[zokier]

But HEIC/AVIF did exactly that, defined image format on top of standard container (isobmff/heif). JPEG-XL is the odd one out because it doesn't have standardized HEIF format, but for example JPEG-XS and JPEG-XR are supported in HEIF.

[jonsneyers]

JPEG XL uses the ISOBMFF container, with an option to skip the container completely and just use a raw codestream. HEIF is also ISOBMFF based but adds more mandatory stuff so you end up with more header overhead, and it adds some functionality at the container level (like layers, or using one codestream for the color image and another codestream for the alpha channel) that is useful for codecs that don't have that functionality at the codestream level — like video codecs which typically only support yuv, so if you want to do alpha you have to do it with one yuv frame and one yuv400 frame, and use a container like HEIF to indicate that the second frame represents the alpha channel. So if you want to use a video codec like HEVC or AV1 for still images and have functionality like alpha channels, ICC profiles, or orientation, then you need such a container since these codecs do not natively support those things. But for JPEG XL this is not needed since JPEG XL already does have native support for all of these things — it was designed to be a still image codec after all. It's also more effective for compression to support these things at the codec level, e.g. in JPEG XL you can have an RGBA palette which can be useful for lossless compression of certain images, while in HEIC/AVIF this is impossible since the RGB and A are in two different codestreams which are independent from one another and only combined at the container level.

It would be possible to define a JPEG XL payload for the HEIF container but it would not really bring anything except a few hundred bytes of extra header overhead and possibly some risk of patent infringement since the IP situation of HEIF is not super clear (Nokia claims it has relevant patents on it, and those are not expired yet).

[zokier]

> JPEG XL uses the ISOBMFF container, with an option to skip the container completely and just use a raw codestream

Hey, thanks for the clarification! I was basing my info on Wikipedia (my bad), ISO BMFF page doesn't mention JXL at all, and even JPEG XL page has only small print in infobox saying that its "based on" ISO BMFF but the main article text doesn't mention that at all.

> But for JPEG XL this is not needed since JPEG XL already does have native support for all of these things — it was designed to be a still image codec after all

I suppose that is bit the thing grand-parent comment was complaining about, format not designed for general-purpose containers but rather as an standalone thing. I suppose it could be fun thought experiment to imagine what JXL would look like if it were specifically designed to be used in HEIF.

Of course it is well understandable that making tailored purpose-built format ends up better in many ways vs trying to fit into some existing generic thing.

> It would be possible to define a JPEG XL payload for the HEIF container but it would not really bring anything except a few hundred bytes of extra header overhead and possibly some risk of patent infringement since the IP situation of HEIF is not super clear (Nokia claims it has relevant patents on it, and those are not expired yet).

I suppose JXL-in-HEIF would allow some image management tools to have common code path for handling JXL and HEIC/AVIF files, grabbing metadata etc, and possibly would not need any specific JXL support. But that is probably not a practical concern in reality.