Warning about AVIF files with only ICC profile and non-default matrix coefficients in Sequence Header OBU

[leo-barnes]

See the following issue: AOMediaCodec/av1-avif#195

In this AVIF file from the issue, we have only an ICC profile and no NCLX box. The file then specifies identity matrix coefficients in the Sequence Header OBU.

• With the wording in the currently published version of MIAF, the expected behaviour is undefined.
• In the currently proposed changes to MIAF, the behaviour is defined, with the expectation being that the matrix coefficients get CICP value 6 (or 5) and that the value in the Sequence Header OBU is ignored. The expectation in the coming version of MIAF is that you would add both an ICC profile and an NCLX box.

It would be great if the ComplianceWarden could detect cases like these and warn that the behaviour is either undefined or potentially unexpected.

[rbouqueau]

See my comment in #36.