IPTC tags in WebP images

[allmyfavoriteusernamesaretaken]

Describe the bug

I have configured some EXIF and IPTC tags to be set when exporting photos from darktable (lighttable → export → preferences → per metadata settings). When exporting the photos in JPEG format, all metadata tags are included in the resulting image, but when exporting in WebP format, only the EXIF tags are included.

Is there any particular reason for this behavior? Or is this a bug?

Steps to reproduce

1. Open the export preferences (lighttable → export → preferences → per metadata settings).
2. Configure some EXIF and some IPTC metadata tags (e.g. Exif.Image.Artist → $(CREATOR) and Iptc.Application2.Byline → $(CREATOR)). Make sure the tags is defined for the photo to export in darktable.
3. Export the same photo twice, once as JPEG and a second time as WebP.
4. Read the metadata of both images, e.g. using Gimp (Image → Metadata → View Metadata).
5. Both images will contain the EXIF metadata, but only the JPEG the IPTC metadata.

Expected behavior

darktable should export IPTC metadata also in WebP images.

Logfile | Screenshot | Screencast

Commit

No response

Where did you obtain darktable from?

flatpak

darktable version

4.6.1

What OS are you using?

Linux

What is the version of your OS?

Ubuntu 22.04

Describe your system?

No response

Are you using OpenCL GPU in darktable?

None

If yes, what is the GPU card and driver?

No response

Please provide additional context if applicable. You can attach files too, but might need to rename to .txt or .zip

No response

[kmilos]

WebP format does not support IPTC metadata, AFAIK:

https://developers.google.com/speed/webp/docs/riff_container

[allmyfavoriteusernamesaretaken]

Thanks a lot!

[kmilos]

See also https://groups.google.com/a/webmproject.org/g/webp-discuss/c/u-zht-Suh-g/m/EXxcPYLlAQAJ

[kmilos]

@allmyfavoriteusernamesaretaken I guess you can (and should) use IPTC properties in XMP though (in combination w/ Dublin Core ones):

https://exiv2.org/tags-xmp-iptc.html
https://exiv2.org/tags-xmp-iptcExt.html

https://www.iptc.org/std/photometadata/specification/IPTC-PhotoMetadata#iptc-core-schema-1-4-specifications
https://www.iptc.org/std/photometadata/specification/IPTC-PhotoMetadata#iptc-extension-schema-1-8-specifications

Note that this seems to be the recommended way now, and modern formats like AVIF and JPEG XL also support only Exif and XMP.

[allmyfavoriteusernamesaretaken]

Thanks, I'll try this.