4 ms·
Careful with the JPEG-XL re-compression, though--depending on how you're re-encoding, jxl may use SSIM to evaluate for visual losslessness, and the whole point
by mceachen 3y ago
Careful with the JPEG-XL re-compression, though--depending on how you're re-encoding, jxl may use SSIM to evaluate for visual losslessness, and the whole point of TFA is that SSIM is blind to posterization, but (some) humans aren't.
Disk space is cheap. It's most likely not worth the 20% compression to lose your original images (and possibly lose metadata as well--it's quite hard to robustly retain all vendor-specific MakerNotes, for example).
- quikee 3y agoJXL has Guetzli lossless JPEG compressor integrated into the standard so it produces reversible and completely standard compliant JXL images that are 15-20% smaller size. Reversible in sense that you can still convert the image back the original JPEG, that is bit exact file as the input JPEG was (it takes care of all the metadata also - it has to). Also if you decide to forgo the reversibility you can get a bit more out of it as JXL is actually a superset of JPEG, so it can read the JPEG stream and convert it to JXL without complete recompression - it will just use more efficient structure of JXL and much more efficient (ANS vs. Huffman) entropy encoding. The additional savings compared to the reversible mode aren't big however.
- JyrkiAlakuijala 3y agoThe lossless thingy is Brunsli. In the last meters of the standardization, Brunsli in JPEG XL was replaced with "Brunsli 2.0", the more natural formalism in JPEG XL format, allowing for a smaller spec and decoder as well as parallel decoding. Guetzli is a slow high quality jpeg encoder. One can use jpegli for that need nowadays, 1000x faster...
- JyrkiAlakuijala 3y agoWe overprovision low frequencies dramatically to avoid posterisation. JPEG XL development was never driven by SSIM, only butteraugli + human viewing. I reviewed manually every quality affecting change during its research and development.