Need a way to signal the JPEG-style "center" chroma sample position for AVIF images

[wantehchang]

Right now there does not seem to be a way to signal the JPEG-style "center" chroma sample position for AVIF images.

The AV1 specification defines the following four values for the chroma_sample_position syntax element (encoded using two bits):

• 0 CSP_UNKNOWN: Unknown (in this case the source video transfer function must be signaled outside the AV1 bitstream)
• 1 CSP_VERTICAL: Horizontally co-located with (0, 0) luma sample, vertical position in the middle between two luma samples
• 2 CSP_COLOCATED: co-located with (0, 0) luma sample
• 3 CSP_RESERVED

We have some options.

1. Set chroma_sample_position to CSP_UNKNOWN in the AV1 bitstream, and amend the AVIF spec to interpret CSP_UNKOWN as the JPEG-style "center" chroma sample position.
2. Set chroma_sample_position to CSP_UNKNOWN in the AV1 bitstream, and amend the AVIF spec to signal the chroma sample position in the AVIF container format. This option allows AVIF to potentially have more than four chroma sample positions (encoded using more than two bits).
3. Amend the AV1 specification to assign the currently reserved value of 3 to the JPEG-style "center" chroma sample position.

I think Option 3 is the best.

[cconcolato]

There is a PR in the AV1 spec to implement option 3: AOMediaCodec/av1-spec#308

[ValeZAA]

Any updated here?