Ignore descriptive properties associated after non-descriptive ones

[y-guyon]

Section 6.5.1 of HEIF says:

Readers shall allow and ignore descriptive properties following the first transformative or unrecognized property, whichever is earlier, in the sequence associating properties with an item.

• I do not think libavif respects that constraint.
• I am not sure what is the definition of an "unrecognized property".
• I guess "ignoring a descriptive property" means treating it like it was not there.
  Because descriptive properties should not impact the rendering of the decoded image, the effect of ignoring one can only be:
  • treating the file as invalid from a missing mandatory descriptive property (such as ispe),
  • not exposing the same image metadata from the libavif API.

[bradh]

I think there could be a distinction between ignoring a property at the application level and ignoring it at the library level. Or maybe this needs to go to MPEG?

[y-guyon]

I think there could be a distinction between ignoring a property at the application level and ignoring it at the library level.

Do you have another meaning in mind than "ignore as if absent"? Otherwise I do not really see the app/lib distinction.

[bradh]

I think there could be a distinction between ignoring a property at the application level and ignoring it at the library level.

Do you have another meaning in mind than "ignore as if absent"? Otherwise I do not really see the app/lib distinction.

I do. I'm thinking about the geospatial case as an example, where the library might treat those properties as opaque. An application that wants to put imagery on a map can use the geospatial-related properties. An application that doesn't care about the spatial (or temporal) aspects can ignore the properties. Same for special production information, or Dublin Core type properties.

In those cases, the library isn't ignoring the descriptive properties after the first one it doesn't recognise. It can pull out whatever properties are useful at the library level, and pass the rest up.

By contrast, an application is actually ignoring the properties.

Still seems like a non-useful requirement though.

Side rabbit-hole topic: this might impact on the mini property ordering. I don't have a current copy, but it would be worth making sure that the non-HDR properties come first.