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A comment on Why JPEGs Still Rule the Web After 30 Years (2024). Parent comment

Re:Why JPEGs Still Rule the Web After 30 Years (2024) by CorrectHorseBat on Tuesday June 17, 2025 @03:22PM (#44302866)
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.

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Re:Why JPEGs Still Rule the Web After 30 Years (2024) by jbverschoor on Wednesday June 18, 2025 @12:09PM (#44311094)
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

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Re:Why JPEGs Still Rule the Web After 30 Years (2024) by adgjlsfhk1 on Wednesday June 18, 2025 @02:18PM (#44312114)
> JXL to JPG is lossless

only if you got the JXL from JPG.

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