[Libav-user] Enquiry about I-Frame rate

[Alex Cohn]

On Wed, Feb 15, 2012 at 07:43,  <amir.rouhi at rmit.edu.au> wrote:
>
> The scenario is that, I have an .mpg video file. When I extract the I-Frames
> from this file by below command:
>
> ffmpeg -i D:\test\1.mpg -vsync 0 -vf select="eq(pict_type\, PICT_TYPE_I)"
> d:\Test\IFrames\I-Frm-%03d.jpeg
>
> I will get 104 images which is about an I-Frame for every 12 frames of the
> 1.mpg stream.
>
> Then I tried to change the video format to MP4 by below command:
>
> ffmpeg -i D:\test\1.mpg  -vcodec libx264  -g 0 d:\Test\11.mp4
>
> and
>
> ffmpeg -i D:\test\1.mpg  -vcodec libx264  -g 10 d:\Test\12.mp4
>
> the question is that the new generated videos, 11.mp4 and 12.mp4, in spite
> of using different –g values , the I-Frames of these two video is same and
> just is 5 images. I extract them by below command:
>
> ffmpeg -i D:\test\11.mp4 -vsync 0 -vf select="eq(pict_type\, PICT_TYPE_I)"
> d:\Test\IFrames\I-Frm-%03d.jpeg
>
> I want to know why there is a  big difference between the mpg and mp4 files
> and why in spite of different –g in last two video, I have exactly same
> I-Frames?

x264 is a different beast. To use it consciously, you should study the
options for this codec which sometimes have parallels in ffmpeg, and
sometimes don't. To begin with, I suggest a page that describes some
of these parameters: http://mewiki.project357.com/wiki/X264_Settings
and another page that shows the parallels between parameters for x264
and ffmpeg: http://sites.google.com/site/linuxencoding/x264-ffmpeg-mapping.

Specifically, you should use -keyint_min parameter with -g if you want
tighter control over GOP in x264.

BR,
Alex