JPEG XL is not a "variant", it is a completely new algorithm that is also fully backwards-compatible with every single JPEG already out there, of which there are probably billions at this point.

It also has pretty much every feature desired in an image standard. It is future-proofed.

You can losslessly re-compress a JPEG into a JPEG-XL file and gain space.

It is a worthy successor (while also being vastly superior to) JPEG.

Is there any risk that if I open a JPEG-XL in something that knows what a JPEG is but not what a JPEG-XL is and then save it, it'll get lossy compressed? Backwards compatibility is awesome, but I know that if I save/upload/share/copy a PNG, it shouldn't change without explicit edits, right?

a sw that does not know what jpeg xl is, will not be able to open jxl files. How would it?

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.

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".

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.

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).

> You can losslessly re-compress a JPEG into a JPEG-XL file and gain space.

Is that gained space enough to account for the fact you now have 2 files? Sure, you can delete the original jpg on the local system, but are you going to purge your entire set of backups?

if you do not want to delete the original jpeg, there is no point in converting them to jpeg xl I would say.

Unless serving jxl and saving bandwidth, while increasing your total storage, is worth it to you.

Yes the whole point of lossless re-compression is that you do not need to keep the original JPEGs. Of course you don't need to "purge" backups, just let them rotate out normally.

Also backup storage is usually cheaper than something that needs to have fast access speeds.

For people that shoot digital cameras saving as JPEG, it will a very cocky suggestion to tell them to toss out their camera original files!

You'll know JPEG-XL if real when camera manufactures allow for XL acquisition instead of legacy JPEG only.

> For people that shoot digital cameras saving as JPEG, it will a very cocky suggestion to tell them to toss out their camera original files!

But they can recover them from the JXL files, so are they really “tossed out”? It’s not really different than any other form of lossless compression.

I was referring to the new PNG, not to JPEG XL.

Looking at TFA, it's placing in the spec a few things that are already widely stacked onto the format (such as animation). This is a very sensible update, and backwards compatible with existing PNG.

Not sure expanding PNG capabilities is sensible, looking at the overall landscape of image formats.

The capabilities are already expanded in most common implementations. This update is largely blessing those features as officially "standard".