default jpeg-xl uses a different color space (XYZ), bigger transform (up to 256x256), rectangular transforms, etc. if you go from jpg to jxl, you can go back (b

default jpeg-xl uses a different color space (XYZ), bigger transform (up to 256x256), rectangular transforms, etc. if you go from jpg to jxl, you can go back (but your jxl file will be less efficient), but if you compress directly to jxl, you can't losslessly go to jpg