JP2024058011A - Point group decoding device, point group decoding method and program - Google Patents

Abstract

[Problem] To improve coding efficiency. [Solution] The point cloud decoding device 200 according to the present invention includes an approximate surface synthesis unit 2030 that performs vertex interpolation processing using vertices of a node with a larger node size at the boundary between nodes with different sizes, and, when the vertex that has undergone the interpolation processing exists on an edge of a node with a smaller node size, performs Trisoup processing with the vertex that has undergone the interpolation processing as a vertex of the edge of the node with the smaller node size. [Selected Figure] Figure 2

Description

The present invention relates to a point cloud decoding device, a point cloud decoding method, and a program.

Non-Patent Document 1 discloses a geometry information encoding technique called Trisoup.

G-PCC Future Enhancement, ISO/IEC JTC1/SC29/WG11 N19328

However, the method in Non-Patent Document 1 has the problem that Trisoup can only be performed with a fixed node size for each slice.

The present invention has been made in consideration of the above-mentioned problems, and aims to provide a point cloud decoding device, a point cloud decoding method, and a program that are expected to improve coding efficiency by using values interpolated from vertices of larger node sizes at the boundaries between nodes of different node sizes, instead of decoding vertices of smaller node sizes.

The first feature of the present invention is a point cloud decoding device that includes an approximate surface synthesis unit that performs vertex interpolation processing at the boundary between nodes having different sizes, using the vertices of the node with the larger node size, and when the vertex that has undergone the interpolation processing exists on an edge of the node with the smaller node size, performs a Trisoup processing with the vertex that has undergone the interpolation processing as the vertex of the edge of the node with the smaller node size.

The second feature of the present invention is a point cloud decoding method that includes the steps of: performing vertex interpolation processing at the boundary between nodes having different sizes, using the vertices of the node with the larger node size; and, when the vertex on which the interpolation processing has been performed is on an edge of the node with the smaller node size, performing a Trisoup processing with the vertex on which the interpolation processing has been performed as the vertex of the edge of the node with the smaller node size.

The third feature of the present invention is a program that causes a computer to function as a point cloud decoding device, and the point cloud decoding device is provided with an approximate surface synthesis unit that performs vertex interpolation processing using vertices of a node with a larger node size at the boundary between nodes with different sizes, and when the vertex that has undergone the interpolation processing is on an edge of a node with a smaller node size, performs a Trisoup processing with the vertex that has undergone the interpolation processing as a vertex of the edge of the node with the smaller node size.

According to the present invention, it is possible to provide a point cloud decoding device, a point cloud decoding method, and a program that can be expected to improve coding efficiency by using values interpolated from vertices of a larger node size at the boundary between nodes of different node sizes instead of decoding vertices of a smaller node size.

FIG. 1 is a diagram illustrating an example of the configuration of a point cloud processing system 10 according to an embodiment. FIG. 2 is a diagram illustrating an example of functional blocks of a point group decoding device 200 according to an embodiment. FIG. 3 is a diagram showing an example of the configuration of encoded data (bit stream) received by the geometric information decoding unit 2010 of the point cloud decoding device 200 according to an embodiment. FIG. 4 is a diagram showing an example of the syntax configuration of GPS2011. FIG. 5 is a diagram showing an example of the syntax configuration of GSH2012. FIG. 6 is a flowchart illustrating an example of a process in the tree synthesis unit 2020 of the point group decoding device 200 according to an embodiment. FIG. 7 is a flowchart showing an example of processing by the approximate surface synthesis unit 2030 of the point cloud decoding device 200 according to an embodiment. FIG. 8 is a flowchart showing an example of the process of step S703 shown in FIG. FIG. 9 is a flowchart showing an example of the process of step S704 shown in FIG. FIG. 10 is a flowchart showing a specific example of a method for generating mask information. FIG. 11 is a diagram showing an example of generation of segments and mask values. FIG. 12 is a flowchart showing an example of the process of step S705 shown in FIG. FIG. 13 is a flowchart showing an example of the process of step S706 shown in FIG. FIG. 14 is a flowchart showing an example of the process of step S708 shown in FIG. FIG. 15 is a diagram for explaining an example of the process of step S1302 in FIG. FIG. 16 is a diagram for explaining an example of the process of step S1305 in FIG. FIG. 17 is a diagram showing an example of functional blocks of the point group encoding device 100 according to this embodiment.

The following describes embodiments of the present invention with reference to the drawings. Note that the components in the following embodiments can be replaced with existing components as appropriate, and various variations, including combinations with other existing components, are possible. Therefore, the description of the following embodiments does not limit the content of the invention described in the claims.

First Embodiment
A point cloud processing system 10 according to a first embodiment of the present invention will be described below with reference to Figures 1 to 17. Figure 1 is a diagram showing a point cloud processing system 10 according to the present embodiment.

As shown in FIG. 1, the point cloud processing system 10 has a point cloud encoding device 100 and a point cloud decoding device 200.

The point cloud encoding device 100 is configured to generate encoded data (bit stream) by encoding an input point cloud signal. The point cloud decoding device 200 is configured to generate an output point cloud signal by decoding the bit stream.

The input point cloud signal and the output point cloud signal are composed of position information and attribute information of each point in the point cloud. The attribute information is, for example, color information and reflectance of each point.

Here, such a bit stream may be transmitted from the point cloud encoding device 100 to the point cloud decoding device 200 via a transmission path. Also, the bit stream may be stored in a storage medium and then provided from the point cloud encoding device 100 to the point cloud decoding device 200.

(Point Cloud Decoding Device 200)
Hereinafter, the point group decoding device 200 according to this embodiment will be described with reference to Fig. 2. Fig. 2 is a diagram showing an example of functional blocks of the point group decoding device 200 according to this embodiment.

As shown in FIG. 2, the point cloud decoding device 200 has a geometric information decoding unit 2010, a tree synthesis unit 2020, an approximate surface synthesis unit 2030, a geometric information reconstruction unit 2040, an inverse coordinate transformation unit 2050, an attribute information decoding unit 2060, an inverse quantization unit 2070, a RAHT unit 2080, an LoD calculation unit 2090, an inverse lifting unit 2100, and an inverse color transformation unit 2110.

The geometric information decoding unit 2010 is configured to receive as input a bit stream related to geometric information (geometric information bit stream) from the bit streams output from the point cloud encoding device 100, and to decode the syntax.

The decoding process is, for example, a context-adaptive binary arithmetic decoding process. Here, for example, the syntax includes control data (flags and parameters) for controlling the decoding process of the position information.

The tree synthesis unit 2020 is configured to receive as input the control data decoded by the geometric information decoding unit 2010 and an occurrence code indicating at which node in the tree (described later) the point group exists, and generate tree information indicating in which area in the decoding target space the point exists.

The decoding process of the occasion code may be configured to be performed within the tree synthesis unit 2020.

This process divides the space to be decoded into rectangular parallelepipeds, refers to the occupancy code to determine whether a point exists in each rectangular parallelepiped, divides the rectangular parallelepiped in which the point exists into multiple rectangular parallelepipeds, and then refers to the occupancy code. This process is repeated recursively to generate tree information.

Here, when decoding such an occasion code, inter prediction, which will be described later, may be used.

In this embodiment, a method called "Octree" can be used, which recursively performs octree division on the above-mentioned rectangular parallelepiped, always treating it as a cube, and a method called "QtBt" can be used, which performs quadtree division and binary tree division in addition to octree division. Whether or not to use "QtBt" is transmitted from the point cloud encoding device 100 as control data.

In this embodiment, the method of decoding geometric information by dividing the above-mentioned cube using "Octree" until it becomes 1x1x1 in size is specifically referred to as "Octree only."

In addition, even if "Octree" is used in combination with "QtBt", the method of decrypting geometric information by dividing the above-mentioned rectangular parallelepiped until it becomes 1x1x1 size using only "Octree" and "QtBt" in the same manner as above may also be called "Octree only".

Alternatively, when the control data specifies that predictive coding is to be used, the tree synthesis unit 2020 is configured to decode the coordinates of each point based on an arbitrary tree configuration determined by the point cloud encoding device 100.

The approximate surface synthesis unit 2030 is configured to generate approximate surface information using the tree information generated by the tree synthesis unit 2020, and to decode the point cloud based on the approximate surface information.

When decoding three-dimensional point cloud data of an object, for example, if the points are densely distributed on the object's surface, approximate surface information is used to represent the area in which the points exist by approximating the area using a small plane, rather than decoding each point individually.

Specifically, the approximate surface synthesis unit 2030 can generate approximate surface information and decode the point cloud using a method called "Trisoup", for example. A specific processing example of "Trisoup" will be described later. In addition, when decoding a sparse point cloud acquired by Lidar or the like, this processing can be omitted.

The geometric information reconstruction unit 2040 is configured to reconstruct the geometric information (position information in the coordinate system assumed by the decoding process) of each point of the point cloud data to be decoded based on the tree information generated by the tree synthesis unit 2020 and the approximate surface information generated by the approximate surface synthesis unit 2030.

The inverse coordinate transformation unit 2050 is configured to receive the geometric information reconstructed by the geometric information reconstruction unit 2040 as input, transform it from the coordinate system assumed by the decoding process to the coordinate system of the output point cloud signal, and output position information.

The attribute information decoding unit 2060 is configured to receive as input a bit stream related to attribute information (attribute information bit stream) from the bit streams output from the point cloud encoding device 100, and to decode the syntax.

The decoding process is, for example, a context-adaptive binary arithmetic decoding process. Here, for example, the syntax includes control data (flags and parameters) for controlling the decoding process of the attribute information.

The attribute information decoding unit 2060 is also configured to decode the quantized residual information from the decoded syntax.

The inverse quantization unit 2070 is configured to perform an inverse quantization process based on the quantized residual information decoded by the attribute information decoding unit 2060 and the quantization parameter, which is one of the control data decoded by the attribute information decoding unit 2060, to generate inverse quantized residual information.

The dequantized residual information is output to either the RAHT unit 2080 or the LoD calculation unit 2090 depending on the characteristics of the point group to be decoded. The control data decoded by the attribute information decoding unit 2060 specifies which unit the information is output to.

The RAHT unit 2080 is configured to receive the inverse quantized residual information generated by the inverse quantization unit 2070 and the geometric information generated by the geometric information reconstruction unit 2040 as input, and to decode the attribute information of each point using a type of Haar transform (inverse Haar transform in the decoding process) called RAHT (Region Adaptive Hierarchical Transform). As a specific example of the RAHT process, the method described in Non-Patent Document 1 can be used.

The LoD calculation unit 2090 is configured to receive the geometric information generated by the geometric information reconstruction unit 2040 as input and generate the LoD (Level of Detail).

LoD is information for defining a reference relationship (a referencing point and a referenced point) to realize predictive coding, such as predicting attribute information of a certain point from attribute information of another point and encoding or decoding the prediction residual.

In other words, LoD is information that defines a hierarchical structure in which each point contained in the geometric information is classified into multiple levels, and the attributes of points belonging to lower levels are encoded or decoded using attribute information of points belonging to higher levels.

As a specific method for determining LoD, for example, the method described in the above-mentioned non-patent document 1 may be used.

The inverse lifting unit 2100 is configured to decode attribute information of each point based on the hierarchical structure defined by the LoD, using the LoD generated by the LoD calculation unit 2090 and the inverse quantized residual information generated by the inverse quantization unit 2070. As a specific example of the inverse lifting process, the method described in the above-mentioned non-patent document 1 can be used.

The inverse color conversion unit 2110 is configured to perform inverse color conversion processing on the attribute information output from the RAHT unit 2080 or the inverse lifting unit 2100 when the attribute information to be decoded is color information and color conversion has been performed on the point cloud encoding device 100 side. Whether or not such inverse color conversion processing is performed is determined by the control data decoded by the attribute information decoding unit 2060.

The point cloud decoding device 200 is configured to decode and output attribute information of each point in the point cloud through the above processing.

(Geometric information decoding unit 2010)
The control data decoded by the geometric information decoding unit 2010 will be described below with reference to FIGS.

Figure 3 shows an example of the structure of the encoded data (bit stream) received by the geometric information decoding unit 2010.

First, the bit stream may include a GPS2011. A GPS2011 is also called a geometry parameter set, and is a collection of control data related to decoding of geometric information. A specific example will be described later. Each GPS2011 includes at least GPS id information for identifying each GPS2011 when multiple GPS2011 exist.

Secondly, the bit stream may include GSH2012A/2012B. GSH2012A/2012B is also called a geometry slice header or geometry data unit header, and is a collection of control data corresponding to a slice, which will be described later. In the following description, the term "slice" will be used, but slice can also be interpreted as a data unit. Specific examples will be described later. GSH2012A/2012B includes at least GPS id information for specifying the GPS2011 corresponding to each GSH2012A/2012B.

Thirdly, the bit stream may include slice data 2013A/2013B following GSH 2012A/2012B. Slice data 2013A/2013B includes data that encodes geometric information. An example of slice data 2013A/2013B is an occasion code, which will be described later.

As described above, the bit stream is structured so that each slice data 2013A/2013B corresponds to one GSH 2012A/2012B and one GPS 2011.

As described above, the GPS ID information is used to specify which GPS 2011 to refer to in GSH 2012A/2012B, so a common GPS 2011 can be used for multiple slice data 2013A/2013B.

In other words, GPS2011 does not necessarily have to be transmitted for each slice. For example, as shown in FIG. 3, the bit stream may be configured so that GPS2011 is not encoded immediately before GSH2012B and slice data 2013B.

Note that the configuration in FIG. 3 is merely an example. As long as GSH 2012A/2012B and GPS 2011 correspond to each slice data 2013A/2013B, elements other than those described above may be added as components of the bit stream.

For example, as shown in FIG. 3, the bitstream may include a sequence parameter set (SPS) 2001. Similarly, when transmitted, the bitstream may be shaped into a configuration different from that shown in FIG. 3. Furthermore, the bitstream may be combined with a bitstream decoded by an attribute information decoding unit 2060 (described later) and transmitted as a single bitstream.

Figure 4 is an example of the syntax configuration of GPS2011.

Note that the syntax names explained below are merely examples. If the syntax functions explained below are similar, the syntax names may be different.

GPS2011 may include GPS ID information (gsps_geom_parameter_set_id) for identifying each GPS2011.

The Descriptor column in Figure 4 indicates how each syntax is coded. ue(v) means that it is an unsigned zeroth-order exponential Golomb code, and u(1) means that it is a 1-bit flag.

GPS2011 may include a flag (trisoup_enabled_flag) that controls whether or not Trisoup is used in the approximate surface synthesis unit 2030.

For example, it may be defined that Trisoup is not used when the value of trisoup_enabled_flag is "0", and that Trisoup is used when the value of trisoup_enabled_flag is "1".

The geometric information decoding unit 2020 may be configured to additionally decode the following syntax when Trisoup is used, i.e., when the value of trisoup_enabled_flag is "1".

Note that trisoup_enabled_flag may be included in SPS2001 instead of GPS2011.

GPS2011 may include a flag (trisoup_multilevel_enabled_flag, first flag) that controls whether or not trisoup is allowed at multiple levels.

For example, when the value of trisoup_multilevel_enabled_flag is "0", multi-level trisoup is not allowed, i.e., trisoup is performed at a single level, and when the value of trisoup_multilevel_enabled_flag is "1", multi-level trisoup is allowed.

If this syntax is not included in GPS2011, the value of this syntax may be considered to be the value when performing a trisoup at a single level, i.e., "0".

Note that trisoup_multilevel_enabled_flag may be defined to be included in SPS2001 instead of GPS2011. In this case, if trisoup_multilevel_enabled_flag is not included in SPS2001, the value of the syntax may be regarded as the value for performing trisoup at a single level, i.e., "0".

Figure 5 shows an example of the syntax configuration of GSH2012. As mentioned above, GSH is also called GDUH (Geometry Data Unit Header).

The geometric information decoding unit 2020 may be configured to additionally decode the following syntax when Trisoup at multiple levels is permitted, i.e., when the value of trisoup_multilevel_enabled_flag is "1".

GSH2012 may include syntax (log2_trisoup_max_node_size_minus2) to specify the maximum Trisoup node size when allowing multi-level Trisoup.

The syntax may be expressed as a value obtained by converting the maximum actual Trisoup node size to a logarithm with base 2. Furthermore, the syntax may be expressed as a value obtained by subtracting 2 from the logarithm with base 2, for the maximum actual Trisoup node size.

GSH2012 may include syntax (log2_trisoup_min_node_size_minus2) to specify a minimum Trisoup node size when multi-level Trisoup is allowed.

This syntax may be expressed as a value obtained by converting the minimum value of the actual Trisoup node size to a logarithm with the base 2. Furthermore, this syntax may be expressed as a value obtained by subtracting 2 from the logarithm obtained by converting the minimum value of the actual Trisoup node size to 4 (=2 ² ) and converting it to a logarithm with the base 2.

The value of this syntax may also be constrained to be greater than or equal to 0 and less than or equal to log2_trisoup_max_node_size_minus2.

In this case, trisoup_depth may be defined as trisoup_depth = log2_trisoup_max_node_size_minus2 - log2_trisoup_min_node_size_minus2 + 1, as shown in Figure 5.

In addition, instead of directly decoding the minimum and maximum Trisoup node sizes, the Depth values corresponding to the maximum and minimum Trisoup node sizes in the Octree processing described below may be decoded.

For example, if the maximum Depth (the Depth at which all nodes are 1x1x1 in size) is 10, and you want the minimum Trisoup node size to be 4 (= ²² ) and the maximum Trisoup node size to be 16 (= ²⁴ ), you may decode the Depth value corresponding to the minimum Trisoup node size as 8 and the Depth value corresponding to the maximum Trisoup node size as 6.

The geometric information decoding unit 2020 may be configured to additionally decode the following syntax when multi-level Trisoup is not permitted, i.e., when the value of trisoup_multilevel_enabled_flag is "0".

GSH2012 may include syntax (log2_trisoup_node_size_minus2) to specify the Trisoup node size when multi-level Trisoup is not allowed and when Trisoup is used.

The syntax may be expressed as a value obtained by converting the actual Trisoup node size to a logarithm with base 2. Furthermore, the syntax may be expressed as a value obtained by subtracting 2 from the logarithm with base 2 with respect to the actual Trisoup node size.

In this case, trisoup_depth may be defined as trisoup_depth = 1, as shown in Figure 5.

When using Trisoup, GSH2012 may include a syntax (trisoup_sampling_value_minus1) that controls the sampling interval of the decoding point. The specific definition of the syntax may be the same as that described in the above-mentioned non-patent document 1, for example.

GSH2012 may include a syntax (trisoup_vertex_number_bits) that specifies the precision (number of bits) of the vertex position of Trisoup, which will be described later. For example, if the value of this syntax is 2, this means that the vertex position is 2 bits, meaning that it can take on four values: 0, 1, 2, and 3.

Here, as described above, trisoup_vertex_number_bits may always transmit only one value regardless of the value of trisoup_depth, or the number to be decoded may be changed depending on the value of trisoup_depth.

For example, trisoup_vertex_number_bits corresponding to each trisoup_depth may be decoded. In other words, the same number of trisoup_vertex_number_bits as trisoup_depth may be decoded. Specifically, for example, when trisoup_depth is 2, two types of values of trisoup_vertex_number_bits may be transmitted.

GSH2012 may include a flag (trisoup_centroid_vertex_residual_flag) indicating whether or not to decode the centroid residuals of the vertices of Trisoup, which will be described later. For example, the flag may be defined so that a value of 1 indicates that the centroid residuals are decoded, and a value of 0 indicates that the centroid residuals are not decoded.

Here, as described above, trisoup_centroid_vertex_residual_flag may always transmit only one value regardless of the value of trisoup_depth, or the number of decoded bits may be changed depending on the value of trisoup_depth.

For example, trisoup_centroid_vertex_residual_flag corresponding to each trisoup_depth may be decoded. In other words, the same number of trisoup_centroid_vertex_residual_flag as trisoup_depth may be decoded. Specifically, for example, when trisoup_depth is 2, two types of values of trisoup_centroid_vertex_residual_flag may be transmitted.

When GSH2012 uses Trisoup and allows Trisoup at multiple levels, it may include a flag (unique_segments_exist_flag[i]) for each hierarchical level i (i=0,..., trisoup_depth-1) indicating whether a unique segment exists in the target hierarchical level.

For example, if the value of unique_segments_exist_flag[i] is "1", it means that at least one unique segment exists in hierarchical layer i. Also, if the value of unique_segments_exist_flag[i] is "0", it means that no unique segments exist in hierarchical layer i.

For each hierarchical layer i (i = 0, ..., trisoup_depth-1), if a unique segment exists in the target hierarchical layer, i.e., if the value of unique_segments_exist_flag[i] is "1", GSH2012 may additionally include syntax (num_unique_segments_bits_minus1[i]) indicating the number of bits of the syntax indicating the number of unique segments in the target hierarchical layer and syntax (num_unique_segments_minus1[i]) indicating the number of unique segments in the target hierarchical layer.

Here, for both num_unique_segments_bits_minus1[i] and num_unique_segments_minus1[i], the original value minus "1" may be encoded as the syntax value.

(Tree synthesis unit 2020)
The processing of the tree merging unit 2020 will be described below with reference to Fig. 6. Fig. 6 is a flowchart showing an example of the processing in the tree merging unit 2020. Note that, below, an example of merging trees using "Octree" will be described.

In step S601, the tree synthesis unit 2020 checks whether processing for all depths has been completed. Note that the depth number may be included as control data in the bit stream transmitted from the point cloud encoding device 100 to the point cloud decoding device 200.

The tree synthesis unit 2020 calculates the node size of the target depth. In the case of an "Octree", the node size of the first depth may be defined as "2 to the power of the depth number". In other words, if the depth number is N, the node size of the first depth may be defined as 2 to the power of N.

In addition, the node size at the second and subsequent depths may be defined by decrementing the number N by one. That is, the node size at the second depth may be defined as "2 to the power of (N-1)", the node size at the third depth may be defined as "2 to the power of (N-2)", and so on.

Alternatively, because node sizes are always defined as powers of 2, you can simply think of the exponent part (N, N-1, N-2, etc.) as the node size. In the following explanation, node size refers to the exponent part of the length of one side of the node.

For simplicity's sake, we'll use the example below where the node shape is a cube, i.e. where all edges of the node are the same length.

When using QtBt, that is, when the node shape is a rectangular prism and the length of each side of the node varies in each axial direction (x, y, z), the length of the shortest side of the three directions may be considered to be the node size. Similarly, the length of the longest side of the three directions may be considered to be the node size.

Here, when the flag (trisoup_enabled_flag) that controls whether or not to use Trisoup indicates that Trisoup is to be used, that is, when the value of trisoup_enabled_flag is "1", the tree synthesis unit 2020 may change the number of Depths to be processed based on the value of the syntax (log2_trisoup_min_node_size_minus2) that specifies the minimum value of the Trisoup node size or the syntax (log2_trisoup_node_size_minus2) that specifies the Trisoup node size. In such a case, for example, it may be defined as follows.

Processing Depth Number=Total Depth Number−(Minimum) Trisoup Node Size Here, the minimum Trisoup node size can be defined as, for example, (log2_trisoup_min_node_size_minus2+2). Similarly, the Trisoup node size can be defined as (log2_trisoup_node_size_minus2+2).

In this case, if processing for all processing depth numbers is completed, the tree synthesis unit 2020 proceeds to step S609, otherwise, the tree synthesis unit 2020 proceeds to step S602.

In other words, if (processing depth number - n) = 0, the tree synthesis unit 2020 proceeds to step S609, and if (processing depth number - n) > 0, the tree synthesis unit 2020 proceeds to step S602.

The tree synthesis unit 2020 may also determine that Trisoup is applied to all nodes having the node size (N-processing depth number) when proceeding to step S609.

In step S602, the tree synthesis unit 2020 determines whether or not it is necessary to decode the Trisoup_applied_flag described below at the target depth.

For example, if "Trisoups at multiple levels are permitted (the value of trisoup_multilevel_enabled_flag is "1")" and "the node size (N-n) of the target depth is equal to or smaller than the maximum trisoup node size," the tree synthesis unit 2020 may determine that "decoding of trisoup_applied_flag is necessary."

For example, if "multiple levels of Trisoup are allowed (the value of trisoup_multilevel_enabled_flag is "1")," and "the node size (N-n) of the target depth is equal to or smaller than the maximum Trisoup node size," and "the node size (N-n) of the target depth is equal to or larger than the minimum Trisoup node size," the tree synthesis unit 2020 may determine that "decoding of Trisoup_applied_flag is necessary."

In addition, if the above conditions are not met, the tree synthesis unit 2020 may determine that "decoding of Trisoup_applied_flag is not necessary."

Here, the maximum Trisoup node size can be defined, for example, as (log2_trisoup_max_node_size_minus2 + 2). Similarly, the minimum Trisoup node size can be defined, for example, as (log2_trisoup_min_node_size_minus2 + 2).

Once the above-mentioned determination is completed, the tree synthesis unit 2020 proceeds to step S603.

In step S803, the tree synthesis unit 2020 determines whether processing of all nodes included in the target Depth has been completed.

If it is determined that processing of all nodes at the target Depth has been completed, the tree synthesis unit 2020 proceeds to step S601 and processes the next Depth.

On the other hand, if processing of all nodes at the target depth has not been completed, the tree synthesis unit 2020 proceeds to step S604.

In step S604, the tree synthesis unit 2020 checks whether or not it is necessary to decode the Trisoup_applied_flag determined in step S602.

If it is determined that decoding of Trisoup_applied_flag is necessary, the tree synthesis unit 2020 proceeds to step S605, and if it is determined that decoding of Trisoup_applied_flag is not necessary, the tree synthesis unit 2020 proceeds to step S608.

In step S605, the tree synthesis unit 2020 decodes Trisoup_applied_flag.

Trisoup_applied_flag is a 1-bit flag (second flag) that indicates whether Trisoup is applied to the target node. For example, it may be defined that Trisoup is applied to the target node when the value of this flag is "1", and that Trisoup is not applied to the target node when the value of this flag is "0".

After the tree synthesis unit 2020 decodes Trisoup_applied_flag, it proceeds to step S606.

In step S606, the tree synthesis unit 2020 checks the value of Trisoup_applied_flag decoded in step S605.

If Trisoup is to be applied to the target node, i.e., if the value of Trisoup_applied_flag is "1", the tree synthesis unit 2020 proceeds to step S607.

If Trisoup is not applied to the target node, i.e., if the value of Trisoup_applied_flag is "0", the tree synthesis unit 2020 proceeds to step S608.

In step S607, the tree synthesis unit 2020 stores the target node as a node to which Trisoup is applied, i.e., as a Trisoup node. No further node splitting by "Octree" is applied to the target node. The tree synthesis unit 2020 then proceeds to step S603 and moves on to processing the next node.

In step S608, the tree synthesis unit 2020 decodes information called the occupancy code.

In the case of "Octree", the occupancy code is information that indicates whether the point to be decoded is contained in each of the child nodes when the target node is divided in half in each of the x, y, and z axis directions and divided into eight nodes (called child nodes).

For example, the occupancy code may assign one bit of information to each child node, and if the one bit of information is "1", it may be defined that the point to be decoded is included in the child node, and if the one bit of information is "0", it may be defined that the point to be decoded is not included in the child node.

When decoding such an occurrence code, the tree synthesis unit 2020 may estimate in advance the probability that the point to be decoded exists in each child node, and entropy decode the bits corresponding to each child node based on that probability.

In addition, in step S608, the tree synthesis unit 2020 may store the position information of nodes to which Trisoup was not applied in a one-dimensional array or the like for each Depth, for use in S901 described below.

Specifically, the tree synthesis unit 2020 may store position information of the target node (node before division) in step S608. For example, the tree synthesis unit 2020 may store the coordinate values of the vertices of the target node (rectangular parallelepiped) that are closest to the origin in a one-dimensional array for each Depth, and this information may be used by the approximate surface synthesis unit 2030 described later.

If the size of the rectangular parallelepiped (the lengths of the sides in the x-, y-, and z-directions) is determined for each Depth, then if the coordinates and Depth of the point closest to the origin within the node can be identified as described above, the position information of the node can be restored.

Similarly, entropy coding may be performed in the point cloud coding device 100.

(Approximate Surface Synthesis Unit 2030)
The approximate surface synthesis unit 2030 is configured to perform a decoding process for each node determined to be a Trisoup node by the tree synthesis unit 2020, as will be described with reference to Figs.

Below, an example of the processing performed by the approximate surface synthesis unit 2030 is explained using Figures 7 to 16.

Figure 7 is a flowchart showing an example of processing by the approximate surface synthesis unit 2030.

As shown in FIG. 7, in step S701, the approximate surface synthesis unit 2030 determines whether processing has been completed for all trisoup_depths.

If processing has been completed for all trisoup_depths, proceed to step S709 and end processing. If processing has not been completed for all trisoup_depths, proceed to step S702.

In step S702, the approximate surface synthesis unit 2030 determines whether the Trisoup node size corresponding to the trisoup_depth is equal to the maximum Trisoup node size.

If the two are equal, the operation proceeds to step S704; if the two are not equal, i.e., if the Trisoup node size corresponding to the trisoup_depth is smaller than the maximum Trisoup node size, the operation proceeds to step S703.

In step S702, the approximate surface synthesis unit 2030 obtains and integrates the vertex positions for each node.

In step S703, the approximate surface synthesis unit 2030 generates segments corresponding to the "interpolated vertices (vertices on which the interpolation process has been performed)" generated in step S708, which will be described later. FIG. 8 is a flowchart showing an example of the process in step S703.

An example of the processing in step S703 is described below with reference to FIG.

As shown in FIG. 8, in step S801, the approximate surface synthesis unit 2030 determines whether processing has been completed for all of the "interpolated vertices" generated in step S708, which will be described later.

If completed, the operation proceeds to step S805 and ends processing. If not completed, the operation proceeds to step S802 to process the next "interpolated vertex."

In step S802, the approximate surface synthesis unit 2030 determines on which segment (x, y, or z direction) the "interpolated vertex" exists.

For example, as described below, if the vertices of a segment and Trisoup are located at a position that is 0.5 offset from the integer coordinate position, then in the x-direction segment, only the x-coordinate of the "interpolated vertex" will be an integer value (x.0), and the y-coordinate and z-coordinate will be decimal values (x.5).

Similarly, in a segment in the y direction, only the y coordinate is an integer value, and in a segment in the z direction, only the z coordinate is an integer value, so the approximate surface synthesis unit 2030 can determine the direction of the segment in which the "interpolated vertex" should exist by checking which axis coordinate is an integer.

When coordinate values are stored in integer format, for example when the true coordinate values are doubled and stored as integer values to represent 0.5, the approximate surface synthesis unit 2030 can determine whether the coordinate values in each axis direction are integers or decimals (x.5) depending on whether the least significant bit is 1.

The approximate surface synthesis unit 2030 stores the results of this determination and proceeds to the next step S803.

In step S803, the approximate surface synthesis unit 2030 determines whether the "interpolated vertex" exists on an edge in the current Trisoup node size. For example, the approximate surface synthesis unit 2030 can perform such a determination in the following manner.
(1) First, the approximate surface synthesis unit 2030 quantizes the x-, y-, and z-axis coordinate values of the “interpolated vertex” to integers.

Specifically, the approximate surface synthesis unit 2030 adds 0.5 to each coordinate value and then rounds down to an integer.

Here, if the coordinate values are stored as integer values that are twice the true value, the approximate surface synthesis unit 2030 adds 1, divides by 2, and truncates to the nearest integer.

Alternatively, if the coordinate values are stored as integer values that are twice the true values, the approximate surface synthesis unit 2030 adds 1 and then shifts them one bit to the right.

Note that, although the case where all of the x, y, and z axial directions are converted to integers has been described here, the approximate surface synthesis unit 2030 may convert only the axes other than the "direction" determined in step S802 into integers.

For example, when the above-mentioned "direction" is the x-axis, the approximate surface synthesis unit 2030 needs to convert only the y-coordinate and the z-coordinate into integers.
(2) Secondly, the approximate surface synthesis unit 2030 checks whether the coordinate values after integer conversion of the axes other than the "direction" determined in step S802 are multiples of the Trisoup node size.

For example, when the above-mentioned "direction" is the x-axis direction and the value of the Trisoup node size is 8 (=2 ³ ), the approximate surface synthesis unit 2030 checks whether the integer-converted y coordinate and z coordinate are each a multiple of 8.

In the above process (2), if the coordinate values after integer conversion of the axes other than the "direction" are all multiples of the Trisoup node size, the approximate surface synthesis unit 2030 determines that the "interpolated vertex" exists on an edge of the current Trisoup node size, and proceeds to step S804.

If this is not the case, that is, if the coordinate value in at least one axis direction is not a multiple of the Trisoup node size, the approximate surface synthesis unit 2030 determines that the "interpolated vertex" does not exist on an edge in the current Trisoup node size, and proceeds to step S801 to process the next "interpolated vertex."

In step S804, the approximate surface synthesis unit 2030 generates a segment corresponding to the "interpolated vertex" using the coordinates of the "interpolated vertex" that were converted to integers in step S803.

Here, a segment may consist of three elements: the coordinates of the start point (x, y, z), the coordinates of the end point (x, y, z), and the vertex position on the segment (0 to Trisoup node size - 1).

The above-mentioned starting point is obtained, for example, by quantizing the coordinate value of the "interpolated vertex" converted to an integer by the Trisoup node size.

Specifically, for example, if the logarithm of the base 2 of the Trisoup node size is TrisoupNodeSizeLog2, it can be derived by performing the following calculations on the coordinate values of the "interpolated vertices".

Quantized coordinate value = (coordinate value >> TrisoupNodeSizeLog2) << TrisoupNodeSizeLog2
Here, >> means a right bit shift, and << means a left bit shift.

When proceeding to the processing of step S804, it is clear that the coordinate values of the axes other than the "direction" determined in step S802 are already multiples of the Trisoup node size, so in step S804, the approximate surface synthesis unit 2030 may perform the above-mentioned quantization processing only on the coordinate values of the axis corresponding to the "direction" determined in step S802.

Next, the coordinates of the end point can be calculated by adding the value of the Trisoup node size to the coordinate value of the axis corresponding to the "direction" determined in step S802 for the coordinates of the start point.

Finally, the vertex position on the segment can be derived by subtracting the coordinate value of the starting point on the axis corresponding to the above-mentioned "direction" from the coordinate value of the "interpolated vertex" that has been converted to an integer on the axis corresponding to the "direction" determined in step S802.

After performing the above processing, the operation proceeds to step S801 and proceeds to processing the next "interpolated vertex."

As described above, the approximate surface synthesis unit 2030 generates segments from the "interpolated vertices" in step S703 of FIG. 7, and then proceeds to step S704.

In step S704, the approximate surface synthesis unit 2030 collects adjacent information to be used for decoding the vertices in the next step S705.

Figure 9 is a flowchart showing an example of the processing of step S704. Below, an example of the processing of step S704 is described using Figure 9.

As shown in FIG. 9, in step S901, the approximate surface synthesis unit 2030 generates mask information. FIG. 10 shows a specific example of a method for generating mask information.

When generating mask information, as shown in FIG. 10, in step S1001, the approximate surface synthesis unit 2030 determines whether processing has been completed for all Trisoup nodes for the Trisoup node size.

If completed, the operation proceeds to step S1003; if not completed, the operation proceeds to step S1002.

In step S1002, the approximate surface synthesis unit 2030 generates 36 segments and mask values corresponding to each segment for the Trisoup node.

Figure 11 shows an example of segment and mask value generation. The shaded areas in Figure 11 show examples of the Trisoup node, each line segment shows an example of a segment, and the numbers written on the segments show examples of mask values.

Note that because it is difficult to provide examples of mask values for all segments, examples of mask values are provided only for some segments, but in reality, mask values are set for all segments as follows.

First, the approximate surface synthesis unit 2030 generates four segments each at adjacent positions with larger coordinate values than the node and adjacent positions with smaller coordinate values for each of the 12 sides of the Trisoup node in the x, y, and z directions, generating a total of 36 segments as shown in Figure 11.

Here, each segment has information about its start and end points, but since there are no vertices, no information about the vertex positions is stored.

Next, we will explain examples of setting mask values for each segment.

The approximate surface synthesis unit 2030 sets a mask value for each segment corresponding to the 12 edges of the Trisoup node such that only one of the lowest 4 bits of the mask value is 1 and the rest are 0.

For example, as shown in FIG. 11, the approximate surface synthesis unit 2030 may set mask values of 1, 2, 4, and 8 for segments in the x-axis direction.

Similarly, the approximate surface synthesis unit 2030 may set mask values of 1+(1<<13), 2+(1<<13), 4+(1<<13), and 8+(1<<13) for segments in the y-axis direction.

Similarly, the approximate surface synthesis unit 2030 may set mask values of 1+(1<<14), 2+(1<<14), 4+(1<<14), and 8+(1<<14) for segments in the z-axis direction.

When set in this way, if any of the first through fourth bits of the mask value is 1, it can be determined that the node corresponding to the segment is a Trisoup node.

Next, the approximate surface synthesis unit 2030 may set mask values of 16, 32, 64, and 128, respectively, for the segments (four in each of the x, y, and z directions, totaling 12) corresponding to adjacent nodes whose coordinate values increase from the Trisoup node in the x, y, and z directions, as shown in FIG. 11.

When set in this way, if any of the 5th to 8th bits of the mask value is 1, it can be determined that the adjacent node with the smaller coordinate value for the segment is a Trisoup node.

Next, the approximate surface synthesis unit 2030 may set mask values of 256, 512, 1024, and 2048, respectively, as shown in FIG. 11, for the segments (four in each of the x, y, and z directions, totaling 12) corresponding to adjacent nodes whose coordinate values are smaller than the Trisoup node in the x, y, and z directions.

When set in this way, if any of the 9th to 12th bits of the mask value is 1, it can be determined that the adjacent node with the larger coordinate value for the segment is a Trisoup node.

As described above, after generating the segments and their corresponding mask values, the approximate surface synthesis unit 2030 proceeds to step S1001 and moves on to processing the next Trisoup node.

Here, the nodes to be processed in step S1001 that were saved in step S608 above and to which Trisoup was not applied at that Depth may be added as the nodes to be processed in this step.

For "nodes to which Trisoup has not been applied at the relevant depth", Trisoup is not applied at the relevant depth, so no Trisoup vertices are generated for the node itself, but it is an area where Trisoup should be applied at a depth greater than the relevant depth. Therefore, when generating a mask for "nodes to which Trisoup has not been applied at the relevant depth" in step S1002, a mask in which "any bit in the 1st to 4th bits of the mask value is 1" as described above is not set, and only a mask in which "any bit in the 5th to 8th bits of the mask value is 1" and a mask in which "any bit in the 9th to 12th bits of the mask value is 1" are set.

In step S1003, the approximate surface synthesis unit 2030 determines whether processing of all segments corresponding to the "interpolated vertices" generated in step S703 has been completed.

If completed, the operation proceeds to step S1005, and the mask generation process of step S901 ends. If not completed, the operation proceeds to step S1004.

In step S1004, the approximate surface synthesis unit 2030 generates a mask value for the segment corresponding to the "interpolated vertex," which indicates that the "interpolated vertex" exists in the segment.

The approximate surface synthesis unit 2030 may set this mask value to, for example, a value of 1<<15. When such a value is set, if the 16th bit of the mask value is 1, it can be determined that an "interpolated vertex" exists in the segment.

After generating the mask value, the approximate surface synthesis unit 2030 proceeds to step S1003 and proceeds to process the next segment.

Note that the mask values generated in steps S1002 and S1004 are both powers of 2. This makes it easy to combine mask values by performing a bit operation (logical sum) when combining mask values of segments that exist in the same position in step S905, which will be described later.

In addition, depending on whether each bit of the mask value synthesized in this way is 1 or 0, as described above, information about the segment (whether it is a Trisoup node, whether there is an adjacent Trisoup node, whether there is an "interpolated vertex" in the segment, etc.) can be obtained.

After generating the mask information as described above, the approximate surface synthesis unit 2030 proceeds to step S902.

In step S902, the approximate surface synthesis unit 2030 collects and sorts all of the segments generated in step S901 and the segments corresponding to the "interpolated vertices."

The approximate surface synthesis unit 2030 sorts, for example, based on the start and end coordinates of each segment. By performing such processing, it is possible to rearrange the segments so that segments having the same start and end points are consecutive.

After completing this sorting, the approximate surface synthesis unit 2030 proceeds to step S903.

In step S903, the approximate surface synthesis unit 2030 determines whether processing of all segments after sorting in step S902 has been completed.

If completed, the operation proceeds to step S909, and the neighbor information collection process of step S704 ends. If not completed, the operation proceeds to step S904.

In step S904, the approximate surface synthesis unit 2030 determines whether the segment is in the same position as the previously processed segment.

Specifically, the approximate surface synthesis unit 2030 determines, for example, whether or not both the coordinates of the start point and the coordinates of the end point of the segment are the same as those of the segment processed immediately before.

If the position is the same as the previously processed segment, the operation proceeds to step S905. If the position is not the same as the previously processed segment, the operation proceeds to step S906.

In step S905, the approximate surface synthesis unit 2030 integrates the mask value corresponding to the segment generated in step S901 with the mask values of each segment at the same position that was processed up to that point.

For example, when the mask values corresponding to each segment are composed of powers of 2 as described in step S901, the approximate surface synthesis unit 2030 can integrate the masks by taking the logical sum of the mask values of each segment.

After integrating the masks, the approximate surface synthesis unit 2030 proceeds to step S907.

In step S906, the approximate surface synthesis unit 2030 stores the segment processed immediately before the segment in question as a unique segment if it satisfies a predetermined condition A.

For example, the specified condition A may be when the previously processed segment is a segment corresponding to a Trisoup node.

For example, if at least one of the lowest four bits of the mask value integrated in step S905 is 1, it can be determined that the previously processed segment is a segment corresponding to the Trisoup node.

The approximate surface synthesis unit 2030 may store, as information about a unique segment, for example, the coordinates of the start point, the coordinates of the end point, information about the interpolation points described in step S907 below, information about adjacent segments described in step S908 below, and the mask value integrated in step S905. This information is used in the vertex decoding process in step S705.

After saving information about the unique segments, the approximate surface synthesis unit 2030 initializes various information (interpolation point information, adjacent segment information, integrated mask values, etc.).

If the predetermined conditions are not met, the approximate surface synthesis unit 2030 performs only initialization. After the above processing is completed, the operation proceeds to step S907.

In step S907, if the predetermined condition B is satisfied, the approximate surface synthesis unit 2030 stores information about the interpolation points.

For example, the predetermined condition B may be that the mask value of the segment indicates that an "interpolated vertex" exists in the segment. Specifically, the predetermined condition B may be that the 16th bit of the mask value is 1.

If the predetermined condition B is satisfied, the approximate surface synthesis unit 2030 stores the coordinates of the "interpolated vertex." Specifically, the approximate surface synthesis unit 2030 stores the value of the "vertex position on the segment" generated in step S804. The value stored here is stored as information about the unique segment in step S906.

If the segment is not determined to be a unique segment in step S906 (if the specified condition A in step S906 is not satisfied), the information about the interpolation points of the segment stored in step S907 is discarded.

The approximate surface synthesis unit 2030 may prepare a separate array to store the values of "vertex positions on a segment", and in this step store an index value for identifying the value corresponding to the segment in the array instead of the coordinate value itself.

After the above processing is completed, the operation proceeds to step S908.

In step S908, the approximate surface synthesis unit 2030 stores information about adjacent segments.

Specifically, for example, the approximate surface synthesis unit 2030 stores the start coordinates of the segment, and information for identifying segments with the same start coordinates or end coordinates (for example, a segment index, etc.).

For example, there are segments with the same start coordinates as the segment, and start coordinates or end coordinates, specifically, one segment in the same axial direction as the segment, whose end coordinates are the same as the start coordinates of the segment, and four segments in the axial direction perpendicular to the segment. The information saved here is saved as information about unique segments in step S906.

If the segment is not determined to be a unique segment in step S906 (if the specified condition A in step S906 is not satisfied), the information on the adjacent segments of the segment generated in step S908 is discarded.

After the above processing is completed, the operation proceeds to step S903 and proceeds to processing the next segment.

As described above, after collecting adjacent information in step S704, the approximate surface synthesis unit 2030 proceeds to step S705.

In step S705, the approximate surface synthesis unit 2030 decodes the vertices of Trisoup at the trisoup_depth.

Figure 12 is a flowchart showing an example of a method for decoding vertices in step S705. Below, an example of the processing in step S705 is described using Figure 12.

As shown in FIG. 12, in step S1201, the approximate surface synthesis unit 2030 determines whether processing has been completed for all unique segments generated in step S703.

If processing has been completed for all unique segments, the operation proceeds to step S1207, and processing of step S703 ends. If processing has not been completed for all unique segments, the operation proceeds to step S1202.

In step S1202, the approximate surface synthesis unit 2030 determines whether or not an "interpolated vertex" exists in the unique segment.

Whether or not an "interpolated vertex" exists can be determined based on the integrated mask value saved in step S906. If an "interpolated vertex" exists, the operation proceeds to step S1203. If an "interpolated vertex" does not exist, the operation proceeds to step S1204.

In step S1203, the approximate surface synthesis unit 2030 saves the "vertex positions on the segment" saved in step S906 as the vertex positions in the unique segment.

In other words, the approximate surface synthesis unit 2030 sets the "vertex position on the segment" saved in step S906 as the decoded value of the vertex position in the unique segment.

Furthermore, the approximate surface synthesis unit 2030 sets the decoded value of "vertex presence/absence" in the unique segment to "vertex present."

In other words, step S1203 is a process in which, instead of decoding information indicating the vertex positions and the presence or absence of vertices from the bitstream, the vertex positions and the information indicating the presence or absence of vertices are implicitly determined using the interpolated values.

After the above processing is completed, the operation proceeds to step S1201 and proceeds to process the next unique segment.

In step S1204, the approximate surface synthesis unit 2030 decodes information indicating the presence or absence of a vertex from the bit stream, indicating whether or not a vertex exists in the unique segment.

The information indicating the presence or absence of a vertex is a one-bit flag, and may be entropy coded in the point cloud coding device 100. The approximate surface synthesis unit 2030 may prepare multiple probability models for entropy coding (coding device)/decoding (decoding device) and select one according to the context. The context may be set, for example, based on the mask value saved and integrated in step S906.

After the approximate surface synthesis unit 2030 decodes the information indicating the presence or absence of a vertex, it proceeds to step S1205.

In step S1205, the approximate surface synthesis unit 2030 determines whether a vertex exists in the unique segment.

If a vertex is present based on the information indicating the presence or absence of a vertex decoded in step S1204, the approximate surface synthesis unit 2030 proceeds to step S1206. If such a vertex is not present, the approximate surface synthesis unit 2030 proceeds to step S1201 and proceeds to processing the next unique segment.

In step S1206, the approximate surface synthesis unit 2030 decodes the "vertex position on the segment" in the unique segment in which it is determined that a vertex exists. Here, the vertex position may be entropy coded. When performing entropy coding (coding device)/decoding (decoding device), the approximate surface synthesis unit 2030 may prepare multiple probability models and select one according to the context. The context may be set, for example, based on the information of the adjacent segment saved in step S906.

Here, the vertex positions may be decoded with the bit precision specified by the syntax (trisoup_vertex_number_bits) that specifies the precision (number of bits) of the vertex positions of the above-mentioned Trisoup.

For example, if trisoup_vertex_number_bits is 2, it may be decoded to take four values: 0, 1, 2, and 3. Furthermore, if trisoup_vertex_number_bits is decoded for each depth, it may be decoded with the bit precision corresponding to that depth.

In addition, the bit precision specified by trisoup_vertex_number_bits may be used only at the Depth corresponding to the minimum Trisoup node size (the largest Depth), and thereafter the bit precision may be increased by 1 for each node size that increases by 1 (Depth that decreases by 1).

For example, if there are two types of node sizes (two levels: minimum and maximum), the vertex position may be decoded using the value of trisoup_vertex_number_bits for the Depth corresponding to the minimum Trisoup node size, and the value of trisoup_vertex_number_bits plus 1 for the Depth corresponding to the maximum Trisoup node size, as the bit precision of the Depth.

After decoding the vertex positions, the approximate surface synthesis unit 2030 proceeds to step S1201 and proceeds to process the next unique segment.

As described above, after the approximate surface synthesis unit 2030 decodes the vertices in step S705, it proceeds to step S706.

In step S706, the approximate surface synthesis unit 2030 generates a decoded point group (reconstructed point group) based on the vertices decoded in step S705. FIG. 13 is a flowchart showing an example of a method for generating a decoded point group. An example of the processing in step S706 will be described below with reference to FIG. 13.

As shown in FIG. 13, in step S1301, the approximate surface synthesis unit 2030 determines whether processing has been completed for all Trisoup nodes at the trisoup_depth.

If processing has been completed for all Trisoup nodes, the operation proceeds to step S1306 and ends. If processing has not been completed for all Trisoup nodes, the operation proceeds to step S1302.

In step S1302, first, the approximate surface synthesis unit 2030 identifies unique segments corresponding to each side (segment) of the Trisoup node. For example, the approximate surface synthesis unit 2030 can identify unique segments whose start point coordinates and end point coordinates of each side (segment) of the Trisoup node are the same, from among the unique segments processed in step S705.

Second, if a vertex exists in the identified unique segment, the approximate surface synthesis unit 2030 adds the vertex to the reconstructed point group in the following procedure.
(1) The approximate surface synthesis unit 2030 shifts the position of the segment by −0.5. Specifically, the approximate surface synthesis unit 2030 subtracts 0.5 from the coordinate values of the start point and end point of the segment other than the axis along the direction of the segment.

For example, if the start coordinates of a segment are (x, y, z) = (12, 100, 32), the node size is 4, and the direction of the segment is the x-axis, the end coordinates will be (16, 100, 32), calculated by adding the node size to the x-coordinate of the start point.

On the other hand, the coordinates other than the x-axis, that is, in this example, the y-coordinate and the z-coordinate are each decreased by 0.5, so that the start point coordinates and the end point coordinates become (12, 99.5, 31.5) and (16, 99.5, 31.5), respectively.
(2) The approximate surface synthesis unit 2030 adds the "vertex position on the segment" of the segment to the above-mentioned starting point coordinates. For example, in the above example, if the "vertex position on the segment" is 2, the result is (14, 99.5, 31.5).
(3) The approximate surface synthesis unit 2030 adds 0.5 to the coordinate values of the axes other than those along the segment direction for the coordinates calculated in (2) above. For example, in the above example, the result is (14, 100, 32). The same result can be obtained by adding 2, the vertex position on the unique segment, to the start coordinates (12, 100, 32) of the unique segment before executing the above procedure (1), so the approximate surface synthesis unit 2030 may perform the calculation in this manner.
(4) If the coordinates calculated in (3) above exist inside the Trisoup node, the approximate surface synthesis unit 2030 generates a point at the coordinates calculated in (3) above and adds it to the reconstructed point group.

Here, inside the Trisoup node is a point whose x, y, and z coordinate values are in the range of the start point of the Trisoup node (the point with the smallest x, y, and z coordinates) + Trisoup node size - 1.

For example, if the start coordinates of the Trisoup node are (x, y, z) = (12, 100, 32) and the Trisoup node size is 4, points that are in the ranges (12 to 15, 100 to 103, 32 to 35) are considered to be inside the Trisoup node.

In this case, if the coordinates calculated in (3) above are (14, 100, 32), these coordinates are determined to be inside the Trisoup node, and the approximate surface synthesis unit 2030 adds the point with these coordinates to the reconstructed point cloud. Figure 15(a) shows such results when projected onto the x-y plane.

On the other hand, for example, if the start coordinates of the Trisoup node are (x, y, z) = (12, 96, 32), points in the ranges (12 to 15, 96 to 99, 32 to 35) are inside the Trisoup node, so in this case, it is determined that (14, 100, 32) does not exist inside the Trisoup node, and the coordinates calculated in (3) above are not added to the reconstructed point group, and this operation ends the process. Figure 15(b) shows an example of such a case.

After completing the above process, the operation proceeds to step S1303.

In step S1303, the approximate surface synthesis unit 2030 calculates the initial coordinates of the centroid from the vertices corresponding to the Trisoup node.

For example, the initial coordinates of the centroid can be calculated by averaging the x, y, and z components of all vertex positions of the Trisoup node. Note that if the Trisoup node has three or fewer vertices, the calculation of the centroid process may be omitted.

After this process is completed, the operation proceeds to step S1304.

In step S1304, the approximate surface synthesis unit 2030 performs a process of sorting the vertices and determining the projection surface. Specifically, for example, the approximate surface synthesis unit 2030 can sort the vertices and determine the projection surface in the following procedure.
(1) The approximate surface synthesis unit 2030 projects the vertices onto the xy plane. This process is equivalent to extracting the x and y coordinate values from the coordinates of each vertex.

Since the vertices are originally on the sides of the nodes, in the projected plane, each vertex will be on the side of a square or rectangle that the nodes are projected onto the plane. Since a square is also a type of rectangle, the following explanation will be given assuming that the shape of the nodes after projection is a rectangle.
(2) The approximate surface synthesis unit 2030 sorts the points on the sides of the rectangle onto which the nodes are projected in clockwise or counterclockwise order.

Furthermore, the approximate surface synthesis unit 2030 assigns a tentative index value (0, 1, 2, . . . ) to each vertex in the sorted order.
(3) The approximate surface synthesis unit 2030 defines a triangle defined by two adjacent vertices in the sorted order and the centroid, and calculates the area of the triangle. The area of the triangle can be calculated, for example, by generating vectors starting from the centroid and pointing to the coordinates of the two vertices, and using the cross product of these vectors.

In this way, the approximate surface synthesis unit 2030 calculates and sums the areas of triangles for all combinations of two adjacent vertices in sorted order: (0, 1), (1, 2), ... (N, 0).

When there are only three vertices, the approximate surface synthesis unit 2030 calculates the area of a triangle formed by the three vertices without using a centroid. This area is the area on the projected plane.
(4) The approximate surface synthesis unit 2030 performs the above steps (1) to (3) on the x-z plane and the y-z plane in the same manner, and selects the plane with the largest area calculated in the above step (3) as the projection plane. At this time, the sorting order and the provisional index assigned in the above step (2) are adopted as the final sorting order and index.

After sorting the vertices and determining the projection surface as described above, the approximate surface synthesis unit 2030 proceeds to step S1305.

In step S1305, the approximate surface synthesis unit 2030 generates a reconstructed point cloud using the triangles generated in step S1304 and ray tracing.

Specifically, the approximate surface synthesis unit 2030 generates the reconstructed point group in the following procedure.
(1) First, the approximate surface synthesis unit 2030 generates triangles based on the index and centroid determined in step S1304, similarly to step S1304. However, whereas the approximate surface synthesis unit 2030 generated triangles on a two-dimensional plane in step S1304, in step S1305 it generates triangles in three-dimensional space using all of the x, y, and z coordinates of each vertex.
(2) Secondly, the approximate surface synthesis unit 2030 defines a normal vector of the projection surface (if the projection surface is an xy plane, a vector in the z direction) and places it on the projection surface.

For example, the approximate surface synthesis unit 2030 sets the initial position to the point on the projection surface that is closest to the origin of the node shape (i.e., a rectangle).
(3) Thirdly, the approximate surface synthesis unit 2030 determines whether or not the normal vector intersects with the triangle generated in (1) above as the norm of the normal vector is increased, and if so, calculates the coordinates of the intersecting point.

For example, the approximate surface synthesis unit 2030 can realize such processing by using a general ray tracing method, etc. Fig. 16A shows an example of a configuration of triangles.
(4) Fourth, if the coordinates where the normal vector calculated in (3) above intersects with the triangle exist inside the Trisoup node, the approximate surface synthesis unit 2030 generates a point at such coordinate position and adds it to the reconstructed point group.
(5) Fifth, the approximate surface synthesis unit 2030 repeats the above steps (3) and (4) for each integer coordinate position within the node (rectangle) on the projection surface.

In this case, the interval at which the normal vectors are placed does not have to be 1 (i.e., all integer coordinate positions). For example, the approximate surface synthesis unit 2030 may place the normal vectors at an interval equal to the value of trisoup_sampling_value_minus1 decoded by the geometric information decoding unit 2010 plus 1.

For example, if the interval is 2 and the x-y plane is the projection surface, the approximate surface synthesis unit 2030 may place normal vectors only at integer positions where both the x and y coordinates are multiples of 2. An example of a reconstructed point cloud generated from the triangle in FIG. 16A is shown in FIG. 16B.

After generating the reconstruction points for the Trisoup node in the above manner, the approximate surface synthesis unit 2030 proceeds to step S1301 and proceeds to process the next Trisoup node.

As described above, in step S706, the approximate surface synthesis unit 2030 generates the decoded point group (reconstructed point group), and then proceeds to step S707.

In step S707, the approximate surface synthesis unit 2030 determines whether the Trisoup node size at the trisoup_depth is the minimum Trisoup node size.

If it is the minimum Trisoup node size, the operation proceeds to step S701. If not, that is, if the Trisoup node size at the trisoup_depth is greater than the minimum Trisoup node size, the operation proceeds to step S708.

In step S708, the approximate surface synthesis unit 2030 interpolates vertices onto the segments of the nodes at the minimum Trisoup node size based on the vertices corresponding to the Trisoup node size in trisoup_depth decoded in step S705.

Figure 14 is a flow chart showing an example of the processing of step S708. Below, an example of the processing of step S708 will be described using Figure 14. Note that the processing of Figure 14 is almost the same as the processing of Figure 13, and the same processing is denoted by the same reference numerals. Below, only the differences with Figure 13 will be described.

As shown in FIG. 14, in step S1402, the approximate surface synthesis unit 2030 stores the vertices of the Trisoup node as "interpolated vertices."

Specifically, the approximate surface synthesis unit 2030 stores the coordinate values obtained after steps (1) and (2) described in step S1302 as "interpolated vertices".

For example, in the example of step S1302, the approximate surface synthesis unit 2030 saves the coordinate values (14, 99.5, 31.5) as the "interpolated vertex." This is because the position of the segment is defined to be 0.5 away from the integer coordinate position.

Also, to store this value as an integer, the approximate surface synthesis unit 2030 may store it as a value that is twice the true coordinate value. In the above example, the approximate surface synthesis unit 2030 may save the value (28, 199, 63) as the "interpolated vertex."

In step S1405, the approximate surface synthesis unit 2030 changes the position at which the normal vector is placed in procedure (2) of step S1305 so that it is placed at the position where a segment exists when the Trisoup node is divided by the minimum Trisoup node size.

If the intersecting coordinates calculated in the same manner as in step (3) of step S1305 are on a segment obtained by dividing the Trisoup node by the minimum Trisoup node size, the approximate surface synthesis unit 2030 stores the coordinate values as "interpolated vertices."

If the coordinate values are doubled to convert them into integers and stored in step S1402, the approximate surface synthesis unit 2030 also stores the doubled coordinate values in step S1405.

The approximate surface synthesis unit 2030 saves the "interpolated vertices" generated by the above process and uses them in step S703 in the next trisoup_depth.

In addition, if the trisoup_depth is greater than 0, i.e., if the trisoup node size at the trisoup_depth is not the maximum trisoup node size, there are "interpolated vertices" generated at a larger node size, but the approximate surface synthesis unit 2030 does not delete the set of these "interpolated vertices," but rather saves the newly generated "interpolated vertices" in step S1402 by adding them to the above set.

As shown in FIG. 16C, the approximate surface synthesis unit 2030 may store only those "interpolated vertices" that exist on the surface of the Trisoup node.

Note that, for convenience, steps S706 and S708 have been described here as separate processes, but since the two have much in common, they may be performed simultaneously.

For example, if the condition of step S707 is satisfied in step S1302, the approximate surface synthesis unit 2030 may also perform step S1402, and similarly, if the condition of step S707 is satisfied in step S1305, the approximate surface synthesis unit 2030 may also perform step S1405.

(Point group encoding device 100)
Hereinafter, the point cloud encoding device 100 according to this embodiment will be described with reference to Fig. 17. Fig. 17 is a diagram showing an example of functional blocks of the point cloud encoding device 100 according to this embodiment.

As shown in FIG. 17, the point cloud encoding device 100 has a coordinate transformation unit 1010, a geometric information quantization unit 1020, a tree analysis unit 1030, an approximate surface analysis unit 1040, a geometric information encoding unit 1050, a geometric information reconstruction unit 1060, a color conversion unit 1070, an attribute transfer unit 1080, a RAHT unit 1090, an LoD calculation unit 1100, a lifting unit 1110, an attribute information quantization unit 1120, and an attribute information encoding unit 1130.

The coordinate conversion unit 1010 is configured to perform a conversion process from the three-dimensional coordinate system of the input point cloud to any different coordinate system. The coordinate conversion may, for example, convert the x, y, and z coordinates of the input point cloud into any s, t, and u coordinates by rotating the input point cloud. In addition, as one variation of the conversion, the coordinate system of the input point cloud may be used as is.

The geometric information quantization unit 1020 is configured to quantize the position information of the input point group after coordinate transformation and remove points with overlapping coordinates. Note that when the quantization step size is 1, the position information of the input point group and the position information after quantization match. In other words, when the quantization step size is 1, it is equivalent to not performing quantization.

The tree analysis unit 1030 is configured to receive position information of the quantized point group as input, and generate an occurrence code that indicates at which node in the encoding target space the point exists, based on the tree structure described below.

In this process, the tree analysis unit 1030 is configured to generate a tree structure by recursively dividing the encoding target space into rectangular parallelepipeds.

If a point exists within a certain rectangular parallelepiped, a tree structure can be generated by recursively dividing the rectangular parallelepiped into multiple rectangular parallelepipeds until the rectangular parallelepiped reaches a specified size. Each such rectangular parallelepiped is called a node. Each rectangular parallelepiped generated by dividing a node is called a child node, and the occurrence code is expressed as 0 or 1 to indicate whether or not a point is included in the child node.

As described above, the tree analysis unit 1030 is configured to generate an occupancy code while recursively splitting the node until it reaches a predetermined size.

In this embodiment, a method called "Octree" can be used that recursively performs octree division on the above-mentioned rectangular parallelepiped, always treating it as a cube, and a method called "QtBt" can be used that performs quadtree division and binary tree division in addition to octree division.

Here, whether or not to use "QtBt" is transmitted to the point cloud decoding device 200 as control data.

Alternatively, predictive coding using an arbitrary tree structure may be specified. In such a case, the tree analysis unit 1030 determines the tree structure, and the determined tree structure is transmitted to the point cloud decoding device 200 as control data.

For example, the tree-structured control data may be configured so that it can be decoded using the procedure described in FIG. 6.

The approximate surface analysis unit 1040 is configured to generate approximate surface information using the tree information generated by the tree analysis unit 1030.

When decoding three-dimensional point cloud data of an object, for example, if the points are densely distributed on the object's surface, approximate surface information is used to represent the area in which the points exist by approximating the area using a small plane, rather than decoding each point individually.

Specifically, the approximate surface analysis unit 1040 may be configured to generate approximate surface information using, for example, a method called "Trisoup." In addition, when decoding a sparse point cloud acquired by Lidar or the like, this process can be omitted.

The geometric information encoding unit 1050 is configured to generate a bit stream (geometric information bit stream) by encoding syntax such as the occupancy code generated by the tree analysis unit 1030 and the approximate surface information generated by the approximate surface analysis unit 1040. Here, the bit stream may include, for example, the syntax described in Figures 4 and 5.

The encoding process is, for example, a context-adaptive binary arithmetic encoding process. Here, for example, the syntax includes control data (flags and parameters) for controlling the decoding process of the position information.

The geometric information reconstruction unit 1060 is configured to reconstruct the geometric information of each point of the point cloud data to be encoded (the coordinate system assumed by the encoding process, i.e., the position information after the coordinate transformation in the coordinate transformation unit 1010) based on the tree information generated by the tree analysis unit 1030 and the approximate surface information generated by the approximate surface analysis unit 1040.

The color conversion unit 1070 is configured to perform color conversion when the input attribute information is color information. Color conversion does not necessarily have to be performed, and whether or not the color conversion process is performed is coded as part of the control data and transmitted to the point cloud decoding device 200.

The attribute transfer unit 1080 is configured to correct the attribute values based on the position information of the input point cloud, the position information of the point cloud after reconstruction in the geometric information reconstruction unit 1060, and the attribute information after color change in the color conversion unit 1070, so as to minimize distortion of the attribute information.

The RAHT unit 1090 is configured to receive the attribute information transferred by the attribute transfer unit 1080 and the geometric information generated by the geometric information reconstruction unit 1060 as input, and to generate residual information for each point using a type of Haar transform called RAHT (Region Adaptive Hierarchical Transform). As a specific example of the RAHT process, the method described in the above-mentioned literature 2 can be used.

The LoD calculation unit 1100 is configured to receive the geometric information generated by the geometric information reconstruction unit 1060 as input and generate the LoD (Level of Detail).

LoD is information for defining a reference relationship (a referencing point and a referenced point) to realize predictive coding, such as predicting attribute information of a certain point from attribute information of another point and encoding or decoding the prediction residual.

In other words, LoD is information that defines a hierarchical structure in which each point contained in the geometric information is classified into multiple levels, and the attributes of points belonging to lower levels are encoded or decoded using attribute information of points belonging to higher levels.

As a specific method for determining LoD, for example, the method described in the above-mentioned literature 2 may be used.

The lifting unit 1110 is configured to generate residual information by a lifting process using the LoD generated by the LoD calculation unit 1100 and the attribute information after attribute transfer in the attribute transfer unit 1080.

Specific lifting processing may be, for example, the method described in the literature (Text of ISO/IEC 23090-9 DIS Geometry-based PCC, ISO/IEC JTC1/SC29/WG11 N19088).

The attribute information quantization unit 1120 is configured to quantize the residual information output from the RAHT unit 1090 or the lifting unit 1110. Here, a quantization step size of 1 is equivalent to no quantization being performed.

The attribute information encoding unit 1130 is configured to perform encoding processing using the quantized residual information, etc. output from the attribute information quantization unit 1120 as syntax, and generate a bit stream related to the attribute information (attribute information bit stream).

The encoding process is, for example, a context-adaptive binary arithmetic encoding process. Here, for example, the syntax includes control data (flags and parameters) for controlling the decoding process of the attribute information.

The point cloud encoding device 100 is configured to perform encoding processing using the position information and attribute information of each point in the point cloud as input, and to output a geometric information bit stream and an attribute information bit stream through the above processing.

Furthermore, the above-mentioned point cloud encoding device 100 and point cloud decoding device 200 may be realized as a program that causes a computer to execute each function (each process).

In each of the above embodiments, the present invention has been described using the point cloud encoding device 100 and the point cloud decoding device 200 as examples, but the present invention is not limited to such examples and can be similarly applied to a point cloud encoding/decoding system having the functions of the point cloud encoding device 100 and the point cloud decoding device 200.

In addition, according to this embodiment, for example, it is possible to improve the overall service quality in video communication, which makes it possible to contribute to Goal 9 of the Sustainable Development Goals (SDGs) led by the United Nations, which is to "build resilient infrastructure, promote sustainable industrialization and foster innovation."

10...Point cloud processing system 100...Point cloud encoding device 1010...Coordinate transformation unit 1020...Geometric information quantization unit 1030...Tree analysis unit 1040...Approximate surface analysis unit 1050...Geometric information encoding unit 1060...Geometric information reconstruction unit 1070...Color conversion unit 1080...Attribute transfer unit 1090...RAHT unit 1100...LoD calculation unit 1110...Lifting unit 1120...Attribute information quantization unit 1130...Attribute information encoding unit 200...Point cloud decoding device 2010...Geometric information decoding unit 2020...Tree synthesis unit 2030...Approximate surface synthesis unit 2040...Geometric information reconstruction unit 2050...Inverse coordinate transformation unit 2060...Attribute information decoding unit 2070...Inverse quantization unit 2080...RAHT unit 2090...LoD calculation unit 2100...Inverse lifting unit 2110...Inverse color conversion unit

Claims (7)

A point cloud decoding device, comprising:
A point cloud decoding device characterized by comprising an approximate surface synthesis unit that performs vertex interpolation processing using the vertices of the node with the larger node size at the boundary between nodes having different sizes, and when the vertex that has been subjected to the interpolation processing exists on an edge of the node with the smaller node size, performs Trisoup processing with the vertex that has been subjected to the interpolation processing as the vertex of the edge of the node with the smaller node size. The approximate surface synthesis unit includes:
If the vertex for which the interpolation process has been performed for each edge of the node is not on the edge of the node, decode the vertex information;
The point cloud decoding device according to claim 1, characterized in that, when the vertex on which the interpolation process is performed for each side of a node is on an edge of the node, vertex information is set based on the vertex on which the interpolation process is performed. The approximate surface synthesis unit includes:
generating mask information for each edge of the node prior to decoding the vertex information;
The point cloud decoding device according to claim 2 , wherein it is determined whether or not the vertex on which the interpolation process is performed is present on the edge of the node based on the mask information. The point cloud decoding device according to claim 2 or 3, characterized in that the vertex information is composed of two pieces of information: information indicating the presence or absence of a vertex and the position of the vertex on an edge. The point cloud decoding device according to claim 4, characterized in that, if the vertex on which the interpolation process is performed is on an edge of the node, the information indicating the presence or absence of the vertex indicates that the vertex is present, and the vertex position on the edge is implicitly regarded as indicating the vertex position on which the interpolation process is performed. A point cloud decoding method, comprising:
performing vertex interpolation processing at the boundary between nodes having different sizes using the vertices of the node having a larger node size;
and when the vertex on which the interpolation process has been performed is on an edge of a node having a smaller node size, performing a Trisoup process with the vertex on which the interpolation process has been performed as a vertex of the edge of the node having a smaller node size. A program for causing a computer to function as a point group decoding device,
The point group decoding device comprises:
A program comprising an approximate surface synthesis unit that performs vertex interpolation processing using a vertex of a node having a larger node size at a boundary between nodes having different sizes, and when the vertex that has been subjected to the interpolation processing is on an edge of a node having a smaller node size, performs a Trisoup processing with the vertex that has been subjected to the interpolation processing as a vertex of the edge of the node having the smaller node size.

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