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Content for  TS 26.118  Word version:  17.0.0

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A  Content Generation GuidelinesWord‑p. 72

A.1  IntroductionWord‑p. 72

This clause collects information that supports the generation of VR Content following the details in the present document. Video and audio related aspects are collected. For additional details and background also refer to TR 26.918.

A.2  VideoWord‑p. 72

A.2.1  OverviewWord‑p. 72

This clause collects information on that support the generation of video bitstreams that conform to operation points and media profile in the present document.

A.2.2  Decoded Texture Signal ConstraintsWord‑p. 72

A.2.2.1  GeneralWord‑p. 72

Due to the restrictions to use a single decoder, the decoded texture signals require to follow the profile and level constraints of the decoder. Generally, this requires a careful balance of the permitted frame rates, stereo modes, spatial resolutions, and usage of region wise packing for different resolutions and coverage restrictions. Details on preferred settings such as frame rates and spatial resolutions are for example discussed in TR 26.918.
This clause provides a summary of restrictions for the different operation points defined in the present document.
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A.2.2.2  Constraints for Main and Flexible H.265/HEVC Operation PointWord‑p. 72

The profile and level constraints of H.265/HEVC Main-10 Profile Main Tier Profile Level 5.1 require careful balance of the permitted frame rates, stereo modes, spatial resolutions, and usage of region wise packing for different resolutions and coverage restrictions. If the decoded texture signal is beyond the Profile and Level constraints, then a careful adaptation of the signal is recommended to fulfil the constraints.
This clause provides a brief overview of potential signal constraints and possible adjustments.
Table A.2-1 provides selected permitted combinations of spatial resolutions, frame rates and stereo modes assuming full coverage and no region-wise packing applied. Note that fractional frame rates are excluded for better readability. Note that the Main H.265/HEVC Operation Point only allows frame rates up to 60 Hz.
Spatial resolution per eye Stereo Permitted Frame Rates in Hz
4096 × 2048None24; 25; 30; 50; 60
3840 × 1920None24; 25; 30; 50; 60
3072 × 1536None24; 25; 30; 50; 60; 90; 100
2880 × 1440None24; 25; 30; 50; 60; 90; 100; 120
2048 × 1024None24; 25; 30; 50; 60; 90; 100; 120
2880 × 1440TaB24; 25; 30; 50; 60
2048 × 1024TaB24; 25; 30; 50; 60; 90; 100; 120
Table A.2-2 provides the maximum percentage of high-resolution area that can be encoded assuming that the low-resolution area is encoded in 2k resolution covering the full 360 degree area, i.e. using 2048 × 1024 or 1920 × 960 and full coverage is provided for different frame rates. Note also that a viewport typically covers about 12-25% of a full 360 video. Note that fractional frame rates are excluded for better readability.
Spatial resolution per eye in VP Spatial resolution per eye in non-VP Stereo Frame Rates in Hz
24 25 30 50 60 90 100 120
6144 × 30722048 × 1024None9.29%9.29%9.29%9.29%9.29%5.24%4.43%3.21%
4096 × 20482048 × 1024None21.67%21.67%21.67%21.67%21.67%12.22%10.33%7.50%
3840 × 19201920 × 960None100.00%100.00%100.00%100.00%100.00%74.12%63.38%47.26%
6144 × 30722048 × 1024TaB3.21%3.21%3.21%3.21%3.21%1.19%0.79%0.18%
4096 × 20482048 × 1024TaB7.50%7.50%7.50%7.50%7.50%2.78%1.83%0.42%
3840 × 19201920 × 960TaB47.26%47.26%47.26%47.26%47.26%20.40%15.02%6.96%
Table A.2-3 provides the maximum percentage of coverage area that can be encoded assuming that the remaining pixels are not encoded for different frame rates. Note that fractional frame rates are excluded for better readability.
Spatial resolution per eye Stereo Frame Rates in Hz
24 25 30 50 60 90 100 120
6144 × 3072None47.22%47.22%47.22%47.22%47.22%31.48%28.33%23.61%
4096 × 2048None100.00%100.00%100.00%100.00%100.00%70.83%63.75%53.13%
3840 × 1920None100.00%100.00%100.00%100.00%100.00%80.59%72.53%60.44%
6144 × 3072TaB23.61%23.61%23.61%23.61%23.61%15.74%14.71%11.81%
4096 × 2048TaB53.13%53.13%53.13%53.13%53.13%35.42%31.88%26.56%
3840 × 1920TaB60.44%60.44%60.44%60.44%60.44%40.30%36.27%30.22%
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A.2.2.2a  Constraints for Main 8K H.265/HEVC Operation Point |R17|Word‑p. 73

The profile and level constraints of H.265/HEVC Main-10 Profile Main Tier Profile Level 6.1 require careful balance of the permitted frame rates, stereo modes, spatial resolutions, and usage of region wise packing for different resolutions and coverage restrictions. If the decoded texture signal is beyond the Profile and Level constraints, then a careful adaptation of the signal is recommended to fulfil the constraints.
This clause provides a brief overview of potential signal constraints and possible adjustments.
Table A.2a-1 provides selected permitted combinations of spatial resolutions, frame rates and stereo modes assuming full coverage and no region-wise packing applied. Note that fractional frame rates are excluded for better readability. Note that the Main 8K H.265/HEVC Operation Point only allows frame rates up to 60 Hz.
Spatial resolution per eye Stereo Permitted Frame Rates in Hz
8192 × 4096None24; 25; 30; 50; 60
7680 × 3840None24; 25; 30; 50; 60
6144 × 3072None24; 25; 30; 50; 60; 90; 100
5760 × 2880None24; 25; 30; 50; 60; 90; 100; 120
4096 × 2048None24; 25; 30; 50; 60; 90; 100; 120
5760 × 2880TaB24; 25; 30; 50; 60
4096 × 2048TaB24; 25; 30; 50; 60; 90; 100; 120
Table A.2a-2 provides the maximum percentage of coverage area that can be encoded assuming that the remaining pixels are not encoded for different frame rates. Note that fractional frame rates are excluded for better readability.
Spatial resolution per eye Stereo Frame Rates in Hz
24 25 30 50 60 90 100 120
12288 × 6144None47.22%47.22%47.22%47.22%47.22%31.48%28.33%23.61%
8192 × 4096None100.00%100.00%100.00%100.00%100.00%70.83%63.75%53.13%
7680 × 3840None100.00%100.00%100.00%100.00%100.00%80.59%72.53%60.44%
12288 × 6144TaB23.61%23.61%23.61%23.61%23.61%15.74%14.71%11.81%
8192 × 4096TaB53.13%53.13%53.13%53.13%53.13%35.42%31.88%26.56%
7680 × 3840TaB60.44%60.44%60.44%60.44%60.44%40.30%36.27%30.22%
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A.2.3  Conversion of ERP Signals to CMPWord‑p. 74

A.2.3.1  GeneralWord‑p. 74

The 3D XYZ coordinate system as shown in Figure A.1 can be used to describe the 3D geometry of ERP and CMP projection format representations. Starting from the center of the sphere, X axis points toward the front of the sphere, Z axis points toward the top of the sphere, and Y axis points toward the left of the sphere.
Copy of original 3GPP image for 3GPP TS 26.118, Fig. A.1: 3D XYZ coordinate definition
Figure A.1: 3D XYZ coordinate definition
(⇒ copy of original 3GPP image)
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The coordinate system is specified for defining the sphere coordinates azimuth (Φ) and elevation (θ) for identifying a location of a point on the unit sphere. The azimuth Φ is in the range [−ϖ, ϖ], and elevation θ is in the range [−ϖ/2, ϖ/2], where ϖ is the ratio of a circle's circumference to its diameter. The azimuth (Φ) is defined by the angle starting from X axis in counter-clockwise direction as shown in Figure A.1. The elevation (θ) is defined by the angle from the equator toward Z axis as shown in Figure A.1. The (X, Y, Z) coordinates on the unit sphere can be evaluated from (Φ, θ) using following equations:
X = cos(θ)*cos(Φ)
Y = cos(θ)*sin(Φ)
Z = sin(θ)
Inversely, the longitude and latitude (Φ, θ) can be evaluated from (X, Y, Z) coordinates using:
Φ = tan-1(Y/X)
θ = sin-1(Z/(sqrt(X2+Y2+Z2)))
A 2D plane coordinate system is defined for each face in the 2D projection plane. Where Equirectangular Projection (ERP) has only one face, Cubemap Projection (CMP) has six faces. In order to generalize the 2D coordinate system, a face index is defined for each face in the 2D projection plane. Each face is mapped to a 2D plane associated with one face index.
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A.2.3.2  Equirectangular Projection (ERP)Word‑p. 75

Equirectangular mapping is the most commonly used mapping from spherical video to a 2D texture signal. The mapping is bijective, i.e. it may be expressed in both directions and is illustrated in Figure A.2.
Copy of original 3GPP image for 3GPP TS 26.118, Fig. A.2: Mapping of spherical video to a 2D texture signal
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ERP has only one face and the face index f for ERP is always set to 0. The sphere coordinates (Φ, θ) for a sample location (i, j), in degrees, are given by the following equations:
Φ = (0.5 - i/pictureWidth)*360
θ = (0.5 - j/pictureHeight)*180
Finally, (X, Y, Z) can be calculated from the equations given above.

A.2.3.3  Cubemap Projection (CMP)Word‑p. 75

Figure A.3 shows the CMP projection with 6 square faces, labelled as PX, PY, PZ, NX, NY, NZ (with "P" standing for "positive" and "N" standing for "negative"). Table A.2-4 specifies the face index values corresponding to each of the six CMP faces.
Copy of original 3GPP image for 3GPP TS 26.118, Fig. A.3: Relation of the cube face arrangement of the projected picture to the sphere coordinates
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Face index Face label Notes
0PXFront face with positive X axis value
1NXBack face with negative X axis value
2PYLeft face with positive Y axis value
3NYRight face with negative Y axis value
4PZTop face with positive Z axis value
5NZBottom face with negative Z axis value
The 3D coordinates (X, Y, Z) are derived using following equations:
lw = pictureWidth / 3
lh = pictureHeight / 2
tmpHorVal = i − Floor( i ÷ lw ) * lw
tmpVerVal = j − Floor( j ÷ lh ) * lh
i' = −( 2 * tmpHorVal ÷ lw ) + 1
j' = −( 2 * tmpVerVal  ÷ lh ) + 1
w = Floor( i ÷ lw )
h = Floor( j ÷ lh )
if( w  = =  1  &&  h  = =  0 ) { // PX: positive x front face
	X = 1.0
	Y = i'
	Z = j'
} else if( w  = =  1  &&  h  = =  1 ) { // NX: negative x back face
	X = −1.0
	Y = −j'
	Z = −i'
} else if( w  = =  2  &&  h  = =  1 ) { // PZ: positive z top face
	X = −i'
	Y = −j'
	Z = 1.0
} else if( w  = =  0  &&  h  = =  1 ) { // NZ: negative z bottom face
	X = i'
	Y = −j'
	Z = −1.0
} else if( w  = =  0  &&  h  = =  0 ) { // PY: positive y left face
	X = −i'
	Y = 1.0
	Z = j'
} else { // ( w  = =  2  &&  h  = =  0 ), NY: negative y right face
	X = i'
	Y = −1.0
	Z = j'
}
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A.2.3.4  Conversion between two projection formatsWord‑p. 76

Denote (fd,id,jd) as a point (id,jd) on face fd in the destination projection format, and (fs,is,js) as a point (is,js) on face fs in the source projection format. Denote (X,Y,Z) as the corresponding coordinates in the 3D XYZ space. The conversion process starts from each sample position (fd,id,jd) on the destination projection plane, maps it to the corresponding (X,Y,Z) in 3D coordinate system, finds the corresponding sample position (fs,is,js) on the source projection plane, and sets the sample value at (fd,id,jd) based on the sample value at (fs,is,js).
Therefore, the projection format conversion process from ERP source format to CMP destination format is performed in the following three steps:
  1. Map the destination 2D sampling point (fd,id,jd) to 3D space coordinates (X,Y,Z) based on the CMP format.
  2. Map (X,Y,Z) from step 1 to 2D sampling point (f0,is,js) based to the ERP format.
  3. Calculate the sample value at (f0,is,js) by interpolating from neighboring samples at integer positions on face f0, and the interpolated sample value is placed at (fd,id,jd) in the destination projection format.
The above steps are repeated until all sample positions (fd,id,jd) in the destination projection format are filled. Note that (Step 1) and (Step 2) can be pre-calculated at the sequence level and stored as a lookup table, and only (Step 3) needs to be performed per sample position for each picture in order to render the sample values.
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