[MEncoder-users] What's the special about the I-frames in libavcodec?

[Giacomo Comes]

On Tue, Oct 31, 2006 at 07:26:15PM +0100, Pepe wrote:
> I have a dvd recorder Samsumg HR-725, with support for playing divx videos. 
> 
> It plays well the videos I created with mencoder using xvid. It also plays ok 
> videos I downloaded from internet with DX50 fourcc (I think they are divx).
> 
> But if I use libavcodec then there's a strange effect: it seems the I-frames 
> are not displayed, so the image is merging with the previous scene.
> 
> I also use the program avidemux to edit videos. This program has a mode call 
> smart copy, it copies the original frames until it gets to a place I edited 
> then it regenerates there the frames until the next I-frame. If I cut some 
> frames right at the beginning of the video, obviously the first frame it has 
> to create is a I-frame. If I edit a xvid video, when I play this edited video 
> in the dvd recorder, the first frames (the ones created by avidemux) have the 
> same problem I mentioned above. When the second I-frame is reached (an 
> original one from xvid) the problem fixes and rest of the video looks ok.
> 
> I think avidemux uses ffmpeg for re-encoding the edited frames, and I think 
> ffmpeg is something related to libavcodec, isn't it?
> 
> So I suppose there should be something special, something different in the 
> I-frames that libavcodec creates. Is there any way to create I-frames more 
> compatible with xvid or divx?
> 
> Here is an example of a command line I use for libavcodec:
> 
> mencoder -noodml -o test_av.avi \
>   -oac copy \
>   -ffourcc DX50 \
>   -ovc lavc \
>   -lavcopts 
> vcodec=mpeg4:vbitrate=1033:mbd=2:vstrict=1:vpass=1:turbo:vqmax=4:v4mv:trell \
>   -vf-add scale=608:464 -zoom -sws 9 \
>   -vf-add harddup \
>   -force-avi-aspect 4:3 \
>   video2.avi
> 
> I tried several things. Setting fourcc to DIVX and XVID, use b-frames, remove 
> the vstrict option (which I need for another dvd player), or using mpeg 
> quantizers. No change at all.
> 
> Any ideas?

Try to use keyint with a lower value. May be the default (250) is too high for
your player. The manual suggest <=132 for a strict MPEG-1/2/4 compliance.

Giacomo