WO2021165566A1

WO2021165566A1 - An apparatus, a method and a computer program for volumetric video

Info

Publication number: WO2021165566A1
Application number: PCT/FI2021/050048
Authority: WO
Inventors: Payman Aflaki Beni; Vinod Kumar MALAMAL VADAKITAL
Original assignee: Nokia Technologies Oy
Priority date: 2020-02-19
Filing date: 2021-01-26
Publication date: 2021-08-26

Abstract

The embodiments relate to a method comprising: projecting a 3D representation of at least one object onto a plurality of 2D patches (600); generating a texture component picture, a depth component picture, an occupancy map and auxiliary patch information from the 2D patches (602); aggregating said plurality of 2D patches with the corresponding texture component picture into an atlas (604); determining a reference value of patch rotation for said atlas based on patch rotation values of said plurality of 2D patches (606); determining a difference value of patch rotation for each of said 2D patches as a difference between said reference value and a patch rotation value of any of the 2D patches (608); and encoding the auxiliary patch information to include said reference value of patch rotation for said atlas and an indication for each of said 2D patches, said indication indicating whether the reference value or the difference value of patch rotation of the 2D patch is to be used for determining the patch rotation value of said 2D patch (610).

Description

AN APPARATUS, A METHOD AND A COMPUTER PROGRAM FOR

VOLUMETRIC VIDEO

TECHNICAL FIELD

[0001 ] The present invention relates to an apparatus, a method and a computer program for volumetric video coding.

BACKGROUND

[0002] Volumetric video data represents a three-dimensional scene or object and can be used as input for virtual reality (VR), augmented reality (AR) and mixed reality (MR) applications. Such data describes the geometry, e.g. shape, size, position in three- dimensional (3D) space, and respective attributes, e.g. colour, opacity, reflectance and any possible temporal changes of the geometry and attributes at given time instances. Volumetric video is either generated from 3D models through computer-generated imagery (CGI), or captured from real-world scenes using a variety of capture solutions, e.g. multi camera, laser scan, combination of video and dedicated depth sensors, and more. Also, a combination of CGI and real-world data is possible.

[0003] Typical representation formats for such volumetric data are triangle meshes, point clouds (PCs), or voxel arrays. In dense point clouds or voxel arrays, the reconstructed 3D scene may contain tens or even hundreds of millions of points. One way to compress a time-varying volumetric scene/object is to project 3D surfaces to some number of pre defined 2D planes. Regular 2D video compression algorithms can then be used to compress various aspects of the projected surfaces. For example, MPEG Video-Based Point Cloud Coding (V-PCC) provides a procedure for compressing a time-varying volumetric scene/object by projecting 3D surfaces onto a number of pre-defined 2D planes, which may then be compressed using regular 2D video compression algorithms. The projection is presented using different patches, where each set of patches may represent a specific object or specific parts of a scene.

[0004] Depending on the viewport to be outputted, the patches may be aligned differently, i.e. in addition to different vertical and horizontal positions and sizes, the patches may also be rotated and/or mirrored. However, the rotation of patches indicating the desired orientation for any patch is signalled separately for each patch. Transmitting this information per patch takes a relatively high amount of bits and therefore increases the size of encoded content bitstream that needs to be transmitted for the presentation of the content to the decoded.

SUMMARY

[0005] Now, an improved method and technical equipment implementing the method has been invented, by which the above problems are alleviated. Various aspects include a method, an apparatus and a computer readable medium comprising a computer program, or a signal stored therein, which are characterized by what is stated in the independent claims. Various details of the embodiments are disclosed in the dependent claims and in the corresponding images and description.

[0006] The scope of protection sought for various embodiments of the invention is set out by the independent claims. The embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention.

[0007] According to a first aspect, there is provided a method comprising projecting a 3D representation of at least one object onto a plurality of 2D patches; generating a texture component picture, a depth component picture, an occupancy map and auxiliary patch information from the 2D patches; aggregating said plurality of 2D patches with the corresponding texture component picture into an atlas; determining a reference value of patch rotation for said atlas based on patch rotation values of said plurality of 2D patches; determining a difference value of patch rotation for each of said 2D patches as a difference between said reference value and a patch rotation value of any of the 2D patches; and encoding the auxiliary patch information to include said reference value of patch rotation for said atlas and an indication for each of said 2D patches, said indication indicating whether the reference value or the difference value of patch rotation of the 2D patch is to be used for determining the patch rotation value of said 2D patch.

[0008] An apparatus according to a second aspect comprises at least one processor and at least one memory, said at least one memory stored with computer program code thereon, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: project a 3D representation of at least one object onto a plurality of 2D patches; generate a texture component picture, a depth component picture, an occupancy map and auxiliary patch information from the 2D patches; aggregate said plurality of 2D patches with the corresponding texture component picture into an atlas; determine a reference value of patch rotation for said atlas based on patch rotation values of said plurality of 2D patches; determine a difference value of patch rotation for each of said 2D patches as a difference between said reference value and a patch rotation value of the 2D patch; and encode the auxiliary patch information to include said reference value of patch rotation for said atlas and an indication for each of said 2D patches, said indication indicating whether the reference value or the difference value of patch rotation of the 2D patch is to be used for determining the patch rotation value of said 2D patch.

[0009] An apparatus according to a third aspect comprises means for projecting a 3D representation of at least one object onto a plurality of 2D patches; means for generating a texture component picture, a depth component picture, an occupancy map and auxiliary patch information from the 2D patches; means for aggregating said plurality of 2D patches with the corresponding texture component picture into an atlas; means for determining a reference value of patch rotation for said atlas based on patch rotation values of said plurality of 2D patches; means for determining a difference value of patch rotation for each of said 2D patches as a difference between said reference value and a patch rotation value of the 2D patch; and means for encoding the auxiliary patch information to include said reference value of patch rotation for said atlas and an indication for each of said 2D patches, said indication indicating whether the reference value or the difference value of patch rotation of the 2D patch is to be used for determining the patch rotation value of said 2D patch.

[0010] According to an embodiment, the reference value is configured to be determined based on at least one of the following: a mean of patch rotation values of said plurality of 2D patches; a median of patch rotation values of said plurality of 2D patches; a mode of patch rotation values of said plurality of 2D patches. a reference value of the reference values of a predetermined number of previous atlases.

[0011] According to an embodiment, the reference value is configured to be updated temporally based on at least one of the following: after a predetermined number of frames, such as corresponding to a group of pictures (GOP) length; when one or more objects appear on a 3D scene; when one or more objects disappear from a 3D scene; after a predetermined number of frames, wherein said number is dependent on an amount of motion appearing on a 3D scene; after a predetermined number of frames, wherein said number is dependent on the change in the determined reference value per frame; after a predetermined number of frames, wherein said number is dependent on the change of the determined reference value from the latest signaled reference value. [0012] According to an embodiment, said indication is configured to be encoded as a flag for each of said plurality of 2D patches, wherein said flag indicates whether a rotation according to the reference value is to be applied for the 2D patch.

[0013] According to an embodiment, a signalling of the reference value of patch rotation for said atlas, and said flag and the difference values of patch rotation for each of said 2D patches are configured to be carried out by at least one syntax element included in an atlas parameter syntax structure.

[0014] According to an embodiment, in response to the difference between said reference value and the patch rotation value of a particular 2D patch is smaller than a threshold value, the apparatus is configured to set the value of the flag for said particular 2D patch as zero.

[0015] According to an embodiment, in response to the difference between said reference value and the patch rotation value of a particular 2D patch is equal to or greater than a threshold value, the apparatus is configured to set the value of the flag for said particular 2D patch as one and include the difference value of patch rotation for said particular 2D patch. [0016] A method according to a fourth aspect comprises receiving a bitstream in a decoder, said bitstream comprising an encoded texture component picture, an encoded depth component picture and an encoded occupancy map indicative of a plurality of 2D patches from a 3D representation of at least one object, wherein said plurality of 2D patches have been aggregated with the corresponding texture component picture into an atlas; receiving, either in said bitstream or in a further bitstream, an encoded auxiliary patch information from said plurality of 2D patches, wherein said auxiliary patch information comprises a reference value of patch rotation for said atlas and an indication for each of said 2D patches of the atlas, said indication indicating whether the reference value or a difference value of patch rotation of the 2D patch is to be used for determining a rotation value of said 2D patch; decoding the texture component picture by determining for each 2D patch associated with the atlas the rotation value of said 2D patch either as reference value of patch rotation for said atlas or as a difference between said reference value and a patch rotation value of said 2D patch; and decoding the depth component picture, the occupancy map and the auxiliary patch information for reconstructing a 3D representation of said at least one object.

[0017] An apparatus according to a fifth aspect comprises at least one processor and at least one memory, said at least one memory stored with computer program code thereon, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: receive a bitstream in a decoder, said bitstream comprising an encoded texture component picture, an encoded depth component picture and an encoded occupancy map indicative of a plurality of 2D patches from a 3D representation of at least one object, wherein said plurality of 2D patches have been aggregated with the corresponding texture component picture into an atlas; receive, either in said bitstream or in a further bitstream, an encoded auxiliary patch information from said plurality of 2D patches, wherein said auxiliary patch information comprises a reference value of patch rotation for said atlas and an indication for each of said 2D patches of the atlas, said indication indicating whether the reference value or a difference value of patch rotation of the 2D patch is to be used for determining a rotation value of said 2D patch; decode the texture component picture by determining for each 2D patch associated with the atlas the rotation value of said 2D patch either as reference value of patch rotation for said atlas or as a difference between said reference value and a patch rotation value of said 2D patch; and decode the depth component picture, the occupancy map and the auxiliary patch information for reconstructing a 3D representation of said at least one object.

[0018] An apparatus according to a sixth aspect comprises: means for receiving a bitstream in a decoder, said bitstream comprising an encoded texture component picture, an encoded depth component picture and an encoded occupancy map indicative of a plurality of 2D patches from a 3D representation of at least one object, wherein said plurality of 2D patches have been aggregated with the corresponding texture component picture into an atlas; means for receiving, either in said bitstream or in a further bitstream, an encoded auxiliary patch information from said plurality of 2D patches, wherein said auxiliary patch information comprises a reference value of patch rotation for said atlas and an indication for each of said 2D patches of the atlas, said indication indicating whether the reference value or a difference value of patch rotation of the 2D patch is to be used for determining a rotation value of said 2D patch; means for decoding the texture component picture by determining for each 2D patch associated with the atlas the rotation value of said 2D patch either as reference value of patch rotation for said atlas or as a difference between said reference value and a patch rotation value of said 2D patch; and means for decoding the depth component picture, the occupancy map and the auxiliary patch information for reconstructing a 3D representation of said at least one object.

[0019] According to an embodiment, said indication is a flag for each of said plurality of 2D patches, wherein said flag indicates whether a rotation according to the reference value is to be applied for the 2D patch.

[0020] According to an embodiment, a signalling of the reference value of patch rotation for said atlas, and said flag and the difference values of patch rotation for each of said 2D patches are configured to be received by at least one syntax element included in an atlas parameter syntax structure.

[0021] According to an embodiment, in response to the value of the flag for said particular 2D patch is zero, the apparatus is configured to apply the reference value of patch rotation as the rotation value for said atlas for said particular 2D patch. [0022] According to an embodiment, in response to the value of the flag for said particular 2D patch is one, the apparatus is configured to apply the difference between said reference value and the patch rotation value of said 2D patch as the rotation value for said atlas for said particular 2D patch.

[0023] Computer readable storage media according to further aspects comprise code for use by an apparatus, which when executed by a processor, causes the apparatus to perform the above methods.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] For a more complete understanding of the example embodiments, reference is now made to the following descriptions taken in connection with the accompanying drawings in which:

[0025] Figs la and lb show an encoder and decoder for encoding and decoding 2D pictures;

[0026] Figs. 2a and 2b show a compression and a decompression process for 3D volumetric video;

[0027] Figs. 3a and 3b show an example of a point cloud frame and a projection of points to a corresponding plane of a point cloud bounding box;

[0028] Fig. 4 shows an illustrative example of the relationship between atlases, patches and view representations;

[0029] Fig. 5 shows a decoder reference architecture for immersive video;

[0030] Fig. 6 shows a flow chart for encoding patch rotation information according to an embodiment;

[0031] Fig. 7 shows a flow chart for decoding patch rotation information according to an embodiment; and

[0032] Figs. 8a and 8b show some embodiments relating to the encoding and decoding of the patch rotation information.

DETAILED DESCRIPTON OF SOME EXAMPLE EMBODIMENTS

[0033] In the following, several embodiments of the invention will be described in the context of point cloud models for volumetric video coding. It is to be noted, however, that the invention is not limited to specific scene models or specific coding technologies. In fact, the different embodiments have applications in any environment where coding of volumetric scene data is required.

[0034] A video codec comprises an encoder that transforms the input video into a compressed representation suited for storage/transmission, and a decoder that can un compress the compressed video representation back into a viewable form. An encoder may discard some information in the original video sequence in order to represent the video in a more compact form (i.e. at lower bitrate).

[0035] Volumetric video may be captured using one or more three-dimensional (3D) cameras. When multiple cameras are in use, the captured footage is synchronized so that the cameras provide different viewpoints to the same world. In contrast to traditional 2D/3D video, volumetric video describes a 3D model of the world where the viewer is free to move and observer different parts of the world.

[0036] Volumetric video enables the viewer to move in six degrees of freedom (6DOF): in contrast to common 360° video, where the user has from 2 to 3 degrees of freedom (yaw, pitch, and possibly roll), a volumetric video represents a 3D volume of space rather than a flat image plane. Volumetric video frames contain a large amount of data because they model the contents of a 3D volume instead of just a two-dimensional (2D) plane. However, only a relatively small part of the volume changes over time. Therefore, it may be possible to reduce the total amount of data by only coding information about an initial state and changes which may occur between frames. Volumetric video can be rendered from synthetic 3D animations, reconstructed from multi-view video using 3D reconstruction techniques such as structure from motion, or captured with a combination of cameras and depth sensors such as LiDAR (Light Detection and Ranging), for example. [0037] Volumetric video data represents a three-dimensional scene or object, and thus such data can be viewed from any viewpoint. Volumetric video data can be used as an input for augmented reality (AR), virtual reality (VR) and mixed reality (MR) applications. Such data describes geometry (shape, size, position in 3D-space) and respective attributes (e.g. color, opacity, reflectance, ...), together with any possible temporal changes of the geometry and attributes at given time instances (e.g. frames in 2D video). Volumetric video is either generated from 3D models, i.e. computer-generated imagery (CGI), or captured from real-world scenes using a variety of capture solutions, e.g. a multi-camera, a laser scan, a combination of video and dedicated depths sensors, etc. Also, a combination of CGI and real-world data is possible. Examples of representation formats for such volumetric data are triangle meshes, point clouds, or voxel. Temporal information about the scene can be included in the form of individual capture instances, i.e. “frames” in 2D video, or other means, e.g. position of an object as a function of time.

[0038] Increasing computational resources and advances in 3D data acquisition devices has enabled reconstruction of highly detailed volumetric video representations of natural scenes. Infrared, lasers, time-of-flight and structured light are all examples of devices that can be used to construct 3D video data. Representation of the 3D data depends on how the 3D data is used. Dense voxel arrays have been used to represent volumetric medical data.

In 3D graphics, polygonal meshes are extensively used. Point clouds on the other hand are well suited for applications, such as capturing real world 3D scenes where the topology is not necessarily a 2D manifold. Another way to represent 3D data is coding this 3D data as a set of texture and depth map as is the case in the multi-view plus depth. Closely related to the techniques used in multi-view plus depth is the use of elevation maps, and multi-level surface maps.

[0039] In 3D point clouds, each point of each 3D surface is described as a 3D point with color and/or other attribute information such as surface normal or material reflectance. Point cloud is a set of data points in a coordinate system, for example in a three- dimensional coordinate system being defined by X, Y, and Z coordinates. The points may represent an external surface of an object in the screen space, e.g. in a three-dimensional space.

[0040] In dense point clouds or voxel arrays, the reconstructed 3D scene may contain tens or even hundreds of millions of points. If such representations are to be stored or interchanged between entities, then efficient compression of the presentations becomes fundamental. Standard volumetric video representation formats, such as point clouds, meshes, voxel, suffer from poor temporal compression performance. Identifying correspondences for motion-compensation in 3D-space is an ill-defined problem, as both, geometry and respective attributes may change. For example, temporal successive “frames” do not necessarily have the same number of meshes, points or voxel. Therefore, compression of dynamic 3D scenes is inefficient. 2D-video based approaches for compressing volumetric data, i.e. multiview with depth, have much better compression efficiency, but rarely cover the full scene. Therefore, they provide only limited 6DOF capabilities.

[0041 ] Instead of the above-mentioned approach, a 3D scene, represented as meshes, points, and/or voxel, can be projected onto one, or more, geometries. These geometries may be “unfolded” or packed onto 2D planes (two planes per geometry: one for texture, one for depth), which are then encoded using standard 2D video compression technologies. Relevant projection geometry information may be transmitted alongside the encoded video files to the decoder. The decoder decodes the video and performs the inverse projection to regenerate the 3D scene in any desired representation format (not necessarily the starting format).

[0042] Projecting volumetric models onto 2D planes allows for using standard 2D video coding tools with highly efficient temporal compression. Thus, coding efficiency can be increased greatly. Using geometry-projections instead of 2D-video based approaches based on multiview and depth, provides a better coverage of the scene (or object). Thus, 6DOF capabilities are improved. Using several geometries for individual objects improves the coverage of the scene further. Furthermore, standard video encoding hardware can be utilized for real-time compression/decompression of the projected planes. The projection and the reverse projection steps are of low complexity.

[0043] Figs la and lb show an encoder and decoder for encoding and decoding the 2D texture pictures, geometry pictures and/or auxiliary pictures. A video codec consists of an encoder that transforms an input video into a compressed representation suited for storage/transmission and a decoder that can uncompress the compressed video representation back into a viewable form. Typically, the encoder discards and/or loses some information in the original video sequence in order to represent the video in a more compact form (that is, at lower bitrate). An example of an encoding process is illustrated in Figure la. Figure la illustrates an image to be encoded (F); a predicted representation of an image block (P'ⁿ); a prediction error signal (Dⁿ); a reconstructed prediction error signal (D'ⁿ); a preliminary reconstructed image (I'ⁿ); a final reconstructed image (R'ⁿ); a transform (T) and inverse transform (T^-1); a quantization (Q) and inverse quantization (Q ¹); entropy encoding (E); a reference frame memory (RFM); inter prediction (Pinter); intra prediction (Pintra); mode selection (MS) and filtering (F).

[0044] An example of a decoding process is illustrated in Figure lb. Figure lb illustrates a predicted representation of an image block (P'ⁿ); a reconstructed prediction error signal (D'ⁿ); a preliminary reconstructed image (I'ⁿ); a final reconstructed image (R'ⁿ); an inverse transform

an inverse quantization (Q ¹); an entropy decoding (E ¹); a reference frame memory (RFM); a prediction (either inter or intra) (P); and filtering (F). [0045] Many hybrid video encoders encode the video information in two phases. Firstly pixel values in a certain picture area (or “block”) are predicted for example by motion compensation means (finding and indicating an area in one of the previously coded video frames that corresponds closely to the block being coded) or by spatial means (using the pixel values around the block to be coded in a specified manner). Secondly the prediction error, i.e. the difference between the predicted block of pixels and the original block of pixels, is coded. This is typically done by transforming the difference in pixel values using a specified transform (e.g. Discrete Cosine Transform (DCT) or a variant of it), quantizing the coefficients and entropy coding the quantized coefficients. By varying the fidelity of the quantization process, encoder can control the balance between the accuracy of the pixel representation (picture quality) and size of the resulting coded video representation (file size or transmission bitrate). Video codecs may also provide a transform skip mode, which the encoders may choose to use. In the transform skip mode, the prediction error is coded in a sample domain, for example by deriving a sample-wise difference value relative to certain adjacent samples and coding the sample-wise difference value with an entropy coder.

[0046] Many video encoders partition a picture into blocks along a block grid. For example, in the High Efficiency Video Coding (HEVC) standard, the following partitioning and definitions are used. A coding block may be defined as an NxN block of samples for some value of N such that the division of a coding tree block into coding blocks is a partitioning. A coding tree block (CTB) may be defined as an NxN block of samples for some value of N such that the division of a component into coding tree blocks is a partitioning. A coding tree unit (CTU) may be defined as a coding tree block of luma samples, two corresponding coding tree blocks of chroma samples of a picture that has three sample arrays, or a coding tree block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples. A coding unit (CU) may be defined as a coding block of luma samples, two corresponding coding blocks of chroma samples of a picture that has three sample arrays, or a coding block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples. A CU with the maximum allowed size may be named as LCU (largest coding unit) or coding tree unit (CTU) and the video picture is divided into non-overlapping LCUs.

[0047] In HEVC, a picture can be partitioned in tiles, which are rectangular and contain an integer number of LCUs. In HEVC, the partitioning to tiles forms a regular grid, where heights and widths of tiles differ from each other by one LCU at the maximum. In HEVC, a slice is defined to be an integer number of coding tree units contained in one independent slice segment and all subsequent dependent slice segments (if any) that precede the next independent slice segment (if any) within the same access unit. In HEVC, a slice segment is defined to be an integer number of coding tree units ordered consecutively in the tile scan and contained in a single NAL unit. The division of each picture into slice segments is a partitioning. In HEVC, an independent slice segment is defined to be a slice segment for which the values of the syntax elements of the slice segment header are not inferred from the values for a preceding slice segment, and a dependent slice segment is defined to be a slice segment for which the values of some syntax elements of the slice segment header are inferred from the values for the preceding independent slice segment in decoding order. In HEVC, a slice header is defined to be the slice segment header of the independent slice segment that is a current slice segment or is the independent slice segment that precedes a current dependent slice segment, and a slice segment header is defined to be a part of a coded slice segment containing the data elements pertaining to the first or all coding tree units represented in the slice segment. The CUs are scanned in the raster scan order of LCUs within tiles or within a picture, if tiles are not in use. Within an LCU, the CUs have a specific scan order.

[0048] Entropy coding/decoding may be performed in many ways. For example, context-based coding/decoding may be applied, where in both the encoder and the decoder modify the context state of a coding parameter based on previously coded/decoded coding parameters. Context-based coding may for example be context adaptive binary arithmetic coding (CABAC) or context-adaptive variable length coding (CAVLC) or any similar entropy coding. Entropy coding/decoding may alternatively or additionally be performed using a variable length coding scheme, such as Huffman coding/decoding or Exp-Go lomb coding/decoding. Decoding of coding parameters from an entropy-coded bitstream or codewords may be referred to as parsing.

[0049] The phrase along the bitstream (e.g. indicating along the bitstream) may be defined to refer to out-of-band transmission, signalling, or storage in a manner that the out- of-band data is associated with the bitstream. The phrase decoding along the bitstream or alike may refer to decoding the referred out-of-band data (which may be obtained from out-of-band transmission, signalling, or storage) that is associated with the bitstream. For example, an indication along the bitstream may refer to metadata in a container file that encapsulates the bitstream.

[0050] A first texture picture may be encoded into a bitstream, and the first texture picture may comprise a first projection of texture data of a first source volume of a scene model onto a first projection surface. The scene model may comprise a number of further source volumes.

[0051 ] In the projection, data on the position of the originating geometry primitive may also be determined, and based on this determination, a geometry picture may be formed. This may happen for example so that depth data is determined for each or some of the texture pixels of the texture picture. Depth data is formed such that the distance from the originating geometry primitive such as a point to the projection surface is determined for the pixels. Such depth data may be represented as a depth picture, and similarly to the texture picture, such geometry picture (such as a depth picture) may be encoded and decoded with a video codec. This first geometry picture may be seen to represent a mapping of the first projection surface to the first source volume, and the decoder may use this information to determine the location of geometry primitives in the model to be reconstructed. In order to determine the position of the first source volume and/or the first projection surface and/or the first projection in the scene model, there may be first geometry information encoded into or along the bitstream. [0052] An attribute picture may be defined as a picture that comprises additional information related to an associated texture picture. An attribute picture may for example comprise surface normal, opacity, or reflectance information for a texture picture. A geometry picture may be regarded as one type of an attribute picture, although a geometry picture may be treated as its own picture type, separate from an attribute picture.

[0053] Texture picture(s) and the respective geometry picture(s), if any, and the respective attribute picture(s) may have the same or different chroma format.

[0054] Terms texture image and texture picture may be used interchangeably. Terms geometry image and geometry picture may be used interchangeably. A specific type of a geometry image is a depth image. Embodiments described in relation to a geometry image equally apply to a depth image, and embodiments described in relation to a depth image equally apply to a geometry image. Terms attribute image and attribute picture may be used interchangeably. A geometry picture and/or an attribute picture may be treated as an auxiliary picture in video/image encoding and/or decoding.

[0055] Figures 2a and 2b illustrate an overview of exemplified compression/ decompression processes. The processes may be applied, for example, in Point Cloud Coding (PCC) according to MPEG standard. MPEG Video-Based Point Cloud Coding (V- PCC), Test Model a.k.a. TMC2vO (MPEG N18017) discloses a projection-based approach for dynamic point cloud compression. For the sake of illustration, some of the processes related to video-based point cloud compression (V-PCC) compression/decompression are described briefly herein. For a comprehensive description of the model, a reference is made to MPEG N 18017.

[0056] Each point cloud frame represents a dataset of points within a 3D volumetric space that has unique coordinates and attributes. An example of a point cloud frame is shown on Figure 3 a.

[0057] The patch generation process decomposes the point cloud frame by converting 3d samples to 2d samples on a given projection plane using a strategy that provides the best compression. The patch generation process aims at decomposing the point cloud into a minimum number of patches with smooth boundaries, while also minimizing the reconstruction error. In the V-PCC test model TMC2vO, the following approach is implemented. [0058] First, the normal per each point is estimated and the tangent plane and its corresponding normal are defined per each point, based on the point’s nearest neighbours m within a predefined search distance. A K-D tree is used to separate the data and find neighbours in a vicinity of a point p_t and a barycenter c = p of that set of points is used to define the normal. The barycenter c is computed as follows:

[0059] The normal is estimated from eigen decomposition for the defined point cloud as:

[0060] Based on this information each point is associated with a corresponding plane of a point cloud bounding box. Each plane is defined by a corresponding normal n_p.dx with values:

- (1.0, 0.0, 0.0),

- (0.0, 1.0, 0.0),

- (0.0, 0.0, 1.0),

- (-1.0, 0.0, 0.0),

- (0.0, -1.0, 0.0),

- (0.0, 0.0, -1.0).

[0061] More precisely, each point is associated with the plane that has the closest normal (i.e., maximizes the dot product of the point normal n_p. and the plane normal ⁿPidx

[0062] The sign of the normal is defined depending on the point’s position in relationship to the “center”. The projection estimation description is shown in Figure 3b. [0063] The initial clustering is then refined by iteratively updating the cluster index associated with each point based on its normal and the cluster indices of its nearest neighbors. The next step consists of extracting patches by applying a connected component extraction procedure. [0064] The packing process aims at mapping the extracted patches onto a 2D grid while trying to minimize the unused space, and guaranteeing that every TxT (e.g., 16x16) block of the grid is associated with a unique patch. Herein, T is a user-defined parameter that is encoded in the bitstream and sent to the decoder.

[0065] TMC2vO uses a simple packing strategy that iteratively tries to insert patches into a WxH grid. W and H are user defined parameters, which correspond to the resolution of the geometry/texture images that will be encoded. The patch location is determined through an exhaustive search that is performed in raster scan order. The first location that can guarantee an overlapping-free insertion of the patch is selected and the grid cells covered by the patch are marked as used. If no empty space in the current resolution image can fit a patch, then the height H of the grid is temporarily doubled and search is applied again. At the end of the process, H is clipped so as to fit the used grid cells.

[0066] The image generation process exploits the 3D to 2D mapping computed during the packing process to store the geometry and texture of the point cloud as images. In order to better handle the case of multiple points being projected to the same pixel, each patch is projected onto two images, referred to as layers. More precisely, let H(u,v) be the set of points of the current patch that get projected to the same pixel (u, v). The first layer, also called the near layer, stores the point of H(u,v) with the lowest depth DO. The second layer, referred to as the far layer, captures the point of H(u,v) with the highest depth within the interval [DO, DO+D], where D is a user-defined parameter that describes the surface thickness.

[0067] The generated videos have the following characteristics: geometry: WxH YUV420-8bit, where the geometry video is monochromatic, and texture: WxH YUV420- 8bit, where the texture generation procedure exploits the reconstructed/smoothed geometry in order to compute the colors to be associated with the re-sampled points.

[0068] The padding process aims at filling the empty space between patches in order to generate a piecewise smooth image suited for video compression. TMC2vO uses a simple padding strategy, which proceeds as follows:

Each block of TxT (e.g., 16x16) pixels is processed independently. If the block is empty (i.e., all its pixels belong to empty space), then the pixels of the block are filled by copying either the last row or column of the previous TxT block in raster order.

If the block is full (i.e., no empty pixels), nothing is done.

If the block has both empty and filled pixels (i.e. a so-called edge block), then the empty pixels are iteratively filled with the average value of their non-empty neighbors.

[0069] The generated images/layers are stored as video frames and compressed using a video codec.

[0070] In the auxiliary patch information compression, the following meta data is encoded/decoded for every patch:

Index of the projection plane o Index 0 for the normal planes (1.0, 0.0, 0.0) and (-1.0, 0.0, 0.0) o Index 1 for the normal planes (0.0, 1.0, 0.0) and (0.0, -1.0, 0.0) o Index 2 for the normal planes (0.0, 0.0, 1.0) and (0.0, 0.0, -1.0).

2D bounding box (uO, vO, ul, vl)

3D location (xO, yO, zO) of the patch represented in terms of depth 50, tangential shift sO and bi-tangential shift rO. According to the chosen projection planes, (dq, sO, rO) are computed as follows: o Index 0, d0= xO, s0=z0 and rO = yO o Index 1, d0= yO, s0=z0 and rO = xO o Index 2, d0= zO, s0=x0 and rO = yO

[0071] Also, mapping information providing for each TxT block its associated patch index is encoded as follows:

For each TxT block, let L be the ordered list of the indexes of the patches such that their 2D bounding box contains that block. The order in the list is the same as the order used to encode the 2D bounding boxes. L is called the list of candidate patches.

The empty space between patches is considered as a patch and is assigned the special index 0, which is added to the candidate patches list of all the blocks. Let I be index of the patch to which belongs the current TxT block and let J be the position of I in L. Instead of explicitly encoding the index I, its position J is arithmetically encoded instead, which leads to better compression efficiency.

[0072] The occupancy map consists of a binary map that indicates for each cell of the grid whether it belongs to the empty space or to the point cloud. Herein, one cell of the 2D grid produces a pixel during the image generation process. When considering an occupancy map as an image, it may be considered to comprise occupancy patches. Occupancy patches may be considered to have block-aligned edges according to the auxiliary information described in the previous section. An occupancy patch hence comprises occupancy information for a corresponding texture and geometry patches.

[0073] The occupancy map compression leverages the auxiliary information described in previous section, in order to detect the empty TxT blocks (i.e., blocks with patch index 0). The remaining blocks are encoded as follows.

[0074] The occupancy map could be encoded with a precision of a BOxBO blocks. B0 is a user-defined parameter. In order to achieve lossless encoding, B0 should be set to 1. In practice B0=2 or B0=4 result in visually acceptable results, while significantly reducing the number of bits required to encode the occupancy map. The generated binary image covers only a single colour plane. However, given the prevalence of 4:2:0 codecs, it may be desirable to extend the image with “neutral” or fixed value chroma planes (e.g. adding chroma planes with all sample values equal to 0 or 128, assuming the use of an 8-bit codec).

[0075] The obtained video frame is compressed by using a video codec with lossless coding tool support (e.g., AVC, HEVC RExt, HEVC-SCC).

[0076] Occupancy map is simplified by detecting empty and non-empty blocks of resolution TxT in the occupancy map and only for the non-empty blocks we encode their patch index as follows:

A list of candidate patches is created for each TxT block by considering all the patches that contain that block.

The list of candidates is sorted in the reverse order of the patches.

For each block, o If the list of candidates has one index, then nothing is encoded. o Otherwise, the index of the patch in this list is arithmetically encoded.

[0077] The point cloud geometry reconstruction process exploits the occupancy map information in order to detect the non-empty pixels in the geometry/texture images/layers. The 3D positions of the points associated with those pixels are computed by levering the auxiliary patch information and the geometry images. More precisely, let P be the point associated with the pixel (u, v) and let (50, sO, rO) be the 3D location of the patch to which it belongs and (uO, vO, ul, vl) its 2D bounding box. P could be expressed in terms of depth d (u, v), tangential shift s(u, v) and bi-tangential shift r(u, v) as follows:

8(u, v) = 50 + g(u, v) s(u, v) = sO - uO + u r(u, v) = rO - vO + v where g(u, v) is the luma component of the geometry image.

[0078] The smoothing procedure aims at alleviating potential discontinuities that may arise at the patch boundaries due to compression artifacts. The implemented approach moves boundary points to the centroid of their nearest neighbors.

[0079] In the texture reconstruction process, the texture values are directly read from the texture images.

[0080] Consequently, V-PCC provides a procedure for compressing a time-varying volumetric scene/object by projecting 3D surfaces onto a number of pre-defined 2D planes, which may then be compressed using regular 2D video compression algorithms. The projection is presented using different patches, where each set of patches may represent a specific object or specific parts of a scene.

[0081 ] For explaining the relationship between patches, view representations, layer pairs, layers, coded video sequences (CVS), decoded picture pairs, and atlases, a reference is made to the document N18576 “Working Draft 2 of Metadata for Immersive Video (MIV)”, ISO/IEC JTC 1/SC 29/WG 11, 6 Aug 2019, and to the attached Figure 4 included therein, which shows an illustrative example, in which two atlases contain five patches (patches 2, 3, 5, 7 and 8), which are mapped to three view representations (ViewO, Viewl, View2). [0082] The bitstream contains one or more layer pairs, each layer pair having a texture layer and a depth layer. Each layer contains one or more consecutive CVSes in a unique single independent video coding layer, such as a HE VC independent layer, with each CVS containing a sequence of coded pictures.

[0083] Each layer pair represents a sequence of atlases. An atlas is represented by a decoded picture pair in each access unit, with a texture component picture and a depth component picture. The size of an atlas is equal to the size of the decoded picture of the texture layer representing the atlas. The depth decoded picture size may be equal to the decoded picture size of the corresponding texture layer of the same layer pair. Decoded picture sizes may vary for different layer pairs in the same bitstream.

[0084] A patch may have an arbitrary shape, but in many embodiments it may be preferable to consider the patch as a rectangular region that is represented in both an atlas and a view representation. The size of a particular patch may be the same in both the atlas representation and the view representation.

[0085] An atlas contains an aggregation of one or more patches from one or more view representations, with a corresponding texture component and depth component. The atlas patch occupancy map generator process outputs an atlas patch occupancy map. The atlas patch occupancy map is a 2D array of the same size as the atlas, with each value indicating the index of the patch to which the co-located sample in the atlas corresponds, if any, or otherwise indicates that the sample location has an invalid value.

[0086] A view representation represents a field of view of a 3D scene for particular camera parameters, for the texture and depth component. View representations may be omnidirectional or perspective, and may use different projection formats, such as equirectangular projection or cube map projection. The texture and depth components of a view representation may use the same projection format and have the same size.

[0087] The decoding process may be illustrated by Figure 5, which shows a decoder reference architecture for immersive video as defined in N18576. The bitstream comprises a CVS for each texture and depth layer of a layer pair, which is input to a 2D video decoder, such as an HEVC decoder, which outputs a sequence of decoded picture pairs of synchronized decoded texture pictures (A) and decoded depth pictures (B). Each decoded picture pair represents an atlas (C). [0088] The metadata is input to a metadata parser which outputs an atlas parameters list (D), and camera parameters list (E). The atlas patch occupancy map takes as inputs the depth decoded picture (B) and the atlas parameters list (D) and outputs an atlas patch occupancy map (F). In the reference architecture, a hypothetical reference Tenderer take as inputs one or more decoded atlases (C), the atlas parameters list (D), the camera parameters list (E), the atlas patch occupancy map sequence (F), and the viewer position and orientation, and outputs a viewport.

[0089] Depending on the viewport to be outputted, the patches of the atlas may be aligned differently, i.e. in addition to different vertical and horizontal positions and sizes, the patches may also be rotated and/or mirrored. Currently, the atlas parameters are defined in Atlas_parameters syntax structure, where patch rotation syntax element is used for indicating the rotation and/or mirroring. The syntax element patch_rotation[ a ][ i ] indicates rotation and mirror of the i-th patch in the a-th atlas relative to the orientation of the patch in the view_id[ a ][ i ]-th view. A 3 -bit table is used for indicating various options for a rotation angle and mirroring along Y axis.

[0090] However, the rotation of patches indicating the desired orientation for any patch is signalled separately for each patch. Transmitting this information per patch takes a relatively high amount of bits and therefore, increases the size of encoded content bitstream that needs to be transmitted for the presentation of the content to the decoded. [0091] In the following, an enhanced method for indicating the rotation of patches for volumetric 3D data will be described in more detail, in accordance with various embodiments.

[0092] A starting point for the method may be considered, for example, that a 3D representation of at least one object, such as a point cloud frame or a 3D mesh, is input in an encoder. The method, which is disclosed in Figure 6, comprises projecting (600) a 3D representation of at least one object onto a plurality of 2D patches; generating (602) a texture component picture, a depth component picture, an occupancy map and auxiliary patch information from the 2D patches; aggregating (604) said plurality of 2D patches with the corresponding texture component picture and the corresponding depth component picture into an atlas; determining (606) a reference value of patch rotation for said atlas based on patch rotation values of said plurality of 2D patches; determining (608) a difference value of patch rotation for each of said 2D patches as a difference between said reference value and a patch rotation value of the 2D patch; and encoding (610) the auxiliary patch information to include said reference value of patch rotation for said atlas and an indication for each of said 2D patches, said indication indicating whether the reference value or the difference value of patch rotation of the 2D patch is to be used for determining the rotation value of said 2D patch.

[0093] Thus, a reference patch rotation for the atlas, which may be referred to herein as atlas rotation, is determined based on the rotation values of each patch in the atlas. The reference patch rotation value may preferably characterize the rotation values of each patch in the atlas as well as possible. Following this, the reference patch rotation value will be communicated to a decoder as a single signal per atlas and the patch rotations, if differing from the reference patch rotation value, will be calculated /communicated based on the difference of them with the reference patch rotation value. This reduces the number of bits that needs to be transmitted for each patch as the rotation angles that needs to be communicated are minimum value for the whole atlas.

[0094] It is further noted that this aspect relates to the encoding of only the auxiliary patch information, which may be encoded into a separate bitstream, which may be stored or transmitted to a decoder as such. The geometry image, the texture image and the occupancy map may each be encoded into separate bitstreams, as well. Alternatively, the auxiliary patch information may be encoded into a common bitstream with one or more of the geometry image, the texture image or the occupancy map.

[0095] Another aspect relates to the operation of a decoder. Figure 7 shows an example of a decoding method comprising receiving (700) a bitstream in a decoder, said bitstream comprising an encoded texture component picture, an encoded depth component picture and an encoded occupancy map indicative of a plurality of 2D patches from a 3D representation of at least one object, wherein said plurality of 2D patches have been aggregated with the corresponding texture component picture and the corresponding depth component picture into an atlas; receiving (702), either in said bitstream or in a further bitstream, an encoded auxiliary patch information from said plurality of 2D patches, wherein said auxiliary patch information comprises a reference value of patch rotation for said atlas and an indication for each of said 2D patches of the atlas, said indication indicating whether the reference value or a difference value of patch rotation of the 2D patch is to be used for determining a rotation value of said 2D patch; decoding (704) the texture component picture by determining for each 2D patch associated with the atlas the rotation value of said 2D patch either as reference value of patch rotation for said atlas or as a difference between said reference value and a patch rotation value of said 2D patch; and decoding (706) the depth component picture, the occupancy map and the auxiliary patch information for reconstructing a 3D representation of said at least one object.

[0096] Thus, the decoder receives and decodes the geometry image, texture image, occupancy map and auxiliary patch information of the plurality of 2D patches as well as the atlas aggregation of said plurality of 2D patches with the corresponding texture component picture and the corresponding depth component picture, received either in a common bitstream or in two or more separate bitstreams. From the auxiliary patch information, the decoder decodes, among other auxiliary patch information, also a reference value of patch rotation for said atlas and an indication for each of said 2D patches of the atlas, said indication indicating whether the reference value or a difference value of patch rotation of the 2D patch is to be used for determining a rotation value of said 2D patch. The texture component picture is decoded, wherein the 2D patches of the atlas aggregated with the texture component picture are rotated according to the auxiliary patch information; i.e. either as reference value of patch rotation for said atlas or as a difference between said reference value and a patch rotation value of said 2D patch. After decoding the depth component picture, the occupancy map and possibly the remining of the auxiliary patch information, the 3D representation of one or more objects is then reconstructed. [0097] According to an embodiment, the reference value is configured to be determined based on at least one of the following: a mean of patch rotation values of said plurality of 2D patches; a median of patch rotation values of said plurality of 2D patches; a mode of patch rotation values of said plurality of 2D patches; a reference value (e.g. mean/median/mode) of the reference values of a predetermined number of previous atlases.

[0098] Thus, various methods for determining the reference rotation value for the atlas, i.e. atlas rotation, may be used, and depending e.g. on the distribution of the patch rotation values of said plurality of 2D patches, it may be advisable to use any of them. For rather equally distributed patch rotation values, a mean (an average) of the patch rotation values of said plurality of 2D patches may be advisable. If the distribution of patch rotation values is biased, using a median value (i.e. the middlemost value in ascending/ descending order) or a mode value (most commonly appearing value) of the patch rotation values of said plurality of 2D patches may provide better results in terms of bit savings. It is noted that one target of the embodiment is to make the difference between the reference value and the patch rotation value of as many as possible of the 2D patches to be zero or at least negligibly small, whereupon the reference value can be used commonly for them instead of patch-specific rotation values.

[0099] As a further option, the reference value may be calculated based on the reference values of N previous atlas frames. Thus, the atlas-specific reference value may be calculated as a mean/median/mode of the reference values of N previous atlas frames or the atlas-specific reference value may be a continuation of said previous reference values. For example, if said previous reference value have changed from 15 degrees to 20 degrees and further to 25 degrees, the next atlas-specific reference value may be determined as a linear continuation of the previous reference values, i.e. 30 degrees.

[0100] According to an embodiment, the reference value to be signalled is configured to be updated temporally based on at least one of the following: after a predetermined number of frames, such as corresponding to a group of pictures (GOP) length; when one or more objects appear on a 3D scene; when one or more objects disappear from a 3D scene; after a predetermined number of frames, wherein said number is dependent on an amount of motion appearing on a 3D scene; after a predetermined number of frames, wherein said number is dependent on the change in the determined reference value per frame; after a predetermined number of frames, wherein said number is dependent on the change of the determined reference value from the latest signaled reference value. [0101] The reference value, atlas rotation, may preferably be signalled to the decoder as at least once per atlas. However, it may preferable to update the reference value, atlas rotation, to be signalled to the decoder temporally for several reasons. It may be defined that the update shall take place after encoding a predetermined number of frames. Herein, the update may take place after every group of pictures (GOP), or at a specific and fixed number of frames e.g. after 32 frames regardless of the GOP size. On the other hand, the predetermined number of frames may depend on an amount of motion appearing on a 3D scene; e.g. if the scene comprises a lot of motion, the atlas rotation value may be updated at every N frames and if the scene comprises little motion, the atlas rotation value may be updated at every M frames where N<M.

[0102] Further, a significant change in the reference value between two or a few consecutive frames may lead to signaling an updated reference value atlas rotation. That is, if the difference between e.g. two temporally consecutive frames is higher than a threshold, then the reference value atlas rotation is to be updated and signaled in or along the current frame. On the other hand, the change may be measured between the determined reference value and the latest signaled reference value; if the difference between atlas rotation value of current frame and the last atlas rotation value that has been communicated to the decoder is higher than a threshold, then the reference value atlas rotation is to be updated and signaled in or along the current frame.

[0103] According to another embodiment, a sequence rotation value will be defined, in a similar manner as the reference value for each atlas, and then only the differences of reference values to this sequence rotation value will be signalled in the bitstream. The sequence rotation value may be valid for at least one coded video sequence (CVS), unless otherwise signalled.

[0104] The update of the reference value to be signalled may also result from one or more objects appearing (entering) on the 3D scene and/or disappearing (leaving) from the 3D scene. Since patches are typically associated specific object(s) on the scene, an object appearing on or disappearing from the scene typically affects to the distribution of patch rotation values of all patches. Thus, it is preferable to update the reference value atlas rotation to be signalled.

[0105] According to an embodiment, the auxiliary patch information is configured to be encoded as a flag for each of said plurality of 2D patches, wherein said flag indicates whether a rotation according to the reference value is to be applied for the 2D patch. [0106] Thus, a flag, which may be referred to herein as patch rotation flag, is used for indicating whether the atlas-specific reference value atlas rotation or the patch-specific difference value of patch rotation of the 2D patch is to be used as the rotation value of said 2D patch. For those patches that are determined to use the common reference value atlas_rotation, significant bit savings are achieved by indicating this with a single flag. [0107] According to an embodiment, a signalling of the reference value of patch rotation for said atlas, and said flag and the difference values of patch rotation for each of said 2D patches are configured to be carried out by at least one syntax element included in an atlas parameter syntax structure or any other suitable syntax structure for ISO/IEC 23090-5 (or similar volumetric video coding technology). Table 1 shows an example of including said at least one syntax element into atlas parameter syntax element.

Table 1: (ISO/IEC 23090-5 example)

[0108] Accordingly, the atlas-specific reference value atlas_rotation[a] may be included in the syntax structure. According to an embodiment, for each patch[i], there may be a further syntax element as a patch-specific flag patch_rotation_flag[a][i] indicating whether the atlas-specific reference value atlas rotation or the patch-specific difference value of patch rotation of the 2D patch is to be used as the rotation value of said 2D patch. If the flag indicates that no atlas-specific reference value atlas rotation can be used for said patch, only then the difference value of patch rotation of said 2D patch patch_rotation_difference[a][i] is included in the syntax structure. Thus, the above syntax structure enables significant bit savings compared to prior solutions.

[0109] According to an embodiment, in response to the difference between said reference value and the patch rotation value of a particular 2D patch is smaller than a threshold value, the apparatus is configured to set the value of the flag for said particular 2D patch as zero.

[0110] Thus, if the difference is smaller than a threshold, wherein the threshold => 0, the patch rotation is to be considered negligible and the patch rotation flag is set to 0, thereby indicating that there is no difference between the patch rotation as compared to the atlas rotation and the common reference value atlas rotation should be used for said patch. [0111] According to an embodiment, in response to the difference between said reference value and the patch rotation value of a particular 2D patch is equal to or greater than a threshold value, the apparatus is configured to set the value of the flag for said particular 2D patch as one and include the difference value of patch rotation for said particular 2D patch. Hence, when the patch rotation value differs significantly (i.e. more than the threshold value) from the common reference value atlas rotation, only then the patch-specific difference value patch_rotation_difference[a][i] is included in the syntax structure.

[0112] According to an embodiment, in response to at least a first and a second patch having a substantially similar difference value of patch rotation, the apparatus is configured to include a reference from the second and any subsequent patches having the substantially similar difference value of patch rotation to the first patch. Thus, for the patches where patch rotation flag is equal to 1 , if the difference value is substantially similar (i.e. within a predefined threshold from the other difference value) to the difference value of another patch, then instead of signaling the difference value, only a reference to the other patch is signaled and the same difference value can be used for the other patch(es) too.

[0113] According to an embodiment, in response to a plurality of patches having a substantially similar difference value of patch rotation, the apparatus is configured to signal only one difference value of patch rotation to be used for all of the plurality of patches. Thus, considering the concept of patch grouping, if the patch rotation flag of all patches in the group is equal to 1 and if the difference value for all/majority of the patches is similar, then only one difference value may be signaled for the whole group of patches indicating that the same difference value is to be applied for every patch in the group of patches.

[0114] The encoding and decoding aspects including at least some of the above embodiments may be illustrated by the flow charts of Figures 8a and 8b. The operation of the encoder is shown in Figure 8a, where a plurality of patches are input (800) in the encoder. A reference value of patch rotation for said atlas, i.e. atlas rotation, is calculated (802) based on patch rotation values of all patches of the atlas. Then the value of patch rotation flag is determined (804) for each patch of the atlas, e.g. by determining if the patch rotation value of said patch is within a threshold value from the atlas rotation value, whereupon a value of zero is assigned to patch rotation flag; otherwise a value of one. In response to the value of patch rotation flag being one (806), the difference between the patch rotation value and the atlas rotation value is determined (808) and the syntax structure to be signalled is provided with (810) the patch rotation difference and the atlas rotation value (812). If the patch rotation flag is assigned with the value of zero (806), only the atlas rotation value (812) needs to be signalled for said patch.

[0115] The operation of the decoder is shown in Figure 8b, where encoded patches and auxiliary patch information are input in the decoder. A reference value of patch rotation for each atlas, i.e. atlas rotation, is obtained (820) from a syntax structure of the auxiliary patch information. Then the value of patch rotation flag is obtained (822) for each patch of the atlas. In response to the value of patch rotation flag being one (822), the difference between the patch rotation value and the atlas rotation value, i.e. patch rotation difference, is obtained (826) and patch rotation value is determined (828) to be the sum of the atlas rotation value and the patch rotation difference value. In response to the value of patch rotation flag being zero (822), and patch rotation value is determined (830) to be the atlas rotation value. Then the patch rotation values of each patch are applied (832) patch-specifically.

[0116] According to an embodiment, in response to a plurality of atlas images being encoded, one atlas rotation may be predicted from another atlas rotation. Thus, a reference or a prediction dependency may be applied, not only patches of the same atlas, but also between different atlases.

[0117] Two or more of the embodiments as described above may be combined, and they may be introduced as one or more indicators in any suitable syntax structure for ISO/IEC 23090-5 (or similar volumetric video coding technology).

[011 ] Consequently, the embodiments as described herein enable to reducing the required bitrate to signal the patch rotations. Moreover, the embodiments enable to reduce the complexity of patch rotation signaling by removing the parameter from patches where patch rotation flag parameter is set to 0.

[0119] The embodiments relating to the encoding aspects may be implemented in an apparatus comprising: means for projecting a 3D representation of at least one object onto a plurality of 2D patches; means for generating a texture component picture, a depth component picture, an occupancy map and auxiliary patch information from the 2D patches; means for aggregating said plurality of 2D patches with the corresponding texture component picture and the corresponding depth component picture into an atlas; means for determining a reference value of patch rotation for said atlas based on patch rotation values of said plurality of 2D patches; means for determining a difference value of patch rotation for each of said 2D patches as a difference between said reference value and a patch rotation value of the 2D patch; and means for encoding the auxiliary patch information to include said reference value of patch rotation for said atlas and an indication for each of said 2D patches, said indication indicating whether the reference value or the difference value of patch rotation of the 2D patch is to be used for determining the rotation value of said 2D patch.

[0120] The embodiments relating to the encoding aspects may likewise be implemented in an apparatus comprising at least one processor and at least one memory, said at least one memory stored with computer program code thereon, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: project a 3D representation of at least one object onto a plurality of 2D patches; generate a texture component picture, a depth component picture, an occupancy map and auxiliary patch information from the 2D patches; aggregate said plurality of 2D patches with the corresponding texture component picture and the corresponding depth component picture into an atlas; determine a reference value of patch rotation for said atlas based on patch rotation values of said plurality of 2D patches; determine a difference value of patch rotation for each of said 2D patches as a difference between said reference value and a patch rotation value of the 2D patch; and encode the auxiliary patch information to include said reference value of patch rotation for said atlas and an indication for each of said 2D patches, said indication indicating whether the reference value or the difference value of patch rotation of the 2D patch is to be used for determining the rotation value of said 2D patch.

[0121] The embodiments relating to the decoding aspects may be implemented in an apparatus comprising means for receiving a bitstream in a decoder, said bitstream comprising an encoded texture component picture, an encoded depth component picture and an encoded occupancy map indicative of a plurality of 2D patches from a 3D representation of at least one object, wherein said plurality of 2D patches have been aggregated with the corresponding texture component picture and the corresponding depth component picture into an atlas; means for receiving, either in said bitstream or in a further bitstream, an encoded auxiliary patch information from said plurality of 2D patches, wherein said auxiliary patch information comprises a reference value of patch rotation for said atlas and an indication for each of said 2D patches of the atlas, said indication indicating whether the reference value or a difference value of patch rotation of the 2D patch is to be used for determining a rotation value of said 2D patch; means for decoding the texture component picture by determining for each 2D patch associated with the atlas the rotation value of said 2D patch either as reference value of patch rotation for said atlas or as a difference between said reference value and a patch rotation value of said 2D patch; and means for decoding the depth component picture, the occupancy map and the auxiliary patch information for reconstructing a 3D representation of said at least one object. [0122] The embodiments relating to the decoding aspects may likewise be implemented in an apparatus comprising at least one processor and at least one memory, said at least one memory stored with computer program code thereon, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform receive a bitstream in a decoder, said bitstream comprising an encoded texture component picture, an encoded depth component picture and an encoded occupancy map indicative of a plurality of 2D patches from a 3D representation of at least one object, wherein said plurality of 2D patches have been aggregated with the corresponding texture component picture and the corresponding depth component picture into an atlas; receive, either in said bitstream or in a further bitstream, an encoded auxiliary patch information from said plurality of 2D patches, wherein said auxiliary patch information comprises a reference value of patch rotation for said atlas and an indication for each of said 2D patches of the atlas, said indication indicating whether the reference value or a difference value of patch rotation of the 2D patch is to be used for determining a rotation value of said 2D patch; decode the texture component picture by determining for each 2D patch associated with the atlas the rotation value of said 2D patch either as reference value of patch rotation for said atlas or as a difference between said reference value and a patch rotation value of said 2D patch; and decode the depth component picture, the occupancy map and the auxiliary patch information for reconstructing a 3D representation of said at least one object.

[0123] Such apparatuses may comprise e.g. the functional units disclosed in any of the Figures la, lb, 2a and 2b for implementing the embodiments.

[0124] In the above, some embodiments have been described with reference to encoding. It needs to be understood that said encoding may comprise one or more of the following: encoding source image data into a bitstream, encapsulating the encoded bitstream in a container file and/or in packet(s) or stream(s) of a communication protocol, and announcing or describing the bitstream in a content description, such as the Media Presentation Description (MPD) of ISO/IEC 23009-1 (known as MPEG-DASH) or the IETF Session Description Protocol (SDP). Similarly, some embodiments have been described with reference to decoding. It needs to be understood that said decoding may comprise one or more of the following: decoding image data from a bitstream, decapsulating the bitstream from a container file and/or from packet(s) or stream(s) of a communication protocol, and parsing a content description of the bitstream,

[0125] In the above, where the example embodiments have been described with reference to an encoder or an encoding method, it needs to be understood that the resulting bitstream and the decoder or the decoding method may have corresponding elements in them. Likewise, where the example embodiments have been described with reference to a decoder, it needs to be understood that the encoder may have structure and/or computer program for generating the bitstream to be decoded by the decoder.

[0126] In the above, some embodiments have been described with reference to encoding or decoding texture pictures, geometry pictures, (optionally) attribute pictures and auxiliary patch information into or from a single bitstream. It needs to be understood that embodiments can be similarly realized when encoding or decoding texture pictures, geometry pictures, (optionally) attribute pictures and auxiliary patch information into or from several bitstreams that are associated with each other, e.g. by metadata in a container file or media presentation description for streaming.

[0127] In general, the various embodiments of the invention may be implemented in hardware or special purpose circuits or any combination thereof. While various aspects of the invention may be illustrated and described as block diagrams or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

[0128] Embodiments of the inventions may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

[0129] Programs, such as those provided by Synopsys, Inc. of Mountain View, California and Cadence Design, of San Jose, California automatically route conductors and locate components on a semiconductor chip using well established rules of design as well as libraries of pre stored design modules. Once the design for a semiconductor circuit has been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or the like) may be transmitted to a semiconductor fabrication facility or "fab" for fabrication.

[0130] The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended examples. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention.

Claims

1. A method comprising: projecting a 3D representation of at least one object onto a plurality of 2D patches; generating a texture component picture, a depth component picture, an occupancy map and auxiliary patch information from the 2D patches; aggregating said plurality of 2D patches with the corresponding texture component picture into an atlas; determining a reference value of patch rotation for said atlas based on patch rotation values of said plurality of 2D patches; determining a difference value of patch rotation for each of said 2D patches as a difference between said reference value and a patch rotation value of any of the 2D patches; and encoding the auxiliary patch information to include said reference value of patch rotation for said atlas and an indication for each of said 2D patches, said indication indicating whether the reference value or the difference value of patch rotation of the 2D patch is to be used for determining the patch rotation value of said 2D patch.

2. An apparatus comprising at least one processor and at least one memory, said at least one memory stored with computer program code thereon, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: project a 3D representation of at least one object onto a plurality of 2D patches; generate a texture component picture, a depth component picture, an occupancy map and auxiliary patch information from the 2D patches; aggregate said plurality of 2D patches with the corresponding texture component picture into an atlas; determine a reference value of patch rotation for said atlas based on patch rotation values of said plurality of 2D patches; determine a difference value of patch rotation for each of said 2D patches as a difference between said reference value and a patch rotation value of the 2D patch; and encode the auxiliary patch information to include said reference value of patch rotation for said atlas and an indication for each of said 2D patches, said indication indicating whether the reference value or the difference value of patch rotation of the 2D patch is to be used for determining the patch rotation value of said 2D patch.

3. An apparatus comprising: means for projecting a 3D representation of at least one object onto a plurality of 2D patches; means for generating a texture component picture, a depth component picture, an occupancy map and auxiliary patch information from the 2D patches; means for aggregating said plurality of 2D patches with the corresponding texture component picture into an atlas; means for determining a reference value of patch rotation for said atlas based on patch rotation values of said plurality of 2D patches; means for determining a difference value of patch rotation for each of said 2D patches as a difference between said reference value and a patch rotation value of the 2D patch; and means for encoding the auxiliary patch information to include said reference value of patch rotation for said atlas and an indication for each of said 2D patches, said indication indicating whether the reference value or the difference value of patch rotation of the 2D patch is to be used for determining the patch rotation value of said 2D patch.

4. The apparatus according to claim 2 or 3, wherein the reference value is configured to be determined based on at least one of the following: a mean of patch rotation values of said plurality of 2D patches; a median of patch rotation values of said plurality of 2D patches; a mode of patch rotation values of said plurality of 2D patches; a reference value of the reference values of a predetermined number of previous atlases.

5. The apparatus according to any of claims 2 - 4, wherein the reference value is configured to be updated temporally based on at least one of the following: after a predetermined number of frames, such as corresponding to a group of pictures (GOP) length; when one or more objects appear on a 3D scene; when one or more objects disappear from a 3D scene; after a predetermined number of frames, wherein said number is dependent on an amount of motion appearing on a 3D scene; after a predetermined number of frames, wherein said number is dependent on the change in the determined reference value per frame; after a predetermined number of frames, wherein said number is dependent on the change of the determined reference value from the latest signaled reference value.

6. The apparatus according to any of claims 2 - 5, wherein said indication is configured to be encoded as a flag for each of said plurality of 2D patches, wherein said flag indicates whether a rotation according to the reference value is to be applied for the 2D patch.

7. The apparatus according to claim 6, wherein a signalling of the reference value of patch rotation for said atlas, and said flag and the difference values of patch rotation for each of said 2D patches are configured to be carried out by at least one syntax element included in an atlas parameter syntax structure.

8. The apparatus according to claim 7, wherein, in response to the difference between said reference value and the patch rotation value of a particular 2D patch is smaller than a threshold value, the apparatus is configured to set the value of the flag for said particular 2D patch as zero.

9. The apparatus according to claim 7, wherein, in response to the difference between said reference value and the patch rotation value of a particular 2D patch is equal to or greater than a threshold value, the apparatus is configured to set the value of the flag for said particular 2D patch as one and include the difference value of patch rotation for said particular 2D patch.

10. A method comprising: receiving a bitstream in a decoder, said bitstream comprising an encoded texture component picture, an encoded depth component picture and an encoded occupancy map indicative of a plurality of 2D patches from a 3D representation of at least one object, wherein said plurality of 2D patches have been aggregated with the corresponding texture component picture into an atlas; receiving, either in said bitstream or in a further bitstream, an encoded auxiliary patch information from said plurality of 2D patches, wherein said auxiliary patch information comprises a reference value of patch rotation for said atlas and an indication for each of said 2D patches of the atlas, said indication indicating whether the reference value or a difference value of patch rotation of the 2D patch is to be used for determining a rotation value of said 2D patch; decoding the texture component picture by determining for each 2D patch associated with the atlas the rotation value of said 2D patch either as reference value of patch rotation for said atlas or as a difference between said reference value and a patch rotation value of said 2D patch; and decoding the depth component picture, the occupancy map and the auxiliary patch information for reconstructing a 3D representation of said at least one object.

11. An apparatus comprising at least one processor and at least one memory, said at least one memory stored with computer program code thereon, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: receive a bitstream in a decoder, said bitstream comprising an encoded texture component picture, an encoded depth component picture and an encoded occupancy map indicative of a plurality of 2D patches from a 3D representation of at least one object, wherein said plurality of 2D patches have been aggregated with the corresponding texture component picture into an atlas; receive, either in said bitstream or in a further bitstream, an encoded auxiliary patch information from said plurality of 2D patches, wherein said auxiliary patch information comprises a reference value of patch rotation for said atlas and an indication for each of said 2D patches of the atlas, said indication indicating whether the reference value or a difference value of patch rotation of the 2D patch is to be used for determining a rotation value of said 2D patch; decode the texture component picture by determining for each 2D patch associated with the atlas the rotation value of said 2D patch either as reference value of patch rotation for said atlas or as a difference between said reference value and a patch rotation value of said 2D patch; and decode the depth component picture, the occupancy map and the auxiliary patch information for reconstructing a 3D representation of said at least one object.

12. An apparatus comprising: means for receiving a bitstream in a decoder, said bitstream comprising an encoded texture component picture, an encoded depth component picture and an encoded occupancy map indicative of a plurality of 2D patches from a 3D representation of at least one object, wherein said plurality of 2D patches have been aggregated with the corresponding texture component picture into an atlas; means for receiving, either in said bitstream or in a further bitstream, an encoded auxiliary patch information from said plurality of 2D patches, wherein said auxiliary patch information comprises a reference value of patch rotation for said atlas and an indication for each of said 2D patches of the atlas, said indication indicating whether the reference value or a difference value of patch rotation of the 2D patch is to be used for determining a rotation value of said 2D patch; means for decoding the texture component picture by determining for each 2D patch associated with the atlas the rotation value of said 2D patch either as reference value of patch rotation for said atlas or as a difference between said reference value and a patch rotation value of said 2D patch; and means for decoding the depth component picture, the occupancy map and the auxiliary patch information for reconstructing a 3D representation of said at least one object.

13. The apparatus according to claims 11 or 12, wherein said indication is a flag for each of said plurality of 2D patches, wherein said flag indicates whether a rotation according to the reference value is to be applied for the 2D patch.

14. The apparatus according to claim 13, wherein a signalling of the reference value of patch rotation for said atlas, and said flag and the difference values of patch rotation for each of said 2D patches are configured to be received by at least one syntax element included in an atlas parameter syntax structure.

15. The apparatus according to claim 14, wherein, in response to the value of the flag for said particular 2D patch is zero, the apparatus is configured to apply the reference value of patch rotation as the rotation value for said atlas for said particular 2D patch.

16. The apparatus according to claim 14, wherein, in response to the value of the flag for said particular 2D patch is one, the apparatus is configured to apply the difference between said reference value and the patch rotation value of said 2D patch as the rotation value for said atlas for said particular 2D patch.