Support copy png description metadata to avif description

[Kieaer]

Same title. When converting png format to avif, png description metadata is lost.

[y-guyon]

Thank you for the report.

Is there a PNG file example that you can share?

Do you know if another format than AVIF keeps the description when converted from PNG?

[kmilos]

AFAIK, there is no specific "AVIF description" metadata in the AVIF container specification?

The equivalent of PNG "Description" would be I guess either the ImageDescription Exif tag, or the dc:description XMP one.

IMHO, it's probably less overkill to create the XMP payload w/ a single tag, than the Exif one (as there are quite a few mandatory tags to be filled in then).

While at it, it could be useful to map other PNG text properties to equivalent XMP dc schema ones if applicable.

But all of this might be out of scope for libavif...

I'd instead encourage you to use XMP or Exif directly (instead of specific PNG properties), as those are readily supported and transferred between PNG and AVIF (and many more formats).

[Kieaer]

@y-guyon I've never heard of copying descriptions between different formats. Here's example png contains description in metadata https://drive.google.com/file/d/11x84VJFjMdYI8M0Eb0lMqvj1e7YNiqcA/view?usp=sharing

[Kieaer]

@kmilos After downloading the Image from the Google Drive link above, you can check the PNG:Description at the site below. https://exifmeta.com/

I already tried using Exiftool, but the only thing that didn't copy was the png description.

[maryla-uc]

I agree with @kmilos that the best way would probably be to convert the metadata to XMP. However, I have a few concerns:

• It would increase the file size since XMP is quite verbose and stored uncompressed in AVIF
• There are alternatives available: use XMP in the first place if you really need this metadata
• This seems like a very niche use case. Other tools don't convert this metadata as far as we know?
• If there is already an XMP chunk, we would have to either merge the extra metadata with it, which is not entirely trivial (needs a dependency on an xml library and might raise new questions e.g. what if there is already a description in the XMP), or add an addtional XMP metadata box to the file, but not all readers would support that, in particular libavif only reads the first XMP box
• It feels out of scope to handle all various PNG keywords (see the "PNG TextualData Tags" table at https://exiftool.org/TagNames/PNG.html for the "known" ones, but a PNG file might also contian arbitrary keywords)

All in all, I think we can keep the issue open to see if there is more interest, but I would consider this low priority.