Adds AVIF support

[LarsFronius]

• This adds a new type AVIF to the supported type list if libvips >= 8.9.0 is installed.
• Calls libheif through libvips with the AV1 compression set to save AVIF images.

This should fix #353.

[LarsFronius]

We'll perhaps need a newer libheif in the CI image if we want to keep the test for it.
We currently install 1.3.2 through upstream packages, but AVIF support only landed in 1.7.0. I'll have a look if we can compile from source.

[LarsFronius]

Test is slow but passing for now. Looks like the libaom en-/decoder is easiest to install for us in CI but a little on the slow side in terms of encoding performance.

There's also the option to use https://code.videolan.org/videolan/dav1d/ as decoder and https://github.com/xiph/rav1e as encoder with libheif. Both have more dependencies and would involve more changes as we'd need to compile them from source it seems, so I'd opt to stay with libaom for this use-case.

[h2non]

Thanks for the PR! Is this ready?

[LarsFronius]

Good to go from my end! 👍

[akmittal]

Tried making it work, Getting following error

heifsave: Unsupported compression

[LarsFronius]

That is the error libheif returns if it isn’t compiled with a AVIF encoder or decoder. Can you try recompiling a recent libheif from source with the rav1e or libaom encoder? I think that should make the error disappear.

I am on the go right now, but if it helps can also supply a container image that works - it works inside the travis CI tests of this PR too btw, where I install libaom and compile libheif from source.

[akmittal]

Got it to work by compiling.
I think this step also should be added in readme to install.
I see 1 issue though. .Metadata() method does not return valid info for me.

[LarsFronius]

First of all: Glad to hear you got AVIF processing working!
Hmm, to be honest: I have not tested the Metadata function. I'll have a go at it sometime this week!

[akmittal]

The latest updates work and Metadata is working fine. It was somewhat of effort to make it work with libheif. I think this step should be added un readme or we should wait for libheif release with avif support to release.

[LarsFronius]

Awesome - can you review the sections I added to the README? Mostly only referring to the documentation on the libheif side - no need to duplicate the information in my opinion.

[akmittal]

This looks good,

Although I have a question. https://github.com/strukturag/libheif/releases mentions that they support avif from version 1.8.0. So why we need to compile libheif for this?

[LarsFronius]

AVIF support has been in libheif from 1.7.0 on:

• In 1.7.0 encoding and decoding via libaom was added.
• In 1.8.0 encoding via rav1e was added.
• In 1.9.0 decoding via dav1d was added.

Any one of these are libraries need to be present for libheif, so that libheif can be compiled supporting them - otherwise libheif throws the "heifsave: Unsupported compression" error.

For now libvips developers have made the choice to encode/decode AVIF via libheif - there have been discussions to directly integrate with an AVIF en- or decoder, e.g. libavif, but that apparently has been dropped. libvips/libvips#1570

So for now the of libheif with one of the three aforementioned AVIF encoders/decoders needs to be made available for libvips for it to encode or decode AVIF.

[LarsFronius]

Hi, is anything still needed to get this merged?

[h2non]

Thanks for the nice work done, and apologies for the latency, OSS is not nowadays something I can spend much time on, unfortunately.

[LarsFronius]

Cheers @h2non, absolutely understandable and relatable 👍
Thanks again for creating and sharing this great library!