https://cs.chromium.org/chromium/src/media/base/video_codecs.cc?rcl=137dc8bc3bed0cbdde708372c4f97aa3732946db&l=399

  if (((subsampling_x == '0' || subsampling_y == '0') &&
       chroma_sample_position != '0') ||
      (subsampling_x == '1' && subsampling_y == '1' &&
       chroma_sample_position == '0')) {
    DVLOG(3) << __func__ << " Invalid chroma subsampling (" << fields[5] << ")";
    return false;
  }

When subsampling_x and subsampling_y are 1, the code does not allow chroma_sample_position to be 0.

In fact, it is possible that chroma_sample_position could be 0. I think we should allow chroma_sample_position to be 0 when subsampling_x == '1' && subsampling_y == '1'.

(1) When mono_chrome == 1, subsampling_x = 1, subsampling_y = 1, chroma_sample_position = 0 [1]

if ( mono_chrome ) {
  color_range f(1)
  subsampling_x = 1
  subsampling_y = 1
  chroma_sample_position = CSP_UNKNOWN
  separate_uv_delta_q = 0
  return
}

(2) Even when mono_chrome is not 1, it seems possible that chroma_sample_position could be 0 [2]

chroma_sample_position of 0 means: Unknown (in this case the source video transfer function must be
signaled outside the AV1 bitstream).

I am not familiar with AV1 color configuration so I could be wrong.

(3) AV1 Codec ISO Media File Format Binding [3] specifies the values shall come from Sequence Header OBU in the bit stream. The binding document [4] also says that the fields in AV1CodecConfigurationRecord shall come from Sequence Header OBU in the bit stream. In file https://cs.chromium.org/chromium/src/media/test/data/bear-av1.mp4?q=bear-av1.mp4&sq=package:chromium&dr, AV1CodecConfigurationRecord has subsampling_x = 1, subsampling_y = 1 and chroma_sample_position = 0. It is actually a bug in the content as in the bit stream, chroma_sample_position is 2. However, it could be a problem in a lot of contents; if content packagers use AV1CodecConfigurationRecord to derive the codec string instead of parsing the actual bit stream, the result content would not play in Chrome.

(4) chromaSubsampling field (== subsampling_x + subsampling_y + chroma_sample_position) is optional in the codecs parameter string, thus Chrome allows it to be absent, which implies unknown subsampling_x, unknown subsampling_y and unknown chroma_sample_position. It is ironic that Chrome does not allow chroma_sample_position to be unknown while subsampling_x and subsampling_y are known.


[1] https://aomediacodec.github.io/av1-spec/av1-spec.pdf#page=44 5.5.2 Color config syntax
[2] https://aomediacodec.github.io/av1-spec/av1-spec.pdf#page=131 6.4.2. Color config semantics
[3] https://aomediacodec.github.io/av1-isobmff/#codecsparam
[4] https://aomediacodec.github.io/av1-isobmff/#av1codecconfigurationbox-syntax