As more of the applications I use, including Emby, MPD, Qmmp, and mpv, support JPEG XL, I’ve been converting my external cover.jpg artwork to cover.jxl to save space. I use Picard extensively for tagging, but the cover art it saves externally is usually a JPEG or PNG. So I made a Picard 3 plugin that converts those files to JPEG XL after Picard saves them:
Picard still decides whether to save an image, what to name it, and whether to overwrite an existing file. The plugin converts only an image Picard actually writes. It leaves embedded artwork alone and requires cjxl and djxl.
I tested converting a copy of my cover artwork to JPEG XL. The JPEG and PNG files used 2.10 GiB; the JXL versions used 1.51 GiB—about 28% less space, or about 600 MB saved, without reducing image quality.
What especially interests me is JXL’s reversible JPEG conversion: the original JPEG can be reconstructed exactly. That seems valuable for large cover art collections as well as personal music libraries.
Before submitting it to the plugin registry, I’d like to know whether others would find this useful. Has external JPEG XL artwork been discussed here? Is there an existing plugin that does something similar, or would this fit better as an improvement to one?