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I sorry to say that you are wrong about this. > Key features of the JPEG XL codec are: > lossless JPEG transcoding, > Moreover, JPEG XL includes several featu

I sorry to say that you are wrong about this. > Key features of the JPEG XL codec are: > lossless JPEG transcoding, > Moreover, JPEG XL includes several features that help transition from the legacy JPEG coding format. Existing JPEG files can be losslessly transcoded to JPEG XL files, significantly reducing their size (Fig. 1). These can be reconstructed to the exact same JPEG file, ensuring backward compatibility with legacy applications. Both transcoding and reconstruction are computationally efficient. Migrating to JPEG XL reduces storage costs because servers can store a single JPEG XL file to serve both JPEG and JPEG XL clients. This provides a smooth transition path from legacy JPEG platforms to the modern JPEG XL. https://ds.jpeg.org/whitepapers/jpeg-xl-whitepaper.pdf https://ds.jpeg.org/whitepapers/jpeg-xl-whitepaper.pdf If you need more profs, you could transcode a JPEG to JPEG XL and convert against to JPEG. The result image would be BINARY IDENTICAL to the original image. 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.

gabrielhidasy 1y ago
> 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. So he was not wrong about this. You have perfect JPEG -> JPEG XL conversion, but not the other way around.
CorrectHorseBat 1y ago
Yes, JPG to JPEG XL and back is lossless, but the reverse is nowhere mentioned. Trying around with some jpg and jxl files I cannot convert jxl losslessly to jpg files even if they are only 8bit. The jxl files transcoded from jpg files show "JPEG bitstream reconstruction data available" with jxlinfo, so I think some extra metadata is stored when going from jpg to jxl to make the lossless transcoding possible. I can imagine not supporting the reverse (which is pretty useless anyway) allowed for more optimizations.
- jbverschoor 1y ago
  JPG to JXL is lossless, and will save around 20% JXL to JPG is lossless as in a bit-for-bit identical file can be generated
  - adgjlsfhk1 1y ago
    > JXL to JPG is lossless only if you got the JXL from JPG.
spider-mario 1y ago
> 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.
adgjlsfhk1 1y ago
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

gabrielhidasy 1y ago

> 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. So he was not wrong about this. You have perfect JPEG -> JPEG XL conversion, but not the other way around.

CorrectHorseBat 1y ago

Yes, JPG to JPEG XL and back is lossless, but the reverse is nowhere mentioned. Trying around with some jpg and jxl files I cannot convert jxl losslessly to jpg files even if they are only 8bit. The jxl files transcoded from jpg files show "JPEG bitstream reconstruction data available" with jxlinfo, so I think some extra metadata is stored when going from jpg to jxl to make the lossless transcoding possible. I can imagine not supporting the reverse (which is pretty useless anyway) allowed for more optimizations.

jbverschoor 1y ago
JPG to JXL is lossless, and will save around 20% JXL to JPG is lossless as in a bit-for-bit identical file can be generated
- adgjlsfhk1 1y ago
  > JXL to JPG is lossless only if you got the JXL from JPG.

spider-mario 1y ago

> 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.

adgjlsfhk1 1y ago

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