c->profile =FF_PROFILE_H264_MAIN ;
FFmpeg and x264 Encoding Guide 參數說明：http://ffmpeg.org/trac/ffmpeg/wiki/x264EncodingGuide
preset 與編碼速度和質量相關
av_opt_set(c->priv_data, "preset", "slow", 0); Current presets in descending order of speed are: ultrafast,superfast, veryfast, faster, fast, medium, slow, slower, veryslow,placebo.
priv_data 屬於每個編碼器特有的設置域，用av_opt_set 設置
2014/5/29 ：今天發現 av_opt_set(enc_ctx->priv_data, "preset", "slow", 0); slow的編碼質量比medium(默認值）好多了。
固定碼率150K,設置slow時：編碼速度：245 fps 4ms
[libx264 @ 014bbb40] frame I:38 Avg QP:33.69 size: 3763
[libx264 @ 014bbb40] frame P:262 Avg QP:35.79 size: 381
meidum 時：編碼速度：335 fps 2ms,
[libx264 @ 018dbb40] frame I:38 Avg QP:34.33 size: 3234
[libx264 @ 018dbb40] frame P:262 Avg QP:37.83 size: 455
veryslow 時：編碼速度：140 fps 7ms
[libx264 @ 0187cb40] frame I:38 Avg QP:35.10 size: 3582
[libx264 @ 0187cb40] frame P:262 Avg QP:37.24 size: 405
如何設置固定碼率編碼
CBR (Constant Bit Rate)
There is no native CBR mode, but you can "simulate" a constant bit rate setting by tuning the parameters of ABR, like
ffmpeg -i input -c:v libx264 -b:v 4000k -minrate 4000k -maxrate 4000k -bufsize 1835k out.m2v
in this example, -bufsize is the "rate control buffer" so it will enforce your requested "average" (4000k in this case) across each 1835k worth of video. So basically it is assumed that the receiver/end player will buffer that much data so it's ok to fluctuate within that much.
Of course, if it's all just empty/black frames then it will still serve less than that many bits/s (but it will raise the quality level as much as it can, up to the crf level).
只設置bit_rate是平均碼率，不一定能控制住
c->bit_rate = 400000;
c->rc_max_rate = 400000;
c->rc_min_rate = 400000;
提示 [libx264 @ 00c70be0] VBV maxrate specified, but no bufsize, ignored
再設置 c->rc_buffer_size = 200000; 即可。
如此控制后編碼質量明顯差了。。。