AVIF files with MC=0,8,16,17 in nclx color box and YUV 4:2:0

[wantehchang]

The AV1 specification has the following requirement:

If matrix_coefficients is equal to MC_IDENTITY, it is a requirement of bitstream conformance that subsampling_x is equal to 0 and subsampling_y is equal to 0.

libavif/src/write.c checks a generalization of this requirement:

If matrix_coefficients in the nclx color box is equal to 0 (the identity matrix), the YUV format shall be equal to 4:4:4.

The rationale behind the generalized requirement is that when the identity matrix is used, the Y plane contains G samples, the U plane contains B samples, the V plane contains R samples, and it does not make sense to subsample the U and V planes.

In ITU-T H.265 (V10) (07/2024), I found two requirements similar to the AV1 requirement:

matrix_coeffs shall not be equal to 0, 16, or 17 unless both of the following conditions are true:
– BitDepthC is equal to BitDepthY.
– chroma_format_idc is equal to 3 (the 4:4:4 chroma format).

and

matrix_coeffs shall not be equal to 8 unless one of the following conditions is true:
– BitDepthC is equal to BitDepthY,
– BitDepthC is equal to BitDepthY + 1 and chroma_format_idc is equal to 3 (the 4:4:4 chroma format).

I wonder if we should impose similar requirements on AV1 and AVIF for matrix coefficients 8, 16, and 17.

I attached a ZIP file containing two AVIF files that violated our generalized requirement. The AVIF files have CICP 1/1/0 and 1/13/0 respectively and use the YUV 4:2:0 format.

identity-420-avif.zip

[y-guyon]

I wonder if we should impose similar requirements on AV1 and AVIF for matrix coefficients 8, 16, and 17.

At encoding, decoding, or both?