[FFmpeg-devel] [PATCH] Animated GIF Support

[Don Moir]

> On 10/12/12, Vitaliy Sugrobov <vsugrob at hotmail.com> wrote:
>>
>>> How will this work with seeking?
>>>
>>> This means that first frame is keyframe and any other frame with
>>> transparency is not.> I think that right place to handle this is not in
>>> decoder but from user.
>>>
>>> AFAIK mplayer should handle it and ffplay not.
>>
>> According to gif spec any
>> subsequent frame depends on all previous frames. And this is not only
>> because of transparency, the other reason is variable frame dimensions. In
>> more detail: gif header provides us initial image dimensions { width,
>> height }. Then, any subsequent frame specifies its { x, y, width, height }
>> characteristics within bounds of the canvas, which means than only portion
>> of the canvas is updated. Thus first frame might be considered as a
>> keyframe and any other frame with or without transparency is not a
>> keyframe. So there is no way to seek frame F without calculating preceding
>> frames A, B, C, D and E.
>
> Then mark packets/frames as such in demuxer/decoder.
>

You could mark keyframes based on the disposal method and size but requires reading thru the file to find out what the size and 
disposal method is for various frames.