[FFmpeg-user] ffmpeg architecture question

[Nicolas George]

Michael Koch (12020-04-21):
> If the blend filter gets two input frames with different timestamps, then
> what's the timestamp of the output frame?

To understand how it works, you need to think of frames not as punctual
instant in time, it is an interval going from the frame's PTS to the
next frame's PTS.

When a filter combines two or more streams of frames, you have to
imagine several graduations on the same time axis: a new frame could be
generated for any graduation. If two graduations fall at the same time,
that means two parts of the output image change simultaneously;
otherwise, a part of the image stays the same and a part changes.

All this is grouped together in the framesync system:
http://ffmpeg.org/ffmpeg-filters.html#Options-for-filters-with-several-inputs-_0028framesync_0029

It lest control which streams triggers a new output frame. For example,
if you want to overlay a clock on a 24000/1001 fps video, you could have
a frame at t =~ 0.918, 0.959, 1.001, but we do not want a frame at t = 1
for the change of the clock.

Unfortunately, this is not yet exposed to users as options.

Regards,

-- 
  Nicolas George
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