sub-points SP1, SP2, SP3, SP4, SP5 are considered, these points may have different distances from the location of the capture device. In some embodiments, the attributes of the sub-points may be combined in dependence on this distance, e.g. so that sub-points nearer to the capture device have higher weightings. However, the possibility of sub-points with substantially different distances raises a potential problem. Typically, in order to determine a distance for a point, the distances for the sub-points are averaged. But where the sub-points have substantially different distances and / or are related to different surfaces in the scene, this may result in the point having a distance that does not correspond to any actual surface in the scene. Therefore, the point may seem to hang in space (e.g. to hang between the front and rear surfaces shown in Figure 4b. Similarly, where the attribute values of the sub-points greatly differ, e.g. if the sub-points SP1 and SP2 are white in colour and the sub-points SP3 and SP4 are black in colour, then the attribute value of the point may be substantially different to the attribute value of other points in the scene. In an example, if the scene were composed of black and white objects, the point may appear as a grey point hanging in space between these objects. In some embodiments, the computer device is arranged to aggregate sub-points so as not to create any floating points. For example, the computer device may determine whether the sub-points are spatially coherent by employing a clustering algorithm (e.g. a k-means clustering algorithm). Where the sub-points are spatially coherent (e.g. where a difference in the distance of the sub-points is below a threshold value), these distances may be averaged to obtain a distance for the point. Where the sub-points are not spatially coherent, the sub-points may be processed to ensure that the distance of any point places it upon a surface; for example, in the system of Figure 4b, sub-points SP1, SP2, and SP3 may be grouped into a first point and sub-points SP4 and SP5 may be grouped into a second point. Since each sub-point is associated with the same capture device and capture angle (all of these sub-points being associated with a capture step that has a particular azimuth angle and elevation angle), these points may be located at the same angle with respect to a capture device. Therefore, to ensure that each sub-point affects the representation considered, the first point (made up of sub-points SP1, SP2, and SP3) may have a smaller distance value than the second point (made up of sub-points SP4 and SP5) and the first point may be assigned a non- zero transparency value so that the second point can be seen through the first point. By capturing points at a plurality of azimuth angles and elevation angles, e.g. using the method described with reference to Figure 3, it is possible to provide a three-dimensional representation of the scene that can later be used to enable a viewer to view the scene from a plurality of angles. More specifically, given the three-dimensional points captured by the capture device, a computer device is able to render a two- dimensional representation (e.g. a two-dimensional image) of the scene for each eye of a viewer so as to provide a representation with an impression of depth. The computer device may render a series of two- dimensional representations to enable the viewer to look around the scene, where the two-dimensional representations are rendered based on an orientation of the viewer’s head. In this way, the determined representation is useable to provide, for example, a virtual reality (VR), mixed reality (MR), augmented reality (AR), and / or extended reality (XR) experience to the viewer. To enable such a display, the display device 17 is typically a virtual reality headset, that comprises a plurality of sensors to track a head movement of the user. By tracking this head movement, the display device is able to update the images being displayed to the viewer as the viewer moves their head to look about the scene. Typically, this involves the display device sensing the sensor data to an external computer device (e.g. a computer connected to the display device via a wire). The external computer device may comprise powerful graphical processing units (GPUs) and / or computer processing units (CPUs) so that the external computer device is able to rapidly render appropriate two-dimensional images for the viewer based on the three-dimensional images and the sensor data. In some embodiments, the external computer device may comprise a server device, where the display device 17 may be connected to this server device wirelessly. This enables the two-dimensional images to be streamed from the server to the display device so as to enable the display of high-quality images without the need for a viewer to purchase expensive computer equipment. In other words, operations that require large amounts of computing power, such as the rendering of two-dimensional images based on the three- dimensional representation, may be performed by the server, so that the display device is only required to perform relatively simple operations. This enables the experience to be provided to a wide range of viewers. In some embodiments, a first two-dimensional image is provided to the display device 17 (and / or a connected device) and this first image is ‘warped’ in order to provide an image for viewing at the display device. The warping of the image comprises processing the image based on the sensor data in order to provide an image that matches a current viewpoint of the viewer. By performing the warping at the display device or another local device, the lag between a head movement of the user and an updating of the two- dimensional representation of the scene can be reduced. One issue with the above-described method of capturing a three-dimensional representation is that it only enables a viewer to make rotational movements. That is, since the points are captured using a single capture device at a single capture location, there is no possibility of enabling translational movements of a viewer through a scene. This inability to move translationally can induce motion sickness within a viewer, can reduce a degree of immersion of the viewer, and can reduce the viewer’s enjoyment of the scene. Therefore, it is desirable to enable translational movements through the scene. To enable such movements, the three-dimensional representation of the scene may be captured using a plurality of capture devices placed at different locations (or the same capture device placed at different locations). A viewer is then able to move around the scene translationally (e.g. by moving between these locations). More generally, by capturing points for every possible surface that might be viewed by a viewer, a three- dimensional representation of a scene may be captured that allows a suitable two-dimensional representation of this scene to be rendered regardless of a location of a viewer (e.g. regardless of where a user is standing within a virtual room). This need to capture points for every possible surface (so as to enable movement about a scene) greatly increases the amount of data that needs to be stored to form the three-dimensional representation. Therefore, as has been described in the application WO 2016 / 061640 A1, which is hereby incorporated by reference, the three-dimensional representation may be associated with a viewing zone, or a zone of viewpoints (ZVP), where the three-dimensional representation is arranged to enable a user to move about the viewing zone so as to view the scene. Figure 5 illustrates such a viewing zone 1 and illustrates how the use of a viewing zone limits the amount of image data that needs to be stored to provide a three-dimensional representation of the scene. With the scene shown in this figure, and the viewing zone 1 shown in this figure, it is not necessary to determine attribute data for the occluded surface 2 since this occluded surface cannot be viewed from any point in the viewing zone. Therefore, by enabling the user to only move within the viewing zone (as opposed to around the whole scene) the amount of data needed to depict the scene is greatly reduced. While Figure 5 shows a two-dimensional viewing zone, it will be appreciated that in practice the viewing zone 1 is typically a three-dimensional zone or volume. The viewing zone 1 may, for example, comprise a rectangular volume, or a rectangular parallelepiped, and the viewing zone may have a height of at least 30 cm, a depth of at least 30 cm, and / or a width of at least 30 cm, where these dimensions enable a user to move their head while remaining in the viewing zone. This is merely an exemplary arrangement of the viewing zone; it will be appreciated that viewing zones of various shapes and sizes may be used (e.g. spherical viewing zones). That being said, it is preferable that the viewing zone is limited so as to cover only a part of the volume of the scene, e.g. no more than 50% of the scene no more than 25% of the scene, and / or no more than 10% of the scene. In this regard, if the viewing zone is the same size as the scene, then the three-dimensional representation will simply be a standard representation for virtual reality (that enables a user to move freely about the scene) – and so the use of the viewing zone will not provide any reduction in file size. The viewing zone 1 enables movement of a viewer around (a portion of) the scene. For example, where the scene is a room, the base representation may enable a user to walk around the room so as to view the room from different angles. In particular, the viewing zone enables a user to move through the scene with six degrees-of-freedom (6DoF) movement through the scene, where this aids in the provision of an immersive experience. In some embodiments, the viewing zone 1 may be four-dimensional, where a three-dimensional location of the viewing zone changes over time – and in such embodiments the size and location of the occluded surface 2 may also change over time. More generally, it will be appreciated that viewing zones may be formed in any size or shape, with different sizes and shapes being suitable for different scenes. The volume of the viewing zone 1 is typically selected so that a user is able to move to a degree sufficient to avoid motion sickness and to provide an immersive sensation, while still only enabling a limited amount of movement (where this leads to a smaller file size as compared to an implementation where a user is able to fully move about the scene). Typically, the viewing zone is arranged to enable a user to move their head while they are sitting or standing, but not to freely roam around a room. The viewing zone 1 may have a (e.g. real-world) volume of less than five cubic metres (5m3), less than one cubic metre (1m3), less than one-tenth of a cubic metre (0.1m3) and / or less than one-hundredth of a cubic metre (0.01m3). The viewing zone 1 may also have a minimum size, e.g. the viewing zone may have a volume of at least 1% of the volume of the scene, at least 5% of the volume of the scene, and / or at least than 10% of the volume of the scene. Similarly, the viewing zone may have a volume of at least one-thousandth of a cubic metre (0.01m3); at least one-hundredth of a cubic metre (0.01m3); and / or at least one cubic metre (1m3). The ‘size’ of the viewing zone 1 typically relates to a size in the real world, where if the viewing zone has a length of one metre this means that a user is able to move one metre in the real world while staying within the viewing zone. The size of the viewing zone in the scene may be greater than, equal to, or less than the size of the viewing zone in the real world. For example, the viewing zone may scale a real-world distance so that moving one metre in the real world moves the user less than (or more than) one metre in the scene. This enables the scene to provide different perceptions to the user (e.g. to make the user feel larger or smaller than they are in real life). Similarly, the viewing zone may scale a real-world angle so that rotating one degree in the real world rotates the user less than (or more than) one degree in the scene. Therefore, a viewing zone with a volume of one cubic metre typically connotes a viewing zone in which the user is able to move about a one cubic metre volume in the real world while remaining in the viewing zone. And this may cause the user to move about a volume that is more than, or less than, one metre in the scene. Referring to Figure 6a, in order to capture points for each surface and location that is visible from the viewing zone 1, a plurality of capture devices C1, C2, …, C9 may be used (e.g. a plurality of virtual scanners and / or a plurality of cameras). Each capture device is typically arranged to perform a capture process, e.g. as described with reference to Figure 3, in which the capture device captures points at a plurality of azimuth angles and elevation angles. By locating the capture devices appropriately, e.g. by locating a capture device at each corner of the viewing zone, it can be ensured that most (or all) points of a scene are captured. Typically, a first capture device C1 is located at a centrepoint of the viewing zone 1. In various embodiments, one or more capture devices C2, C3, C4, C5 may be located at the centre of faces of the viewing zone; and / or one or more capture devices C6, C7, C8, C9 may be located at edges of and / or corners of the viewing zone. Figure 6a shows a two-dimensional view (e.g. a plan view) of a rectangular viewing zone. It will be appreciated that within this viewing zone each capture device may be located on a shared plane. Equally, the various capture devices may be located on different planes. Referring, for example, to Figure 6b, there is shown a three-dimensional view of a cuboid viewing zone, where there is a capture device located: at the centre of the viewing zone; at the centre of each face of the viewing zone; and at each corner of the viewing zone. With this arrangement, many locations in the scene (e.g. specific surfaces) will be captured by a plurality of capture devices so that there will be overlapping points relating to different capture devices. This is shown in Figure 7, which shows a first point P1 being captured by each of a first capture device C1, a sixth capture device C6, and a seventh capture device C7. Each capture device captures this point at a different angle and distance and may be considered to capture a different ‘version’ of the point. Typically, only a single version of the point is stored, where this version may be the highest quality version of the point and / or may be the version of the point associated with the nearest and / or least angled capture device. In this regard, the highest ‘quality’ version of the point is captured by the capture device with the smallest distance and smallest angle to the point (e.g. the smallest solid angle). In this regard, as described with reference to Figures 4a and 4b, capturing a point for a given azimuth angle and elevation angle typically comprises capturing a plurality of sub-points at varying sub-point azimuth and elevation angles spread around the point azimuth and elevation angles. Due to the different spreads of sub-points, each capture device will capture a different version of the point (that has a different attribute) even when the points are at the same location. Capture devices that are close to the point and less angled with respect to the point typically have a smaller spread of sub-points and so typically obtain a version of a point that is sharper than a version of that point captured by more distant capture devices. In some embodiments, a quality value of a version of the point is determined based on the spread of sub- points associated with this version (e.g. based on the perimeter formed by these sub-points and / or based on a surface area or volume bounded by these sub-points). The version of the point that is stored may depend on the respective quality values of possible versions of the points. Regarding the ‘versions’ of the points, it will be appreciated that two ‘points’ in approximately the same location captured by each capture device may not have exactly the same location in the three-dimensional representation. More specifically, since each capture device typically projects a ‘ray’ at a given angle, the rays of differing capture devices may contact the surface at different locations for each capture device. Two points may be considered to be two ‘versions’ of a single point when they are within a certain proximity, e.g. a threshold proximity. For example, where the first capture device C1 captures a first point and a second point at subsequent azimuth angles, and the sixth capture device C6 captures a further point that is in between the locations of the first point and the second point, this further point may be considered to be a ‘version’ of one of the first point and the second point. This difference in the points captured by different capture devices is illustrated by Figures 8a and 8b, which show the separate captured grids that are formed by two different capture devices. As shown by these figures, each capture device will capture a slightly different ‘version’ of a point at a given location and these captured points will have different sizes. Each capture step is associated with a particular range of angles (e.g. a nominal capture angle of 1° might encompass angles from 0.9° to 1.1°), and therefore capture devices that are far from a point to be captured represent a wider region at the capture distance than capture devices closer to that point to be captured. As shown in Figure 8a, the capture device C1 would capture the points P1 and P2 in separate brackets, whereas for the capture device C2 these points are in the same bracket. Therefore, the capture device C2 might determine a single point that encompasses both points P1 and P2, whereas the capture device C1 would determine separate points for these two points. Considering then a situation in which points P1 and P2 are captured separately, and capture device C1 is used to capture point P1 while capture device C2 being used to capture point P2, it should be apparent that the ‘sizes’ of these captured points, and the locations in space that are encompassed by the captured points will be based on different grids. For example, the width of the captured point P2 captured by the capture device C2 will be larger than the width of the captured point P1 captured by the capture device C1. The capture process may be determined based on the existence of these different grids, and on the different bracket widths that occur at different distances from a capture device. Figure 8a shows an exaggerated difference between grids for the sake of illustration. Figure 8b shows a more realistic embodiment in which the three-dimensional representation comprises a plurality of points associated with different capture devices, where these points lie on different grids associated with these different capture devices. In order to store the points of the three-dimensional representation, the points may be stored as a string of bits, where a first portion of the string indicates a location of the point (e.g. using x, y, z coordinates) and a second portion of the string locates an attribute of the point. In various embodiments, further portions of the string may be used to indicate, for example, a transparency of the point, a size of the point, and / or a shape of the point. A computer device that processes the three-dimensional representation after the generation of this representation is then able to determine the location and attribute of each point so as to recreate the scene. This location and attribute may then be used to render a two-dimensional representation of the scene that can be displayed to a viewer wearing the display device 17. Specifically, the locations and attributes of the points of the three-dimensional representation can be used to render a two-dimensional image for each of the left eye of the viewer and the right eye of the viewer so as to provide an immersive extended reality (XR) experience to the viewer. The present disclosure considers an efficient method of storing the locations of the points (e.g. at an encoder) and of determining the locations of the points (e.g. at a decoder). As has been described with reference to Figures 5a and 5b, the points of the three-dimensional representation are determined using a set of capture devices placed at locations about the viewing zone, where these capture devices are arranged to capture points at a series of azimuth angles and elevation angles. Typically, each of the capture devices is arranged to use the same capture process (e.g. the same series of azimuth angles and elevation angles), though it will be appreciated that different series of capture angles are possible. For example, there may be a plurality of possible series of capture angles, where different capture devices use different capture angles. In general, the present disclosure considers a method in which points are stored based on a capture device identifier and an indication of a distance of the point from the capture device associated with this capture device identifier. Typically, the point is also associated with an angular indicator, which indicates an azimuth angle and / or an elevation angle of the point relative to the identified capture device. It will be appreciated that the storage of the distance and the angle may take many forms. For example, the distance and the angle of each point may be converted into a universal coordinate system, where each capture device has a different location in this universal coordinate system. In particular, each point may be stored with reference to a centre of this universal coordinate system, which centre may be co-located with a central capture device. Where a point is determined based on a distance and an angle from a capture device of a known location in this universal coordinate system, the coordinates of the point in this universal coordinate system can be determined trivially – and the location of the point may then be stored either relative to the capture device or as a coordinate in the universal coordinate system. The capture device identifier may comprise a location of a capture device (e.g. a location in a co-ordinate system of the three-dimensional representation). Equally, the capture device identifier may comprise an index of a capture device. Similarly, the indication of the azimuth angle and the elevation angle for a point may comprise an angle with reference to a zero-angle of a co-ordinate system of the three-dimensional representation. Equally, the azimuth angle and / or the elevation angle may be indicated using an angle index. In some embodiments, the three-dimensional representation is associated with configuration information, which configuration information comprises one or more of: a set of capture device indexes; locations associated with the capture devices and / or the capture device indexes; a spacing of capture devices (e.g. so that locations of the capture devices can be determined from a location of a first capture device and the spacing); angles associated with a capture process for the capture devices; an azimuth angle increment and / or an elevation angle increment associated with the capture process; and a set of angle indexes (e.g. to match an angle index to an angle). With this configuration information, it is possible to determine a location of each capture device from an index of that capture device and / or to determine a capture angle from a known capture process. Therefore, given two numbers: a capture device index and an angle index (that is associated with a combination of a specific azimuth angle and a specific elevation angle), a location of a capture device and a direction of a point from this capture device can be determined. By also signalling a distance of the point from the signalled capture device, a precise location of the point in the three-dimensional space can be signalled efficiently. Typically, the point is associated with each of: a camera index, a distance, an first angular index (e.g. a first azimuth), and a second angle (e.g. a second elevation) This method of indicating a location of a point enables point locations to be identified using a much smaller number of bits than if each point location is identified using x, y, z coordinates. Referring to Figure 9, there is shown a method of determining a location of a point. This method is carried out by a computer device, e.g. the image generator 11 and / or the decoder 15. In a first step 41, the computer device identifies an indicator of a capture device used to capture the point. Typically, this comprises identifying a portion of a string of bits associated with a capture device index. In a second step 42, the computer device identifies an indicator of an angle of the point from the capture device. Typically, this comprises identifying an angle index, e.g. an azimuth index and / or an elevation index and / or a combined azimuth / elevation index, which index(es) identifies a step of the capture process during which the point was captured. In a third step 43, based on the identifiers, the computer device determines the location of the capture device and the angle of the point from the capture device. The capture device identifier is typically a capture device index, which is related to a capture device location based on configuration information that has been sent before, or along with, the point data. For example, the configuration information may specify: - Location of first capture device is (0,0,0). - Step between capture devices is (0,0,1) along the grid, then across the grid, then up the grid. - The grid is (10,10,10). With this information, a capture device with an index of 1 can be determined to be located at (0,0,0); a capture device with an index of 5 can be determined to be located at (0,0,4); a capture device with an index of 12 can be determined to be located at (0,1,0), and so on. Equally, the configuration information may specify a list of camera indexes and locations associated with these indexes, where this enables the use of a wide range of setups of capture devices. Typically, the three-dimensional representation is associated with a frame of video. The configuration information may be constant over the frames of the video so that the configuration information needs to be signalled only once for an entire video. Therefore, the configuration information may be transmitted alongside a three-dimensional representation of a first frame of the video, with this same information being used for any subsequent frames (e.g. until updated configuration information is sent). The angle identifier may similarly be related to an angle by a location and an increment that are signalled in a configuration file. For example, the configuration information may specify: - An azimuth increment and an elevation increment are each 1°. - There are 359 increments for each angle type. With this information: a capture angle with an index of 1 can be determined to be at an azimuth angle of 0° and an elevation angle of 0°; a capture angle with an index of 10 can be determined to be at an azimuth angle of 10° and an elevation angle of 0°; a capture angle with an index of 360 can be determined to be at an azimuth angle of 0° and an elevation angle of 1°; and a capture angle with an index of 370 can be determined to be at an azimuth angle of 9° and an elevation angle of 1°; etc. In a fourth step 44, based on the determined location of the capture device and the determined angle, a location of the point is determined. Typically, this comprises determining the location of the point based on the location of the capture device, the capture angle, and a distance of the point from the capture device (where this distance is specified in the point data for the point). Determining the location of the point typically comprises determining the location of the point relative to a centrepoint of the three-dimensional representation. This location of the point may then be converted into a desired coordinate system and / or the point may be processed based on its location (e.g. to stitch together adjacent points). The angular identifier typically comprises a first angular identifier and a second angular identifier, where the first identifier provides the azimuthal angle of the point and the second identifier provides the elevation angle of the point. Referring to Figure 10, each angular identifier may be provided as an index of a segment of the three- dimensional representation, where, for example, an index of 0 may identify the point as being in a first angular bracket 101 and an index of 1 may identify the point as being in a second angular bracket 102. In this regard, the capture devices are arranged to perform a capture process, e.g. as described with reference to Figure 3, with a non-infinite angular resolution. Given this non-infinite resolution, each point is not a one-dimensional point located at a precise angle. Instead, each point is a point for a particular area of space, with the size of this area being dependent on the angular resolution as well as the distance of the point from the capture device. In other words, each capture angle determines a point for an angular range (with the range being dependent on the angular resolution). That is, if the capture process leads to points being captured at angles of 10°, 11°, and 12° then this can equally be considered to relate to points being captured at a first range of 9.5°-10.5°, a second range of 10.5°-11.5°, and a third range of 11.5°-12.5°. This is shown in Figure 10, which shows a series of angular brackets, with the size of these angular brackets at a given distance being dependent on the angular resolution. The angular identifier(s) typically comprise a reference to such an angular bracket. Consider, for example, a cube placed with the capture device C1 at the centre of this cube. By dividing this cube into x segments at regular azimuth angles and y segments at regular elevation angles, it is possible to identify any angular range of the representation by reference to an x segment and a y segment (and then the space bracketed by this angular range will depend on both the angular resolution (e.g. the angle between adjacent brackets) and the distance of the point from the capture device). Typically, each capture device has the same capture pattern so that the angular bracketing of each device is the same (albeit centred differently at the location of the relevant capture device). For example, in an embodiment with 1000 equal angular brackets, the angle for each bracket may be 360 / 1000. In some embodiments, different capture devices are associated with different capture patterns, where this may be signalled in configuration information relating to the three-dimensional representation. In some embodiments, each capture device is arranged to capture a point for a plurality of angular brackets, where each bracket is associated with a different angle. The angular spread of each bracket (that is, the angle between a first, e.g. left, angular boundary of the bracket and a second, e.g. right, angular boundary of the bracket) may be the same; equally, this angular spread may vary. In particular, the angular spread may vary so as to be smaller for points which are directly in front of (or behind, or to a side of) the capture device. For example, the embodiment shown in Figure 7 shows an angular bracketing system that is based on a cube. With this system, a cube is placed such that a capture device is located at the centre of the cube and the cube is then split into 1000 sections of equal size (it will be appreciated that the use of 1000 sections is exemplary and any number of sections may be used). Each of these sections is then associated with an angular index. With this arrangement, the angular spread of each section (or bracket) varies, as has been described above. Figure 10 shows a two-dimensional square, where each angular bracket of the square is referenced by an index number (between 1 and 100). In a three-dimensional implementation, an angular bracket of a cube could be indicated with two separate numbers (with a first azimuthal indicator that identifies a ‘column’ of the cube and a second elevational indicator that identifies a ‘row’ of the cube). Equally, a singular indicator may be provided that indicates a specific bracket of the cube. Therefore, for a cube that is divided into 1000 elevational sections and 1000 azimuthal sections, the bracket may be indicated with two separate indicators that are each between 0 and 999 or with a single indicator that is between 0 and 999999. It will be appreciated that the use of a cube to define the brackets is exemplary and that other bracketing systems are possible. For example, a spherical bracketing system may be used (where this leads to curve angular brackets). Equally, a lookup table may be provided that relates angular indexes to angles, where this enables irregularly spaced brackets to be used. Typically, determining the location of the point comprises determining the location of the point so as to be at the centre of the angular bracket identified by the angular identifier(s). Depth buffers In order to render a two-dimensional image that can be presented to a viewer so that the viewer can view the scene, a computer device (e.g. the image generator 11 and / or the display device 17) determines a plurality of pixel values for the two-dimensional image based on the points of the three-dimensional representation. This may be considered the rendering of a point of a two-dimensional image. This rendering process may comprise rasterization, in which shapes within the three-dimensional representation are converted into raster images within the two-dimensional image. Referring to Figures 11a and 11b, there is shown an exemplary scene for which a plurality of surfaces (e.g. points of the three-dimensional representation) are being rendered to form a two-dimensional image for displaying to a user. Figure 11a shows a scene with a background surface BS1, a first and second opaque surface OS1, OS2, and a first, second, and third non-opaque (e.g. transparent or translucent) T1, T2, T3 surface. These non-opaque surfaces may, for example, be associated with glass surfaces in the scene. In the description below, these surfaces will be referred to as translucent surfaces for the sake of an example. Figure 11b illustrates how a (line of a) two-dimensional image may be formed based on this scene. Specifically, Figure 11b shows a plurality of pixel brackets that may be used to determine values for individual pixels of the two-dimensional image based on the points of the three-dimensional representation. Each pixel bracket is associated with a segment of the three-dimensional representation. While Figure 11b shows a plurality of parallel pixel brackets, it will be appreciated that in practice these pixel brackets are dependent on a position of a viewer of the scene (e.g. the pixel brackets may be arranged similarly to the angular brackets of the points of the three-dimensional representation that have been described above, where the pixel brackets spread out from a position of a viewer). Figure 11b shows how each pixel bracket may cover a plurality of surfaces. For example, for the second pixel, there are two opaque surfaces OS1, OS2 present in a bracket associated with that pixel. These surfaces return attribute (e.g. colour) values P2b and P2a. Since the surface OS1 associated with the pixel P2a is opaque, it is the value of this pixel P2a that should be shown to the user (and not the value of the pixel P2b). To render an image based on the surfaces, a computer device may consider each pixel bracket and consider the points / surfaces within that pixel bracket in order to identify the foremost surface. A value for this pixel bracket can then be rendered based on this foremost surface. Referring then to Figure 12, there is described such a method of determining a value of a pixel in a two- dimensional representation. This method is applicable to a situation in which there are a plurality of (possibly overlapping) surfaces associated with points of the three-dimensional representation. It will be appreciated that each point of the three-dimensional representation may be associated with a single pixel value in a rendered two-dimensional image, but equally one or more of the points of the three-dimensional representation may be associated with a plurality of pixel values (depending on the resolution of the two- dimensional image). This method of Figure 12 is typically carried out by a computer device, e.g. the image generator 11 and / or the display device 17. In a first step 51, the computer device initialises a depth buffer (or a ‘z’ buffer). The depth buffer typically comprises a two-dimensional matrix, where each pixel of the two-dimensional image is associated with an entry or a value of the depth buffer (e.g. so that interrogating the depth buffer provides a depth value for each pixel of the image). The methods below are typically described with reference to a single (or a few) pixel value(s) and a single (or a few) corresponding value(s) of the depth buffer, but it will be appreciated that typically these methods comprise determining a value for each pixel and for each entry of the depth buffer. The depth buffer is initialised with an initial value initially stored in (each entry of) the depth buffer, where this initial value is typically a maximum value and / or a value that is greater than a maximum distance of an object of the three-dimensional representation from the viewing zone. This ensures that any surface that is evaluated (as described below) will have a depth that is lower than the initial value in the buffer. In a second step 52, the computer device identifies a next surface in the three-dimensional representation, which next surface lies within a pixel (or a pixel ‘bracket’) that is being considered. Using the second pixel bracket for the sake of example, the computer device may identify the second opaque surface OS2. The ‘surface’ is associated with a point of the three-dimensional representation, where the computer device may iterate through the points of the three-dimensional representation to determine one or more points that lie within the pixel bracket and the evaluate these points. In a third step 53, the computer device determines whether the depth of this surface is less than an present value that is saved in a corresponding entry of the depth buffer (e.g. the entry of the depth buffer that is associated with this pixel bracket). If the depth of the surface is less than the saved value in the depth buffer, then this means that the surface is closer to the viewer than any previously considered surfaces. Therefore, in a fourth step 54, the computer device updates a pixel value of the two-dimensional image and saves the depth of the surface into the depth buffer. For example, the computer device may identify that the distance to the second opaque surface OS2 is less than the initial maximum value that is stored in the depth buffer and the computer device may then save this depth into the depth buffer and update a pixel value for the second pixel bracket based on this second opaque surface. For example, the pixel value may be set to the value of P2b. The computer device then returns to the second step 52 and identifies a next surface in the pixel bracket, e.g. the first opaque surface OS1. Since the depth of this first opaque surface (the distance of the surface from the viewer) is less than the value stored in the depth buffer (which value is, continuing with the above example, the depth of the second opaque surface OS2), the computer device updates the value in the depth buffer and also updates the pixel value based on the first opaque surface. In particular, the pixel value may be updated to be the value of P2a, which value is associated with the first opaque surface. Example of the use of a z-buffer for a rasterization process (albeit in different, purely two-dimensional, contexts) are described at https: / / web.eecs.utk.edu / ~huangj / cs452 / notes / 452_rasterization.pdf and https: / / www.scratchapixel.com / lessons / 3d-basic-rendering / rasterization-practical-implementation / visibility- problem-depth-buffer-depth-interpolation.html. In some embodiments, the computer device is arranged to implement an ordering step prior to the performance of the method of Figures 12, where this ordering step typically involves ordering the points by a depth of these points (e.g. a distance of the points from a viewing location associated with the two- dimensional image). This may involve ordering the points from high depth to low depth or from low depth to high depth. Ordering from high depth to low depth may simplify the rendering of non-opaque surfaces (as described further below), since this enables an opaque surface behind a non-opaque surface to be rendered first so that consideration of the non-opaque surface can modify an existing value. The above-described use of a depth buffer enables suitable pixel values to be determined that show the closest surfaces to a user. The depth buffer also provides a convenient map that indicates the distances of the surfaces associated with each pixel. In some embodiments, the depth buffer may be determined and output (with or without any determining or rendering of pixel values), where this depth buffer may be used to analyse an image and / or to determine suitable effects / overlays that may be combined with the image. Furthermore, while the method of Figure 12 has described a method in which a depth buffer and pixel values are determined simultaneously, these two data structures may instead be determined at different times. For example, the depth buffer may be determined at a first time and then the pixel values may be determined at a later, second, time based on the determined depth buffer. A potential issue with this method of rendering a two-dimensional image is the presence of non-opaque (e.g. transparent or translucent) surfaces. In such a situation, updating the depth buffer based on the depth of the transparent surface may result in a mis-determination of the value of a pixel. For example, for the fifth pixel bracket, if the first translucent surface T1 associated with the value P5a is evaluated prior to the background surface BS1 associated with the value P5b, then the first value recorded in the depth buffer will be the (relatively low) depth of the first translucent surface and so the value P5b of the background surface will not be rendered. This will lead to an inaccurate two-dimensional image (where the user is able to see through the translucent surface T1 but is not able to see the background surface behind this translucent surface). Therefore, referring to Figure 13, there is described a method of determining a depth buffer that accounts for non-opaque (e.g. transparent or translucent) surfaces. It will be appreciated that any features described herein with reference to transparent or translucent surfaces may more generally be applied to any non- opaque surfaces. The first step 61, the second step 62, and the third step 63 of the method of Figure 13 are the same as the first step 51, the second step 52, and the third step 53 of the method of Figure 12. In a fourth step 64, if the depth of the surface is less than a saved value in the depth buffer, then the computer device updates a pixel value. Where the surface is an opaque surface, updating the pixel value may comprise overwriting an existing pixel value. Where the surface is a non-opaque surface, updating the pixel value may comprise applying a mask or a modification to the pixel value based on the surface value (e.g. a translucent surface may alter a colour of an existing pixel value without entirely overwriting this value). In a fifth step 65, the computer device determines whether the surface is opaque. If the surface is opaque, then in a sixth step 66 the computer device saves the depth of the surface into the depth buffer. If the surface is not opaque, then the computer device returns to the second step 62 without updating the value in the depth buffer. Considering again the example above, in which the first translucent surface T1 is considered before the background surface BS1, with the method of Figure 13 the evaluation of the first translucent surface would not result in the depth buffer being update and therefore the computer device would still render the pixel based on the background surface BS1. This method of Figure 13 therefore provides an image with pixel values as shown in Figure 14. Typically, the method of determining the depth buffer (and / or rendering one or more pixels or an image) comprises determining a first set of opaque points and a second set of non-opaque points. The computer device may then first determine a depth buffer (and / or render one or more pixels) based on the opaque points and then update the depth buffer (and / or the image) based on the non-opaque points. This method ensures that the opaque points are each considered before the non-opaque points. In some embodiments, the method of determining the depth buffer (and / or rendering an image) comprises ordering the points before the determination of the depth buffer based on a depth of the points. In particular, the method may comprise ordering the points based on a distance of the points from a viewing position of a viewer and / or from a viewing zone of the scene. Typically, the points are ordered from furthest to nearest, so that the computer device first considers distant points before either overwriting or modifying an image based on nearer points. Overlapping points With the three-dimensional representation that has been described herein, one or more pairs of adjacent points may be defined so as to overlap. For example, each point may be defined to cover an area that is (e.g. a little) larger than a width / height of an angular bracket associated with that point (with the width / height being the width / height of the angular bracket at the location of the point). The use of such overlapping points – which can be seen in Figures 11a, 11b, and 14 as the overlap between the first and second translucent surfaces T1, T2 ensures that there are no gaps between the points of the three-dimensional representation (and this overlap ensures the lack of any gaps between surfaces, whether opaque or non-opaque). Figures 11a and 11b and 14 are arranged to clearly show the overlap between the two translucent surfaces T1, T2 and so these surfaces are shown with differing distances. In practice, the translucent surfaces are typically located at similar depths (so as to join a continuous surface). This is shown in Figure 15, which shows the first and second translucent surfaces T1, T2 and an overlap O1 that is formed by overlapping portions of the first and second surfaces. Where the points are opaque, the overlapping of the points typically has no substantial effect since rendering two similarly-coloured opaque pixels at the same location provides the same effect as rendering a single pixel of this colour. However, as shown in Figure 15, when the points are non-opaque (e.g. are translucent), rendering pixels based on both points can have an undesirable effect on the final image. In practice, this situation typically occurs where there are a plurality of points in the three-dimensional representation that are located on a single translucent (e.g. glass) surface. These points may overlap in order to ensure that the image contains a single unbroken surface, but, problematically, the pixels at the location of the overlap may be a different colour than the pixels adjacent the overlap (since at the overlap, the viewer is effectively looking through two translucent surfaces instead of one). Therefore, referring to Figure 16a, there is described a method of determining a plurality of depth buffers and / or determining values for pixels of a two-dimensional image. As with Figures 12 and 13 above, while this method of Figure 16a describes determining depth buffers and the pixel values at the same time, it will be appreciated that the method may involve determining only one of these features (e.g. the method of Figure 16a may be used only to obtain the depth buffers). Ina first step 71, the computer device initialises both a first depth buffer and a second depth buffer, with each buffer having an initial (e.g. maximum) value. In a second step 72, the computer device identifies a next surface (and / or point) in the three-dimensional representation. In a third step 73, the computer device determines whether a depth of this surface is less than a value saved in the first buffer. If the depth of the surface is not less than the depth in the first depth buffer, then the computer device returns to the second step 72 and identifies a next surface. If the depth of the surface is less than a value saved in the first buffer, then in a fourth step 74, the computer device determines whether a depth of this surface is less than (and / or is substantially different to) a value saved in the second buffer. If the depth is not less than a saved value in the second depth buffer, then the computer device returns to the second step 72. Then, in a fifth step 75, the computer device determines whether the surface is opaque. If the surface is opaque, then in a sixth step 76, the computer device updates a pixel value of the two- dimensional image and saves the depth of the surface into the first depth buffer (and, in some embodiments, also into the first buffer). The computer device then returns to the second step 72. If (e.g. only if), the surface is not opaque, then in a seventh step 77 the computer device updates the pixel value and saves the depth of the surface into the second depth buffer. The computer device then returns to the second step 72. It will be appreciated that the steps of the above-described method(s) could be performed in a variety of orders, for example the determination of the opacity (or non-opacity) of a point may be performed prior to the comparisons of depth values. The steps of this method (and the other methods herein) may be performed on a pixel-by-pixel basis where the computer device performs one of the disclosed methods in order to determine a depth buffer value (and / or a pixel value) for a first pixel before performing the method again for a second pixel. Equally, the method may be performed on a surface-by-surface (or object-by-object) basis where the computer device may compare values for a first surface to current depth values buffer for a plurality of pixels (and may update the depth buffer values and / or pixel values as appropriate for each pixel) before moving on to a second surface. With either implementation, surfaces are typically considered from furthers to nearest. For example, the computer device may iterate through the points of the three-dimensional representation from furthest to closest and, for each point, determine whether any entries of the depth buffer and / or any pixel values should be updated based on the values of this point. A point may cover a plurality of pixels / pixel brackets, so that this method may comprise evaluating a plurality of entries of the depth buffer for a first surface and then moving on to a second surface. It will be appreciated that further methods of selecting pixels and / or surfaces to consider are also possible. This determination of opacity is typically based on an alpha value associated with a point of the three- dimensional representation, where the alpha value indicates a level of opacity of the point. Identifying that the surface is not opaque may comprise identifying that the surface and / or a point associated with the surface has an alpha value of less than 1. In some embodiments, the computer device is arranged to determine an alpha value for a point and / or surface and to determine whether a surface is opaque based on this alpha value (e.g. this alpha value may be compared to a transparency threshold). The value of alpha that is required for a surface to be deemed opaque may depend on the distance of the point and / or surface from a viewing zone of the three-dimensional representation. For example, a point with an alpha value of 0.99 may be deemed to be translucent if a surface is close to the viewing zone but to be opaque if a surface is far from the viewing zone (since the surfaces beyond such an almost opaque surface are unlikely to be distinguishable by a viewer if this almost opaque surface is far away). Equally, the opacity check may deem points with an alpha value of 1 to be opaque and all other points to be non-opaque. With this method, the pixel value is updated only if the depth of a surface is less than values in both of a first depth buffer and a second buffer. The depth of this surface is then saved to the first buffer if the surface is opaque and to the second buffer if a surface is non-opaque. Considering the example shown in Figure 15, when the computer device is considering the sixth pixel bracket, the computer device may first consider the opaque background surface, may second consider the first transparent surface T1 and then third consider the second transparent surface T2. With the method of Figure 13, the computer device would: - In a first stage, update a pixel value based on the opaque background surface and update the depth buffer based on the depth of this opaque background surface. - In a second stage, update the pixel value based on the depth of the first transparent surface T1. - In a third stage, update the pixel value based on the second transparent surface T2. With the method of Figure 13, the computer device would instead: - In a first stage, update a pixel value based on the opaque background surface and update the first depth buffer based on this opaque background surface. - In a second stage, update the pixel value based on the first transparent surface T1 and update the second depth buffer based on this first transparent surface. - In a third stage, identify based on the value in the second depth buffer that the second transparent surface T2 is co-located with the first transparent surface, thereby identify that these two surfaces relate to the same surface in the scene, and not update the pixel value based on the second transparent surface. Typically, the seventh step 77 of saving the depth of the surface into the second depth buffer involves saving a value into the depth buffer that is a combination of the depth of the surface and an offset, which offset is typically a negative offset. For example, the computer device may save a value into the seconddepth buffer that is ^^^^^^^^^^ = ^^^^^^^^ℎ − ^^^^^^^^^^^^.When the offset is used, or when the offset is not used, the computer device may apply a scaling factor to the depth before saving that depth into the first buffer or the second buffer. For example, the computerdevice may save a value into the second buffer that is ^^^^^^^^^^  =  0.996 ⋅ ^^^^^^^^ℎ or ^^^^^^^^^^  =  0.996 ⋅^^^^^^^^ℎ  −  ^^^^^^^^^^^^ .In some embodiments, the computer device saves depths (e.g. scaled depths) of the surfaces into the buffer without applying the offset. In such embodiments, the computer device may, during the fourth step 74, determine whether a difference between a depth of an identified surface and a value in the second buffer exceeds the offset value. In other words, in the fourth step the computer device may determine that an identified surface is closer to the viewer than a value in the second buffer, and the computer device may determine that this identified surface is closer to the viewer by a value that is more than the offset value. It will be appreciated that these two methods of implementing the offset are functionally equivalent in most situations. That being said, applying the offset during the comparison of the depth of a surface and the second buffer (applying the offset during the fourth step) provides final first and second depth buffers that more accurately indicate the depths of surfaces, and in particular non-opaque surfaces, in the scene. These buffers may thereafter be used to analyse and modify the scene. This offset then defines a maximum distance by which two non-opaque surfaces can be separated while still being considered to be part of the same surface. For example, the second transparent surface T2 may be slightly in front of the second surface due to, for example, noise, quantisation errors, or small variations in the surface on which the first and second transparent surfaces each lie. By including the offset, such small variations may be accounted for. In some embodiments, a value of the offset is dependent on a distance of the surface from a viewing zone of the three-dimensional representation. In particular, the value of the offset may increase as the distance of the surface from the viewing zone increases. In some embodiments, the fourth step 74 comprises determining whether a depth of a surface is substantially different to a depth saved in the second buffer (e.g. whether the depth of the surface is within a range of the depth in the buffer, which range may be based on the offset). This ensures that if the computer device considers a foremost translucent surface first, then the computer device will not discount translucent surfaces behind the foremost translucent surface. Typically, the points / surfaces in the representation are pre-sorted by depth so that the identified surface is known to be either closer than or substantially equal to the value in the second buffer. Therefore, the fourth step only needs to consider whether the identified surface has a depth less than a depth saved in the second buffer. Considering again the example shown in Figure 15, when the computer device is considering the seventh pixel bracket, the computer device may first consider the opaque background surface, may second consider the third transparent surface T3 and then third consider the second transparent surface T2. With the method of Figure 13, the computer device would: - In a first stage, update a pixel value based on the opaque background surface and update the first depth buffer based on the depth of this opaque background surface. - In a second stage, update the pixel value based on the third transparent surface T3 and update the second depth buffer based on the depth of this third transparent surface. - In a third stage, compare the depth of the second transparent surface T2 to the value in the second buffer (the depth of the third transparent surface T3). Then, assuming this depth of the second surface is substantially less than the depth in the second buffer, update the pixel value based on the second transparent surface T2 and update the second depth buffer based on the depth of this second transparent surface. This leads to the situation shown in Figure 16b (and it will be appreciated that this figure does not show a two-dimensional image that is rendered based on these surfaces. Instead, this figure illustrates the surfaces that will be shown in the rendered two-dimensional image). As can be seen in this figure, the use of a plurality of depth buffers as disclosed above enables the computer device to accurately render a two- dimensional image in situations where adjacent points of the three-dimensional representation overlap. The above description has primarily considered determining that two overlapping transparent surfaces are a part of the same surface in the scene based on the depths of these surfaces. The method may additionally and / or alternatively comprise identifying overlapping surfaces based on a comparison of the attributes of these surfaces (e.g. the colours of the surfaces and / or the transparencies of the surfaces), where the updating of the pixel value and / or the updating of a value of the second depth buffer may be dependent on this comparison where the pixel value and the second depth buffer may be updated in dependence on a first non-opaque surface and a second non-opaque surface having different attribute values and / or transparencies (e.g. in dependence on a difference of the attributes and / or transparencies exceeding a threshold value). The first and second buffer may be stored separately. Equally, the first and second buffer may be different parts of a combined three-dimensional matrix (e.g. the first buffer may occupy the values (X, Y, 0) and the second buffer may occupy the values (X, Y, 1). Alternatives and modifications It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention. The representation is typically arranged to provide an extended reality (XR) experience (e.g. a representation that is useable to render a XR video). The term extended reality (XR) covers each of virtual reality (VR), augmented reality (AR), and mixed reality (MR) and it will be appreciated that the disclosures herein are applicable to any of these technologies. The representation may be encoded into, and / or transmitted using, a bitstream, which bitstream typically comprises point data for one or more points of the three-dimensional representation. The point data may be compressed or encoded to form the bitstream. The bitstream may then be transmitted between devices before being decoded at a receiving device so that this receiving device can determine the point data and reform the three-dimensional representation (or form one or more two-dimensional images based on this three-dimensional representation). In particular, the encoder 13 may be arranged to encode (e.g. one or more points of) the three-dimensional representation in order to form the bitstream and the decoder 14 may be arranged to decode the bitstream to generate the one or more two-dimensional images. In some embodiments, the scene comprises a stationary scene, where the scene itself does not change but a viewer is able to move through the scene to view different parts of the scene. Alternatively, in some embodiments the scene comprises a video and / or a moving (e.g. non-static) scene. That is, in some embodiments the scene comprises a stationary scene, such as a building, where a viewer is able to move through this scene, e.g. to view different rooms of the building, but where the scene itself does not change. In some embodiments, the scene comprises a moving scene, where elements of the scene vary in time even when the viewer remains stationary. It will be appreciated that typically the scene comprises both stationary and moving elements where, for example, non- stationary elements move in front of a stationary background. Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims

Claims 1. A method of updating a depth buffer of a two-dimensional image of a scene, the method comprising: identifying a surface in the scene; identifying a depth of the surface; and in dependence on the surface being non-opaque, updating a value of a depth buffer based on the identified depth.

2. The method of any preceding claim, comprising: identifying a current value of the depth buffer; and updating the value of the depth buffer in dependence on the depth of the surface being different to, preferably less than, the current value of the depth buffer.

3. The method of any preceding claim, wherein the depth buffer is a second depth buffer and wherein the image is further associated with a first depth buffer.

4. The method of claim 3, wherein: the first depth buffer is associated with opaque surfaces; and the second depth buffer is associated with non-opaque surfaces.

5. The method of claim 3 or 4, comprising: identifying a current value of the first depth buffer; comparing the depth of the surface to a value of the first depth buffer; and in dependence on the depth of the surface being different to, preferably less than, the current value of the first depth buffer: updating the value of the first depth buffer; and / or updating a value of a pixel of the two-dimensional image.

6. The method of any of claims 3 to 5, comprising updating the value of the first depth buffer based on the surface being opaque.

7. The method of any of claims 3 to 6, comprising, in dependence on the depth of the surface being less than the value of the first depth buffer, comparing the depth of the surface to a value of the second depth buffer.

8. The method of claim 7, comprising: updating the value of the second depth buffer based on the identified depth in dependence on said identified depth being different to, preferably less than, the current value of the second depth buffer; and / or updating a value of a pixel of the two-dimensional image in dependence on the depth of the surface being different to, preferably less than, a current value of the second depth buffer.

9. The method of claim 8, comprising updating the value of the second depth buffer based on the surface being non-opaque.

10. The method of any of claims 7 to 9, comprising: applying an offset to the value of the second depth buffer to determine a comparison value; and comparing the depth of the surface to the comparison value;preferably, comprising updating the depth buffer and / or the second depth buffer in dependence on the comparison.

11. The method of any preceding claim, comprising updating the value of the depth buffer and / or the second depth buffer based on a combination of the depth of the surface and an offset value, preferably: wherein the method comprises updating the value based on a combination of the depth of the surface and a predetermined offset value; and / or wherein a value of the offset is dependent on a distance of the surface from a viewing zone of the three-dimensional representation.

12. The method of any preceding claim, wherein the two-dimensional image is associated with a three- dimensional representation of a scene, and wherein identifying the depth of the surface comprises identifying a distance of a point of the three-dimensional representation from a viewing location associated with the two-dimensional image; preferably, wherein the viewing location is associated with a viewing zone of the three-dimensional representation.

13. The method of any preceding claim, comprising: identifying a plurality of surfaces of the scene; and ordering the surfaces in dependence on a distance of each surface from a viewing location and / or a viewing zone of the scene.

14. The method of any preceding claim, comprising determining a transparency value of the surface, preferably determining the transparency value by determining a transparency value of a point of a three- dimensional representation that defines the surface.

15. The method of claim 14, wherein determining the transparency value comprises comparing an alpha value of the surface to an alpha threshold, preferably wherein: the surface is deemed to be opaque if the alpha value exceeds the alpha threshold; and / or the surface is deemed to be non-opaque if the alpha value is below the alpha threshold.

16. The method of claim 15, wherein the alpha threshold is dependent on a distance of the surface from a viewing zone of the three-dimensional representation.

17. The method of any preceding claim, wherein: the method comprises initialising a first depth buffer and a second depth buffer, preferably initialising the first depth buffer and the second depth buffer with maximum depth values; and / or the depth buffer comprises a matrix with a number of rows and columns that is equal to a number of rows and columns of pixels of the two-dimensional image.

18. The method of any preceding claim, further comprising determining and / or updating a value for a pixel of the two-dimensional image based on the depth of the surface.

19. The method of any preceding claim, comprising: identifying a plurality of surfaces of the scene; identifying a first set of opaque surfaces; Identifying a second set of non-opaque surfaces; for each surface in the first set of surfaces, comparing each of the surfaces to a present value of a depth buffer; andthereafter, for each surface in the second set of surfaces, comparing each of the surfaces to a present value of a depth buffer.

20. The method of any preceding claim, comprising one or more of: outputting the depth buffer and / or the two-dimensional image; rendering the two-dimensional image based on the depth buffer; and forming a bitstream comprising the depth buffer and / or the two-dimensional image.

21. The method of any preceding claim, wherein the three-dimensional representation is associated with a viewing zone, the viewing zone comprising a subset of the scene and / or the viewing zone enabling a user to move through a subset of the scene, preferably wherein: the user is able to move within the viewing zone with six degrees of freedom (6DoF); and / or the viewing zone has a volume of less than 50% of the volume of the scene, less than 20% of the volume of the scene, and / or less than 10% of the volume of the scene; and / or the viewing zone has, or is associated with, a volume, preferably a real-world volume, of less than five cubic metres (5m3), less than one cubic metre (1m3), less than one-tenth of a cubic metre (0.1m3) and / or less than one-hundredth of a cubic metre (0.01m3).

22. A method of updating a value of a pixel of a two-dimensional image of a scene, the method comprising: identifying a surface in the scene; identifying a depth of the surface; comparing the depth of the surface to a value of a first depth buffer, wherein the first depth buffer is associated with opaque surfaces; in dependence on the depth of the surface being less than the value of the first depth buffer, comparing the depth of the surface to a value of a second depth buffer, wherein the second depth buffer is associated with non-opaque surfaces; and in dependence on the depth of the surface being different to a current value of the second depth buffer, updating a value of a pixel of the two-dimensional image.

23. A method of rendering a two-dimensional image based on a plurality of depth buffers, the method comprising: for one or more pixels of the image: determining a depth of a surface; determining a value in a first depth buffer; determining a value in a second depth buffer; and determining a value for the pixel in dependence on: a comparison between the depth of the surface and the value in the first depth buffer; and a comparison between the depth of the surface and the value in the second depth buffer; preferably wherein the second depth buffer is associated with non-opaque surfaces and the method comprises: determining the value for the pixel based on the depth of the surface being beneath the value in the first depth buffer; and / or determining the value for the pixel based on the depth of the surface being substantially different to the value in the second depth buffer.

24. A computer program product comprising software code that, when executed on a computer device, causes the computer device to perform the method of any preceding claim.

25. A machine-readable storage medium that includes instructions that, when executed by one or more processors of a machine, cause the machine to perform the method of any of claims 1 to 23.

26. A system for carrying out the method of any of claims 1 to 23 comprising one or more of: a processor; a communication interface; and a display.

27. An apparatus for updating a depth buffer of a two-dimensional image of a scene, the apparatus comprising: means for identifying a surface in the scene; means for identifying a depth of the surface; and means for, in dependence on the surface being non-opaque, updating a value of a depth buffer based on the depth of the surface.

28. A bitstream comprising a plurality of depth buffers for a two-dimensional image, preferably comprising: a first depth buffer associated with opaque surfaces and a second depth buffer associated with non-opaque surfaces; and / or one or more two-dimensional images associated with the depth buffers.

29. An encoder for forming and / or encoding the bitstream of claim 28.

30. A decoder for receiving and / or decoding the bitstream of claim 28.

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