[Libav-user] Setting libx264 bitrate via API
jettoblack at gmail.com
Mon Jul 23 23:12:39 CEST 2012
> 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:
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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