[FFmpeg-user] Extract all keyframes

Matthew Bondy bondy1z at uwindsor.ca
Sat Jun 28 00:54:49 CEST 2014

Thanks Simon. Any suggested resource to learn more on this topic? Searching
for encoding fps on google is not so helpful unfortunately with the
ubiquity of those two terms.

I have also noticed something about duplicate frames is displayed on the
command line while the extraction of frames is in progress. The number of
frames obtained is not consistent with the number of frames I expect from
simple calculations. I tried increasing the precision of the specified
frame rate (i.e. -r 17.1 instead of -r 17) but the result was worse in
terms of the number of frames extracted/expected.

When I bring the images into the other software I mentioned I observe a
small shift in the data. Unfortunately, I am interested in some of the last
frames so a progressive insertion of extra (or deletion) of frames stacks
up against me.

One alternative I will try is MATLAB and the frame2im command. I will be
interested to see how many frames MATLAB is able to extract (I cannot
recall the command to open an AVI file, it might be 'aviread') and what
they contain.

Best regards

On Fri, Jun 27, 2014 at 6:22 PM, Simon Thelen <ffmpeg-user at c-14.de> wrote:

> On 27/06/14 at 11:37, Matthew Bondy wrote:
> > 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
> > analysis).
> That's just the encoding fps. ie how many frames are being encoded every
> second and has absolutely nothing to do with the fps of either the input
> or the output file.
> --
> Simon Thelen
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