[FFmpeg-user] Meaning of input frame rate

[Dzung Nguyen]

On Fri, Aug 23, 2013 at 1:09 PM, Lou <lou at lrcd.com> wrote:

> On Fri, Aug 23, 2013, at 08:15 AM, Dzung Nguyen wrote:
> > What's the meaning of input frame rate. For example, what does this
> > command
> > do?
> >
> > ffmpeg -loop 1 -r 1 -i image.png -q:v 0 -r 29.97 out.mpg
>
> This will use an input frame rate of 1 frame per second. The output will
> be 29.97 frames per second. ffmpeg will duplicate frames to reach your
> desired output frame rate. In the opposite case, if your input -r was a
> larger value than your output -r, then ffmpeg would drop frames to reach
> your output frame rate. You can see the number of dropped or duped
> frames at the end of your console output:
>
> frame=  150 fps=0.0 q=0.0 Lsize=    1604kB time=00:00:04.97
> bitrate=2643.0kbits/s dup=25 drop=0
>
> The result will show each input image for a second, but your output will
> still be 29.97.
>
> Since there is no -t, -vframes, or any other option to limit the
> duration or number of output frames in this command the encoding will
> continue indefinitely.
>
> "-q:v" values of 0-2 all appear to produce the same results with this
> encoder, mpeg1video, and my sample (I did not refer to any code):
>
> $ ffmpeg -loop 1 -i 001.png -q:v 0 -t 5 -c:v mpeg1video -f md5 -
> ...
> MD5=71fdaa434d28777735a85d4aa383d88c
>
> $ ffmpeg -loop 1 -i 001.png -q:v 2 -t 5 -c:v mpeg1video -f md5 -
> ...
> MD5=71fdaa434d28777735a85d4aa383d88c
>
> > Instead of
> >
> > ffmpeg -loop 1 -r 29.97 -i image.png -q:v 0 -r 29.97
>
> This will use an input frame rate of 29.97 frames per second. The output
> -r 29.97 is not needed since the output should inherit the input frame
> rate, so declaring the same -r for both input and output is superfluous
> in this case. No frames will be dropped or duplicated because there is
> no difference between your input and output frame rates.
>
> Use "-r ntsc" or "-r 30000/1001" for NTSC video frame rate.
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Thanks! It helps me a lot. One question, then why second command ffmpeg
-loop 1 -r 29.97 -i image.png output.mpg runs much slower than the first
one?