libheif: support for shared auxl alpha images

[bradh]

Initial work (for discussion / review) on support for shared alpha images.

Relates #1147

I've shown some commented out code that returns an error if the alpha requires scaling (which is valid in HEIF). Until we support that, the options are silently ignoring the alpha plane, or not supporting the image at all.

We do have some scaling code, but my attempt to apply it failed - its designed for an image, not an image plane, and the refactoring got away.

Fixes / suggestions appreciated.

[leo-barnes]

I see the same issue for cdsc:

        if (ref.header.get_short_type() == fourcc("cdsc")) {
          std::vector<uint32_t> refs = ref.to_item_ID;
          if (refs.size() != 1) {
            return Error(heif_error_Invalid_input,
                         heif_suberror_Unspecified,
                         "Metadata not correctly assigned to image");
          }

Presumably the same holds for thmb references.

[bradh]

I see the same issue for cdsc:

Yes (and a bit lower for the region case too). Not too hard to fix.

Presumably the same holds for thmb references.

This (and depth images) will require a bit more refactoring.

[bradh]

Looks like I need to address the fuzzing issue....

[bradh]

This should now fully address #1147

Also cleans up some unused members that made the problem look more complicated.

[leo-barnes]

Hi @bradh!

Do you have a time-line for when this will land?

[bradh]

Do you have a time-line for when this will land?

I think it is complete but cannot progress it beyond this point.

[farindk]

Yes, I know I'm the bottleneck. I'm currently completely blocked by other projects. Will have a look as soon as I can spend time on this.

[bradh]

Yes, I know I'm the bottleneck. I'm currently completely blocked by other projects. Will have a look as soon as I can spend time on this.

@farindk I think you should only do libheif when it suits you. Better that it gets done right, and that the long term for both you and the code is healthy and happy. So no rush from me! Sorry if it looked like criticism.

[leo-barnes]

Got it, no worries. Was just wondering what the plan was and whether there was anything spec or info-wise that was blocking you.

[farindk]

No worries at all. I didn't think that for a single second.
Just wanted to let you know that I'm fine, healthy and happy. Just working on something different and urgent at the moment.

[farindk]

Thanks, this is great.

One thing: I'm not sure if libheif should automatically scale the alpha channel. It is not clear what scaling filter should be used and whether the user wants this. At least, we might want to enable this as a decoding option. But we'll probably have to postpone this since currently, even libheif assumes that the alpha channel has the same resolution.