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

amir.rouhi at rmit.edu.au amir.rouhi at rmit.edu.au
Thu May 17 04:05:38 CEST 2012

I am confusing guys, i encountered a new problem in regard with adaptive
and fixed rate I-Frames by ffmpeg under windows. it is as below:
first i convert the original video (mpeg1) to the pure h.264 format by
below command for adaptive I-Frame rate:

ffmpeg -i C:\1.mpg -r 25 -an -vcodec libx264 -threads 0  -f h264 -r 25 -g 0
C:\1adapt.264  (-g 0 for adaptive I-frame rate)
ffmpeg -i C:\1.mpg -r 25 -an -vcodec libx264 -threads 0  -f h264 -r 25 -g
25 C:\1fixed.264  (-g 25 for fixed I-frame rate every 25 frame one i-frame)

I dont involve you about my first problem which was related to the twice
tbr value in the h264 transformed videos and their shorter time length
(with twice faster speed)

my current problem is that when i want to use select filter to extract the
i-frames from the above 264 videos, it is natural that i expect different
results and for a 50sec video with -g 25 and 25fps, i expected 50 i-frames
but the result was exactly same and just 5 frames for both videos. the
select filter which i used can be seen at below:

ffmpeg -i C:\1adapt.264 -vsync 0 -vf select="eq(pict_type\, PICT_TYPE_I)"
ffmpeg -i C:\1fixed.264 -vsync 0 -vf select="eq(pict_type\, PICT_TYPE_I)"

both produce 5 image for a originally 50 sec video with 25fps tbr 25 which
after transformed reduced to 25 sec with 25fps but tbr 50. Its strange ,
isnt it?

On 16 May 2012 18:48, Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:

> Alex Cohn <alexcohn at ...> writes:
> > If I understand correctly, Amir complains about the
> > encoder, not the decoder.
> Then your answer was clearly more helpful than mine!
> Carl Eugen
> _______________________________________________
> Libav-user mailing list
> Libav-user at ffmpeg.org
> http://ffmpeg.org/mailman/listinfo/libav-user

Amir H. Rouhi
PhD Student/ CSIT RMIT University
Room: 14-09-04
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