[FFmpeg-user] Image extraction broken with ffmpeg, but not with mencoder. How can I fix this?
nicolas.george at normalesup.org
Tue Sep 3 19:51:50 CEST 2013
Le septidi 17 fructidor, an CCXXI, James Board a écrit :
> 1: I'm using ffmpeg to separate a video file into a sequence of frames.
You should probably not be doing that in the first place, since this is
> 2: Ultimately I must use frame numbers because I need to know which
> frames (which are stored as distinct files) to include and which not to
That is completely untrue. A frame is identified by its timestamp just as
well, and in fact better, than any kind of "frame number".
> 3: Timestamps are floating-point numbers, and floating-point numbers are
> not exact.
That is untrue. Timestamps in audio-video formats are expressed as an exact
integer multiple of a constant (rational time base). Floating-point numbers
are used only for user interaction, and they are accurate enough to uniquely
identify a frame in a video with room to spare.
> Frame numbers are integers and integers are exact. I asked previously for a
> formula to map timestamps to frame numbers and didn't get an answer. Here's
You did not get an answer because it does not exist.
> the issue: If I try to use a time stamp that falls midway between two
> frames, which frame will ffmpeg select?
Depends on the place you use the timestamps. You seem to intend to use the
select filter to extract part of the video. Surely you know the meaning of
the "<" and ">" operators?
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