A.2.3.4 Conversion between two projection formats

26.1183GPPRelease 17TSVirtual Reality (VR) profiles for streaming applications

Denote (f_d,i_d,j_d) as a point (i_d,j_d) on face f_d in the destination projection format, and (f_s,i_s,j_s) as a point (i_s,j_s) on face f_s 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 (f_d,i_d,j_d) on the destination projection plane, maps it to the corresponding (X,Y,Z) in 3D coordinate system, finds the corresponding sample position (f_s,i_s,j_s) on the source projection plane, and sets the sample value at (f_d,i_d,j_d) based on the sample value at (f_s,i_s,j_s).

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 (f_d,i_d,j_d) 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,i_s,j_s) based to the ERP format.

3) Calculate the sample value at (f₀,i_s,j_s) by interpolating from neighboring samples at integer positions on face f₀, and the interpolated sample value is placed at (f_d,i_d,j_d) in the destination projection format.

The above steps are repeated until all sample positions (f_d,i_d,j_d) 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.

Annex B (informative):
Example External Binaural Renderer