Support copying monochrome YUV image.

[ledyba-z]

Hi! I' m working at Link-U, a company that publishes digital comics for smart phones.

When monochrome images are decoded by dav1d, U and V planes will be NULL.

However, avifImageCopy assumes that all Y, U and V planes are filled with non-NULL values if and only if the image has YUV planes:

libavif/src/avif.c

Lines 152 to 154 in c541cf9

	if (srcImage->yuvPlanes[AVIF_CHAN_Y] && srcImage->yuvPlanes[AVIF_CHAN_U] && srcImage->yuvPlanes[AVIF_CHAN_V]) {
	avifImageAllocatePlanes(dstImage, AVIF_PLANES_YUV);

Thus, when avifDecoderRead reads a monochrome image (that has only Y plane), the last call of avifImageCopy does not copy at all, and all planes in the returned copied image will be NULL.

libavif/src/read.c

Lines 1924 to 1939 in c541cf9

	avifResult avifDecoderRead(avifDecoder * decoder, avifImage * image, avifROData * input)
	{
	avifResult result = avifDecoderParse(decoder, input);
	if (result != AVIF_RESULT_OK) {
	return result;
	}
	result = avifDecoderNextImage(decoder);
	if (result != AVIF_RESULT_OK) {
	return result;
	}
	if (!decoder->image) {
	return AVIF_RESULT_NO_IMAGES_REMAINING;
	}
	avifImageCopy(image, decoder->image);
	return AVIF_RESULT_OK;
	}

In this patch, just check Y plane to determine an image has YUV planes, and free U, V planes if they are unused.

Please take a look.

P.S.

I also have an idea adding AVIF_PLANES_Y enum to avifPlanesFlags, to tell avifImageAllocatePlanes to allocate just only Y plane. How about this idea?

[joedrago]

I'm currently unavailable to properly fix the copy issue until Tuesday, but if you are trying to avoid unnecessary allocations and copying on decode, I highly recommend using the "advanced" decoding example in the README. It only ends up being one more function call, and it directly gives you the Y plane allocated by dav1d in decoder->image.

If you use this path instead, you should no longer have the copy issue (which I will fix next week), and you will save 3 allocations and copies.

[ledyba-z]

Thanks you for telling me that "advanced" decoding way.
I will try that in SDWebImageAVIFCoder, the project I'm currently working on.

I am going to wait your fix, so I made a separated Issue about this:
#40

And I will close this issue!

[dreampiggy]

@ledyba-z I’m the code owner of SDWebImageAVIFCoder. If we can, choose the advanced decoding API is OK

PR is welcomed. I’ll release a new version with Swift Package Manager support, if you can implements the advanced API, let me merge and release.

Yesterday I spend much time on porting the libavif(https://github.com/SDWebImage/libavif-Xcode), libaom, libdav1d with SwiftPM support :( That’s a hard and boring work....So I need a little time to relax.

Or if this is harder to implements, I can release a minor version firstly, then another minor version with you PR

[dreampiggy]

@joedrago Hi, for iOS user, developer prefers to use the Swift Package Manager dependency tool, not CMake. I maintain a wrapper with SwiftPM support. But everytine you release a new version, I have to update the git submodule and tag a new version...This work is a little tired. Can SwiftPM be officially supported ?

If you want to support, just need one file named Package.swift. It’s a DSL to declare compile unit, not hard to learn. I’ve already done with 0.4.8 libavif, you can have a reference to this : https://github.com/SDWebImage/libavif-Xcode/blob/master/Package.swift.

Or even, if you want, I can create a PR to implements SwiftPM, even the future maintain of Travis-CI to ensure SwiftPM build works. SwiftPM can works for Linux/macOS as well.

[ledyba-z]

@dreampiggy
It is easy to use advanced decoding API. I already added to my wip branch:

link-u/SDWebImageAVIFCoder@7ae9c81

When using this advanced API, we can't call avifImageYUVToRGB (it crashes).
So I am going to include this advanded decoding API patch to SDWebImage/SDWebImageAVIFCoder#7 . Please wait a moment!

[dreampiggy]

@ledyba-z Today I up-to-date with the CocoaPods's libavif to the latest release v0.5.3. Fix the potential issue related to libavif itself (The last time I updated was 0.4.4 I remember). Try pod update again on your local branch for SDWebImageAVIFCoder demo to debug.

[ledyba-z]

@dreampiggy Thank you for your effort!
Unfortunately, this issue is not solved in v.0.5.3 yet, but I made the pull request to use advanced decoding way.

Please take a look: SDWebImage/SDWebImageAVIFCoder#7