[Libav-user] Extracted I-Frames seems not correct, Why?

[Alex Cohn]

On Fri, May 11, 2012 at 7:09 AM, <amir.rouhi at rmit.edu.au> wrote:

> Hi to ffmpeg seniors and experts
> I am using windows platform and I have created a series of video in pure
> format of *.264 from raw format by the below command:
>
> ffmpeg -f rawvideo -pix_fmt yuv420p -s 360x240 -i C:\1.yuv -an -vcodec
> libx264 -threads 0  -s 360x240 -f h264 C:\1.264
>
> I want to extract I-Frames which are created in adaptive mode in encoding
> phase. i use the below command:
>
> ffmpeg -i  C:\ 1.264 -vsync 0 -vf select="eq(pict_type\, PICT_TYPE_I)"
> C:\ Test\IFrames\I-Frm-%03d.jpeg
> Its a short video and i just get 4 images but the images does not looks
> correct images, They are not sharp and clear images. BTW, adaptive I-Frame
> are expected t be located at the start of a new group of pictures, after
> any big changes in the scene, but these extracted image are not showing
> this. do you know whats my mistake or whats wrong?
> --
> Amir
>

Hi,

Adaptive GOP is not the default for x264 encoder. See
http://mewiki.project357.com/wiki/X264_Settings for all options. The
relevant command line arguments are also discussed at
http://www.digital-digest.com/articles/x264_options_page5.html.

This said, you should also learn the way to specify x264 parameters via
ffmpeg command line. A good guide can be found here:
http://sites.google.com/site/linuxencoding/x264-ffmpeg-mapping

Regarding sharpness of the I-frames, it really does happen that key frames
display lower quality than the rest of the video. The reason is that when
the encoder tries to maintain certain bitrate, it prefers to apply more
lossy compression to the I-frames, because otherwise they get very big.
Again, this bias can be controlled with x264 parameters.

BR,
Alex Cohn
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