JPEG XL has the mode that in layman's word, allow bit-by-bit round-trip with JPEG.
Original JPEG -> JPEG XL -> Recreated JPEG.
Sha256(Original JPEG) == Sha256(Recreated JPEG).
That's what people meant by "backward compatible".
Re:PNG Is Back
by colejohnson66 on Wednesday June 25, 2025 @12:56PM
(#44379451)
That’s not “backwards compatible”, but “round tripable” or “lossless reencode”
exactly, it is absolutely not backward compatible. It is lossless-at-bit-level conversion of JPEG, but that doesn’t help older SW in any way.
Re:PNG Is Back
by BobaFloutist on Wednesday June 25, 2025 @01:44PM
(#44379945)
Ah, got it. I assumed it was a losselessly compressed JPEG with metadata telling modern software not to compress differently but that older software would open as a normal JPEG, but I guess they meant something else with "backward compatible".
I guess I meant losslessly round-trippable. In other words, you can go from jpeg -> jxl -> jpeg without any loss in quality, potentially (although with jxl -> jpeg -> jxl, you will lose space while it is a jpeg, and you'd probably have to pick a high compression quality in order to not lose information... you may also lose information such as metadata that jxl accommodates but jpeg does not, like transparency)
So backwards-compatible in the sense that the jpeg-xl algorithm spec can read jpg and store the same pixel data more efficiently as jxl if you like. You gain space and lose nothing (except perhaps encode/decode speed).
Not sure what the previous poster meant with “backward compatible” here. jxl is a different format. It can include every information a jpeg includes, which then maybe qualifies as “backward compatible” but it still is a different format.