同码率下的图像质量或同图像质量下的码率。
AVCodecContext
/**
* the average bitrate
* - encoding: Set by user; unused for constant quantizer encoding.
* - decoding: Set by libavcodec. 0 or some bitrate if this info is available in the stream.
*/
// int bit_rate;
/**
* CODEC_FLAG_*.
* - encoding: Set by user.
* - decoding: Set by user.
*/
int flags;
/**
* CODEC_FLAG2_*
* - encoding: Set by user.
* - decoding: Set by user.
*/
int flags2;
}
static int
X264_init(AVCodecContext *avctx)函数中，libx264.c中
x4->params.i_keyint_max = avctx->gop_size;
x4->params.rc.i_bitrate = avctx->bit_rate / 1000;
x4->params.rc.i_vbv_buffer_size = avctx->rc_buffer_size / 1000;
x4->params.rc.i_vbv_max_bitrate = avctx->rc_max_rate / 1000;
x4->params.rc.b_stat_write = (avctx->flags & CODEC_FLAG_PASS1);
//#define CODEC_FLAG_PASS1 0x0200 ///< Use internal 2pass ratecontrol in first pass mode.
//#define CODEC_FLAG_PASS2 0x0400 ///< Use internal 2pass ratecontrol in second pass mode.
if(avctx->flags & CODEC_FLAG_PASS2) x4->params.rc.b_stat_read = 1;//h->param.rc.i_rc_method == X264_RC_ABR && h->param.rc.b_stat_read;
else{
if(avctx->crf){//crf的默认值为24或是26；
x4->params.rc.i_rc_method = X264_RC_CRF;//参数i_rc_method表示码率控制，CQP(恒定质量)，CRF(恒定码率)，ABR(平均码率)
x4->params.rc.f_rf_constant = avctx->crf;
}else if(avctx->cqp > -1){
x4->params.rc.i_rc_method = X264_RC_CQP;//CQP生效，必须要设置crf=0 //固定量化模式并设置使用的量化值，范围0~51，0为无损压缩，默认26
x4->params.rc.i_qp_constant = avctx->cqp;
}
}
// if neither crf nor cqp modes are selected we have to enable the RC
// we do it this way because we cannot check if the bitrate has been set
if(!(avctx->crf || (avctx->cqp > -1))) //只有设置CRF=0和cqb为-1时，ABR才有效
x4->params.rc.i_rc_method = X264_RC_ABR;//#define X264_RC_ABR 平均码率 2
x4->params.i_bframe = avctx->max_b_frames;
