Just to clarify….
Although I rarely use PNG, —and, yes, I can think of many use cases, but that is not the point— metadata does work with PNG, so that is not my issue. Most of the things for which I might possibly use PNG, I tend to use OpenEXR. The thing is that PNG is smaller than OpenEXR & TIFF, and TIFF is not a perfect “standard,” as not all applications use the entire standard, just the subset which they see as necessary. (Just as how DT does not see metadata in WebP as necessary???)
But all that is mostly besides the point. …Metadata is also exported to OpenEXR!
My issue is that I export to WebP quite often. It is cleaner than JPEG, and smaller than JPEG & TIFF. Eventually one day, I may be exporting to AVIF quite often. Right now, when I export to JPEG, PNG, or OpenEXR all the metadata is kept, (except the ones I told DT NOT to export). So I know the metadata export options is working, insofar as JPEG, PNG, and OpenEXR is concerned.
The issue arises where WebP is concerned. If I can fix that, I will be good. Their are certain bits of metadata which I do not want lost when files are distributed to certain end uses. I can always send them larger JPEGs, but I do notice better colour integrity in the WebP files vs the JPEGs as exported by DT.
I can actually take a lossless PNG, and export it from The GIMP as a JPEG JFIF with 4:4:4 colour and get a better (and larger file size) result, compared to DT, and have my metadata.
Okay. I just did some tests. Part of my problem is that my viewer will show me metadata in JPEG, PNG, and OpenEXR files, but it does not show me metadata in WebP files. …So I used the CLI exiv2.
I exported a PNG, an OpenEXR, and a WebP file directly from DT, and exiv2 found no metadata in the WebP file. I then used The GIMP to export a WebP file from the PNG file, and again from the OpenEXR file, and exiv2 found some metadata in both.
So the fact that my viewer was not showing the metadata is irrelevant, because it is not there in the WebP images directly exported from DT. As a workaround, (emphasis on, “work”), I can use The GIMP, but that is not a fix.
P.s., anyone knows of a good image viewer for Linux which will show both the WebP files and the metadata therein? (I am currently using gThumb, and Deepin, neither of which show the metadata for WebP. Thanks).