> However, perhaps are you talking about an image on JPEG XL, using features only in JPEG XL (24 bit, HDR, etc...) that obviously couldn't be converted in a lossless way to a JPEG.
A lot of those features (non-8×8 DCTs, Gaborish and EPF filters, XYB) are enabled by default when you compress a non-JPEG image to a lossy JXL. At the moment, you really do need to compress to JPEG first and then transcode that to JXL if you want the JXL→JPEG direction to be lossless.
A lot of those features (non-8×8 DCTs, Gaborish and EPF filters, XYB) are enabled by default when you compress a non-JPEG image to a lossy JXL. At the moment, you really do need to compress to JPEG first and then transcode that to JXL if you want the JXL→JPEG direction to be lossless.