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

[Bjorn Roche]

On Tue, Oct 24, 2017 at 8:56 PM, Michael Niedermayer <michael at niedermayer.cc
> wrote:

> 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
>

Ah, thanks -- I thought that was internal only. Is it safe to add to the
end?

bjorn


-- 


Bjorn Roche

Sr. Video Pipeline Engineer

bjorn at giphy.com