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.
I think there is some bug in ffmpeg or I might be missing something.
Can anyone tell whats the problem?
Thanks
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of jettoblack
Sent: Saturday, July 21, 2012 3:56 AM
To: [email protected]
Subject: [Libav-user] Setting libx264 bitrate via API
Hello,
I'm trying to do the equivalent of this via the API:
ffmpeg -i test.m2t -c:v libx264 -b:v 500000 -s 640x360 -ac 2 test.mov
This gives a ABR file at 500kbits/sec.
I thought setting the video bitrate should be as simple as:
out_vcodec->bit_rate = 500000;
When using CODEC_ID_MPEG2VIDEO or CODEC_ID_MPEG4, this works fine, and the
resulting file has the specified ABR. However with libx264 (CODEC_ID_H264)
the bitrate parameter seems to have no effect, even very small values like
50,000 or very large like 50,000,000 (for a 640x360 video) give the same
output of around 200kbits/sec. The input video is not a static picture and
it's not like 200kbit/sec is giving a lossless result and more bits are
unnecessary.
I also tried:
out_vcodec->bit_rate = 500000;
out_vcodec->rc_min_rate = 500000;
out_vcodec->rc_max_rate = 500000;
out_vcodec->rc_buffer_size = 500000;
But still, there is no change in the output bitrate no matter what values I
put. So I suspect these values are getting ignored or I'm not passing them
into the codec correctly.
The only parameter that actually affects the libx264 output bitrate is
setting out_vcodec->qmin and out_vcodec->qmax. By playing with qmin and
qmax I can at least affect the bitrate, but it doesn't give a predictable
ABR or CBR result.
Any ideas? Thanks!
I initialize the codec like this:
AVCodec *vcodec = avcodec_find_encoder(CODEC_ID_H264); // or
CODEC_ID_MPEG2VIDEO, etc.
AVCodecContext *out_vcodec = avcodec_alloc_context3(vcodec);
avcodec_get_context_defaults3(out_vcodec, vcodec);
if (vcodec->id == CODEC_ID_H264) {
av_opt_set(out_vcodec->priv_data, "profile", "main", 0);
}
out_vcodec->bit_rate = 500000;
out_vcodec->width = 640;
out_vcodec->height = 360;
out_vcodec->pix_fmt = PIX_FMT_YUV420P;
r = avcodec_open2(out_vcodec, vcodec, NULL);
Thanks!
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