Yes, every raw file has thumbnails at multiple sizes embedded in it. Windows (and probably MacOS) uses LibRaw for accessing (or generating, idk) it. But in the foss world, all of the apps that deal with raw files have moved or are moving from LibRaw to Exiv2. Exiv2 is supposed to be much faster and robust as far as I’ve heard.
Another problem I forgot to mention with the current thumbnailer in gnome is that it’s not generating or getting thumbnails from the metadata at all for the HEIF, HEIC, AVIF and CR3 images.
Mobile phones are moving rapidly from jpeg to heif and even cameras like any Canon R model shoots CR3 raw and HEIF as lossy compressed format. Basically the gnome thumbnailer afaik doesn’t support the ISOBMFF formats at all, so users can’t get any previews.
Exiv2 just landed support for ISOBMFF a while ago. It would be so great if gnome raw thumbnailer was replaced with a solution based on Exiv2 or developed into one that supports all of the most used formats and isn’t unusably slow.
If anyone is interested in exploring this, you can perhaps start by exploring metadata with exiftool.
To view image metadata you can just:
$ exivtool <rawfile.ext>
There are multiple previews or thumbnails in raws these days, you can extract them by quality by running:
$ exiftool -b -JpgFromRaw [anyraw]>JpgFromRaw.jpg
or
$ exiftool -b -PreviewImage [anyraw]>PreviewImage.jpg
or
$ exiftool -b -ThumbnailImage [anyraw]>ThumbnailImage.jpg
Depending on the camera, some may have a differently named tags. The above is the standard but beware if you don’t get any output with exiftool. So I think Exiv2 abstracts that in some way for ease of use.
But when developing a software I see Darktable and DigiKam and others use Exiv2, unfortunately I’m not familiar how those commands go. It can be used from terminal like the exiftool but afaik it’s meant to be a lib in a software and not a standalone app.
Also, a bunch of raw files for testing and development purposes released under open license can be found here:
https://raw.pixls.us/
So you don’t even need to own a camera.
I really hope someone gets interested in this and picks it up.