Re: [FFmpeg-devel] [PATCH] avformat/movenc: Support alpha channel for AVIF

[James Zern <jzern-at-google.com@ffmpeg.org>]

On Wed, Jun 1, 2022 at 11:06 AM Vignesh Venkatasubramanian
<vigneshv-at-google.com@ffmpeg.org> wrote:
>
> AVIF specification allows for alpha channel as an auxiliary item (in
> case of still images) or as an auxiliary track (in case of animated
> images).  Add support for both of these. The AVIF muxer will take
> exactly two streams (when alpha is present) as input (first one being
> the YUV planes and the second one being the alpha plane).
>
> The input has to come from two different images (one of it color and
> the other one being alpha), or it can come from a single file
> source with the alpha channel extracted using the "alphaextract"
> filter.
>
> Example using alphaextract:
> ffmpeg -i rgba.png -filter_complex "[0:v]alphaextract[a]" -map 0 -map "[a]" -still-picture 1 avif_with_alpha.avif
>
> Example using two sources (first source can be in any pixel format and
> the second source has to be in monochrome grey pixel format):
> ffmpeg -i color.avif -i grey.avif -map 0 -map 1 -c copy avif_with_alpha.avif
>
> The generated files pass the compliance checks in Compliance Warden:
> https://github.com/gpac/ComplianceWarden
>
> libavif (the reference avif library) is able to decode the files
> generated using this patch.
>
> They also play back properly (with transparent background) in:
> 1) Chrome
> 2) Firefox (only still AVIF, no animation support)
>
> Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
> ---
>  libavformat/movenc.c | 188 +++++++++++++++++++++++++++++--------------
>  libavformat/movenc.h |   4 +-
>  2 files changed, 130 insertions(+), 62 deletions(-)
>

lgtm.
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