A.2.3.4 Conversion between two projection formats

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

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.

Annex B (informative):
Example External Binaural Renderer