[FFmpeg-user] Extract all keyframes

Matthew Bondy bondy1z at uwindsor.ca
Fri Jun 27 17:37:11 CEST 2014

Background on my particular application: I have an AVI file with a frame
rate in the header of 1000 fps. It was not obtained with a highspeed camera
and the actual frame rate is 17 fps. The video was captured by an optical
system for measuring material deformation (quasi-static - low loading
rate). The system computes various quantities in real time in addition to
encoding the AVI. I would guess that the AVI is generated in such a form
for reasons associated with computational expense. Each keyframe is an
uncompressed image and I am able to confirm with simple calculations the
total number of keyframes knowing the bit rate, resolution, grayscale
resolution (is there better terminology for this property, i.e. the shade
of a pixel is determined by an integer between 0 and 2^8?), and the file

My question: When I do not specify the frame rate I assume ffmpeg uses some
default (20, 25?). The number of frames generated is in excess of the known
number of keyframes by the ratio of 20/17 (or is it 25?, I cannot recall).
When I use the option -r 17 (should I be more precise? '-r 17.1'?) I obtain
a suitable number of frames but on the command line ffmpeg indicates the
frame rate as it extracts the frames. This number fluctuates. Is the
effective frame rate for keyframes a variable or is this a source of error
in my analysis (I bring the images into software for material deformation

Any assistance would be greatly appreciated,
- Matt

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