[JyrkiAlakuijala]

They originally chose to use x265 to calibrate the bitrates, possibly something went wrong there and the 'Tiny', 'Big', etc. are somewhat meaningless.

At 'Large' and 'Big' settings of this image -- which are still in much less than 1 bpp bitrates, i.e., below internet image quality -- you can still observe significant differences in the clouds even if balloons are relatively well rendered.

[jonsneyers]

Nothing went wrong there, it's just what you get if you configure an encoder using just some quantization setting and not a visual target. The same will happen if you would encode images with libjpeg quality 50 (and then derive all other bitrates from there). In some cases the image will look OK-ish at that setting, in other cases it will be complete garbage.

JPEG XL is the first codec to have a practical encoder that can be configured by saying "I want the worst visual difference to be X units of just-noticeable-difference". All other encoders are basically configured by saying "I want to use this scaling factor for the quantization tables, and let's hope that the result will look OK".

[astrange]

> All other encoders are basically configured by saying "I want to use this scaling factor for the quantization tables, and let's hope that the result will look OK".

crf in x264/x265 is smarter than that, but it's still a closed-form solution. That's probably easier to work with than optimizing for constant SSIM or whatever, it always takes one pass and those objective metrics are not actually very good.