[FFmpeg-devel] [PATCH] fix for transparencies in animated gifs (requires feedback)

[Michael Niedermayer]

On Tue, Oct 24, 2017 at 12:40:22PM -0400, Bjorn Roche wrote:
> Support for transparencies in animated gifs requires modifying both
> libavcodec/gif.c and libavformat/gif.c because both the graphics
> control extension (handled by libavformat/gif.c) and the raw frame data
> (handled by libavcodec/gif.c) must be changed. This is because
> transparencies in GIF can be used both to create a transparent image,
> and to provide optimization.
> 
> How transparencies are interpreted in a given frame is controlled by
> the “disposal method”, which must be set appropriately in the graphics
> control extension.
> 
>  The “in place” disposal method is used when transparency indicates
> optimization, and the “background” disposal method is used when
> transparency is intended to be preserved. In order to support both
> disposal methods, libavcodec/gif.c must signal to libavformat/gif.c
> which disposal method is required for every frame. This is done with a
> new side data type: AV_PKT_DATA_GIF_FRAME_DISPOSAL. This requires a
> change to avcodec.h
> 
> Unfortunately, the addition of a new side data type causes some of the
> FATE tests to fail. This is not addressed here.
> 
> This patch assumes paletteuse has already been patched to support
> transparency. (e.g. lavfi/paletteuse: fix to support transparency)
> 
> Feedback I definitely need:
> - I’ve done a fair bit of testing, but I’m sure I missed some important
> cases.
> - I don’t know if/how to update the FATE tests.
> ---
>  libavcodec/avcodec.h |   6 ++
>  libavcodec/gif.c     | 196 +++++++++++++++++++++++++++++++++++++++++++++++++--
>  libavformat/gif.c    |  16 ++++-
>  3 files changed, 212 insertions(+), 6 deletions(-)
> 
> diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
> index 52cc5b0ca0..82a5328ce1 100644
> --- a/libavcodec/avcodec.h
> +++ b/libavcodec/avcodec.h
> @@ -1599,6 +1599,12 @@ enum AVPacketSideDataType {
>       */
>      AV_PKT_DATA_CONTENT_LIGHT_LEVEL,
>  
> +    /**
> +     * The disposal method that should be used with the frame. If missing,
> +     * the frame will not be disposed. This contains exactly one byte.
> +     */
> +    AV_PKT_DATA_GIF_FRAME_DISPOSAL,
> +
>      /**
>       * ATSC A53 Part 4 Closed Captions. This metadata should be associated with
>       * a video stream. A53 CC bitstream is stored as uint8_t in AVPacketSideData.data.

you cannot add values in the middle of a public enum, that would
change the ABI

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Those who are too smart to engage in politics are punished by being
governed by those who are dumber. -- Plato 
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