[Libav-user] Setting libx264 bitrate via API

[jettoblack]

rsg wrote
> 
> Hi,
> 
> I am also facing a similar issue. 
> When I set the bit rate using 
> 
> 	out_vcodec->bit_rate = 200000;
> 
> the quality of the video goes very bad. But when I set qmin and qmax, the
> bitrate setting is ignored.
> Only when i set a high value (approx 100MB) for the bit rate, the quality
> of the video improves.
> 
> 

Yes that is the same thing I'm seeing.  If qmin/qmax are left at -1
(default), then no matter what bitrate is specified, quality is very bad
(and the bitrate is not honored).

The upper half/third of the image looks good and the lower area looks really
bad.  The dividing line also changes every couple of frames. For example see
this screenshot: https://dl.dropbox.com/u/7730988/encode_prob.png

The only way I found to avoid this problem is to set qmin/qmax but then the
resulting video is not ABR/CBR.  Here is the same frame compressed with all
settings the same as before, except set qmin=1, qmax=31:
https://dl.dropbox.com/u/7730988/qminqmax.png

Obviously ffmpeg can do it right so I'm sure we are doing something wrong,
but I haven't yet found the difference in how ffmpeg is setting the libx264
options such as bitrate differently than what we are doing here.

Hope someone else has an idea...  Thanks.




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