轉載自:http://blog.csdn.net/maopig/article/details/6718157
碼率控制的理論知識:
碼率控制的目的和意義:
圖像通信中碼率控制的目的:通過調節編碼參數,控制機關時間内的編碼視訊流的資料量,以使産生的比特流符合各種應用的需求。視訊壓縮的效率和視訊内容有很大的關系,對于變化多樣的畫面,視訊編碼的輸出的碼流變化較大,在信道環境不好的時候就容易導緻解碼端顯示的品質的不穩定。
率失真理論:
由于傳輸帶寬和存儲空間的限制,視訊應用對壓縮比有較高的要求。而無損編碼較低的壓縮比無法滿足視訊在實際應用中的需求。但如果給視訊引入一定程度的失真,通常可以獲得較高的壓縮比。
率失真理論對有損壓縮編碼下的失真和編碼性能之間的關系的描述,為碼率控制的研究提供了堅實的理論依據。率失真理論主旨是描述編碼失真度和編碼資料速率的關系。該理論建立在圖像是連續的基礎上的,在有限資料速率下,由于存在量化誤差,必然存在失真。當使用有損編碼方法時,重建圖像g(x,y)和原始圖像f(x,y)之間存在差異,失真度D的函數形式在理論上是可以根據需要自由選取的,在圖像編碼中,D常用均方差形式表示的,典型的率失真曲線。R(D)為D的凸減函數。
對于怎麼選擇哪個函數的率失真效果更好,則是比較哪個函數的率失真函數更為接近典型的率失真函數的曲線。
x264碼率控制方法:采用的碼率控制算法并沒有采用拉格朗日代價函數來控制編碼,而是使用一種更簡單的方法,即利用半精度幀的SATD(sum of absolute transformed difference)作為模式選擇的依據。SATD即将殘差經哈德曼變換的4×4塊的預測殘差絕對值總和,可以将其看作簡單的時頻變換,其值在一定程度上可以反映生成碼流的大小。SATD是将殘差經哈達曼變換4*4塊的預測殘差絕對值總和。自适應宏塊層碼率控制政策:X264的宏塊沒有任何碼率控制的機制,其在幀層得到一個QP後,屬于該幀的所有宏塊都用着統一的QP進行量化。
碼率控制性能測度:
1、比特率誤差|ABR-TBR|/TBR ,越小越好。
2、編碼器性能。
3、緩沖區滿度與TBL的比對程度。
4、跳幀數。
5、PSNR波動越小越好。
x264中碼率控制的流程(對于重點函數在下面有注釋):
1.在進行編碼時,Encode--->x264_encoder_open(主要是進行參數的修訂設定,進行初始化)---->x264_ratecontrol_new
2.encode--->Encode_frame--->x264_encoder_encode--->x264_ratecontrol_slice_type
3.encode--->Encode_frame--->x264_encoder_encode--->x264_ratecontrol_start**************
4.encode--->Encode_frame--->x264_encoder_encode--->x264_ratecontrol_qp
5.encode--->Encode_frame--->x264_encoder_encode--->x264_slices_write--->x264_slice_write
--->x264_ratecontrol_mb********************
6.encode--->Encode_frame--->x264_encoder_encode--->x264_ratecontrol_end(在編完一幀過後)
7.在編完過後,encode--->x264_encoder_close---->ratecontrol summary/x264_ratecontrol_delete
函數注釋:
在編碼中所用的編碼方式:
#define X264_RC_CQP 0
#define X264_RC_CRF 1
#define X264_RC_ABR 2
1.
x264_ratecontrol_new( x264_t *h )
{ // 擷取RC方式,FPS,bitrate,rc->buffer_rate,rc->buffer_size
// 在碼率控制的時候會出現2pass,參數的初始化
rc = h->rc;
rc->b_abr = h->param.rc.i_rc_method != X264_RC_CQP && !h->param.rc.b_stat_read;
rc->b_2pass = h->param.rc.i_rc_method == X264_RC_ABR && h->param.rc.b_stat_read;
..........
if( h->param.rc.b_mb_tree )//這裡設定mb_tree
{
h->param.rc.f_pb_factor = 1;
rc->qcompress = 1;
}
else
rc->qcompress = h->param.rc.f_qcompress;
..............
rc->ip_offset = 6.0 * log(h->param.rc.f_ip_factor) / log(2.0);
rc->pb_offset = 6.0 * log(h->param.rc.f_pb_factor) / log(2.0);
rc->qp_constant[SLICE_TYPE_P] = h->param.rc.i_qp_constant;
rc->qp_constant[SLICE_TYPE_I] = x264_clip3( h->param.rc.i_qp_constant - rc->ip_offset + 0.5, 0, 51 );
rc->qp_constant[SLICE_TYPE_B] = x264_clip3( h->param.rc.i_qp_constant + rc->pb_offset + 0.5, 0, 51 );
}
2.
int x264_ratecontrol_slice_type( x264_t *h, int frame_num )
{
//根據不同類型來擷取不同的qp_constant
h->param.rc.i_qp_constant = (h->stat.i_frame_count[SLICE_TYPE_P] == 0) ? 24
: 1 + h->stat.f_frame_qp[SLICE_TYPE_P] / h->stat.i_frame_count[SLICE_TYPE_P];
rc->qp_constant[SLICE_TYPE_P] = x264_clip3( h->param.rc.i_qp_constant, 0, 51 );
rc->qp_constant[SLICE_TYPE_I] = x264_clip3( (int)( qscale2qp( qp2qscale( h->param.rc.i_qp_constant ) / fabs( h->param.rc.f_ip_factor )) + 0.5 ), 0, 51 );
rc->qp_constant[SLICE_TYPE_B] = x264_clip3( (int)( qscale2qp( qp2qscale( h->param.rc.i_qp_constant ) * fabs( h->param.rc.f_pb_factor )) + 0.5 ), 0, 51 );
}
3.
x264_ratecontrol_start( h, h->fenc->i_qpplus1, overhead*8 );
這個函數的目的就是在一幀的編碼前就選擇QP
x264_ratecontrol_start( h, h->fenc->i_qpplus1, overhead*8 );
對x264_ratecontrol_start函數的解析如下:
x264_zone_t *zone = get_zone( h, h->fenc->i_frame );//找到h->fenc->i_frame所在的zone
....................
//由各種不同的slice類型,vbv等等參數擷取的q值
if( i_force_qp )
{
q = i_force_qp - 1;//
}
else if( rc->b_abr )
{
q = qscale2qp( rate_estimate_qscale( h ) );//下面有注解
}
else if( rc->b_2pass )
{
rce->new_qscale = rate_estimate_qscale( h );
q = qscale2qp( rce->new_qscale );
}
else
{
if( h->sh.i_type == SLICE_TYPE_B && h->fdec->b_kept_as_ref )
q = ( rc->qp_constant[ SLICE_TYPE_B ] + rc->qp_constant[ SLICE_TYPE_P ] ) / 2;
else
q = rc->qp_constant[ h->sh.i_type ];
if( zone )
{
if( zone->b_force_qp )
q += zone->i_qp - rc->qp_constant[SLICE_TYPE_P];
else
q -= 6*log(zone->f_bitrate_factor)/log(2);
}
//
static inline double qp2qscale(double qp)
{
return 0.85 * pow(2.0, ( qp - 12.0 ) / 6.0);
}
static inline double qscale2qp(double qscale)
{
return 12.0 + 6.0 * log(qscale/0.85) / log(2.0);
}
rate_estimate_qscale( h )
// update qscale for 1 frame based on actual bits used so far(即根據所需BIT來計算qscale)
static float rate_estimate_qscale( x264_t *h )
{
//這裡是分别針對B,P幀分别進行,因為I幀是已經設定
if( pict_type == SLICE_TYPE_B )
{
//這裡B幀的q的大小是由參考幀求的
.....................
.....................
// 由predict_size獲得幀的size
rcc->frame_size_planned = predict_size( rcc->pred_b_from_p, q, h->fref1[h->i_ref1-1]->i_satd );
x264_ratecontrol_set_estimated_size(h, rcc->frame_size_planned);
//
void x264_ratecontrol_set_estimated_size( x264_t *h, int bits )
{
x264_pthread_mutex_lock( &h->fenc->mutex );
h->rc->frame_size_estimated = bits;//
int x264_ratecontrol_end( x264_t *h, int bits )
{
///統計ipb類型的Mb的個數,并計算平均QP
h->fdec->f_qp_avg_rc = rc->qpa_rc /= h->mb.i_mb_count;
h->fdec->f_qp_avg_aq = rc->qpa_aq /= h->mb.i_mb_count;
}
7.
void x264_ratecontrol_summary( x264_t *h )
{
x264_ratecontrol_t *rc = h->rc;
//ABR
if( rc->b_abr && h->param.rc.i_rc_method == X264_RC_ABR && rc->cbr_decay > .9999 )
{
double base_cplx = h->mb.i_mb_count * (h->param.i_bframe ? 120 : 80);
double mbtree_offset = h->param.rc.b_mb_tree ? (1.0-h->param.rc.f_qcompress)*13.5 : 0;
x264_log( h, X264_LOG_INFO, "final ratefactor: %.2f\n",
qscale2qp( pow( base_cplx, 1 - rc->qcompress )
* rc->cplxr_sum / rc->wanted_bits_window ) - mbtree_offset );
}
}
/
void x264_ratecontrol_delete( x264_t *h )///釋放RC開辟的空間
通過以上的流程總結x264碼率控制的過程基本是有以下三步:
1.對碼率控制的相關變量進行初始化,如,I,P,B的初始QP值,RC的方式,VBV的初始狀态等等;
2.擷取編碼幀的複雜度,x264用SATD表示,對于采用的不同參數的碼率控制的方式,由前面已編碼的Bits,複雜度,目标比特的設定等一些條件來擷取編碼目前幀的qp值。
3.在編碼過程中,由獲得qp值得到預測的bits;
實驗部分:
1.簡單參數設定:
參數設定:
--frames 10 --qp 26 -o test.264 F:\.......\akiyo_qcif.yuv 176x144
其他的參數采用預設設定(在預設設定時采用的碼率控制模型是X264_RC_CQP),所得的實驗結果:
x264 [info]: 176x144 @ 25.00 fps
x264 [info]: using cpu capabilities: MMX2 SSE2 Cache64 Slow_mod4_stack
x264 [info]: profile High, level 1.1
x264 [info]: frame I:1 Avg QP:23.00 size: 4189
x264 [info]: frame P:3 Avg QP:26.00 size: 62
x264 [info]: frame B:6 Avg QP:28.00 size: 38
x264 [info]: consecutive B-frames: 11.1% 0.0% 0.0% 88.9%
x264 [info]: mb I I16..4: 3.0% 41.4% 55.6%
x264 [info]: mb P I16..4: 0.0% 0.0% 0.0% P16..4: 3.0% 1.3% 1.7% 0.0% 0
.0% skip:93.9%
x264 [info]: mb B I16..4: 0.0% 0.0% 0.0% B16..8: 4.4% 0.2% 0.3% direct:
0.7% skip:94.4% L0:56.0% L1:40.5% BI: 3.4%
x264 [info]: 8x8 transform intra:41.4% inter:25.9%
x264 [info]: coded y,uvDC,uvAC intra: 83.6% 81.8% 68.7% inter: 1.1% 0.1% 0.0%
x264 [info]: i16 v,h,dc,p: 100% 0% 0% 0%
x264 [info]: i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 25% 30% 15% 4% 4% 4% 7% 5% 6%
x264 [info]: i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 35% 18% 8% 5% 5% 10% 5% 8% 6%
x264 [info]: ref P L0: 88.9% 0.0% 11.1%
x264 [info]: kb/s:92.08
encoded 10 frames, 24.33 fps, 92.08 kb/s
2.改變碼率控制的模型:
--frames 10 --qp 26 --crf 2 -o test.264 F:\......\akiyo_qcif.yuv 176x144
x264 [info]: 176x144 @ 25.00 fps
x264 [info]: using cpu capabilities: MMX2 SSE2 Cache64 Slow_mod4_stack
x264 [info]: profile High, level 1.1
x264 [info]: frame I:1 Avg QP:10.00 size: 10246
x264 [info]: frame P:3 Avg QP:11.48 size: 847
x264 [info]: frame B:6 Avg QP:12.10 size: 172
x264 [info]: consecutive B-frames: 11.1% 0.0% 0.0% 88.9%
x264 [info]: mb I I16..4: 1.0% 44.4% 54.5%
x264 [info]: mb P I16..4: 0.0% 0.0% 0.0% P16..4: 30.0% 3.0% 4.7% 0.0% 0
.0% skip:62.3%
x264 [info]: mb B I16..4: 0.0% 0.0% 0.0% B16..8: 6.9% 1.0% 1.2% direct:
4.0% skip:86.9% L0:34.7% L1:55.6% BI: 9.7%
x264 [info]: 8x8 transform intra:44.4% inter:34.8%
x264 [info]: coded y,uvDC,uvAC intra: 100.0% 99.0% 94.9% inter: 11.6% 7.6% 4.9%
x264 [info]: i16 v,h,dc,p: 100% 0% 0% 0%
x264 [info]: i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 30% 33% 16% 3% 2% 3% 4% 4% 5%
x264 [info]: i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 28% 12% 11% 5% 7% 11% 6% 12% 8%
x264 [info]: ref P L0: 95.8% 1.6% 2.7%
x264 [info]: ref B L0: 96.3% 3.7%
x264 [info]: kb/s:276.36
encoded 10 frames, 14.27 fps, 276.36 kb/s
針對1,2兩個實驗,所采用的RC模型不一樣,1:X264_RC_CQP,2:X264_RC_CRF,其他參數的設定一樣,從IPB的平均QP,編碼Bits可以看出和對于實際的應用來說,CRF的效果不如CQP。
3.
--frames 10 --qp 26 --pass 1 -o test.264 F:\.....\bin\akiyo_qcif.yuv 176x144
x264 [info]: 176x144 @ 25.00 fps
x264 [info]: using cpu capabilities: MMX2 SSE2 Cache64 Slow_mod4_stack
x264 [info]: profile Main, level 1.1
x264 [info]: frame I:1 Avg QP:23.00 size: 4068
x264 [info]: frame P:3 Avg QP:26.00 size: 59
x264 [info]: frame B:6 Avg QP:28.00 size: 31
x264 [info]: consecutive B-frames: 11.1% 0.0% 0.0% 88.9%
x264 [info]: mb I I16..4: 15.2% 0.0% 84.8%
x264 [info]: mb P I16..4: 0.0% 0.0% 0.0% P16..4: 7.1% 0.0% 0.0% 0.0% 0
.0% skip:92.9%
x264 [info]: mb B I16..4: 0.0% 0.0% 0.0% B16..8: 1.2% 0.0% 0.0% direct:
1.5% skip:97.3% L0:100.0% L1: 0.0% BI: 0.0%
x264 [info]: coded y,uvDC,uvAC intra: 87.4% 77.8% 68.7% inter: 1.1% 0.1% 0.0%
x264 [info]: i16 v,h,dc,p: 47% 20% 27% 7%
x264 [info]: i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 29% 27% 10% 5% 4% 8% 5% 6% 5%
x264 [info]: kb/s:88.58
encoded 10 frames, 52.63 fps, 88.58 kb/s
4.
--frames 10 --qp 26 --pass 2 -o test.264 F:\.....\bin\akiyo_qcif.yuv 176x144
x264 [info]: 176x144 @ 25.00 fps
x264 [info]: using cpu capabilities: MMX2 SSE2 Cache64 Slow_mod4_stack
x264 [info]: profile High, level 1.1
x264 [info]: frame I:1 Avg QP:23.00 size: 4189
x264 [info]: frame P:3 Avg QP:26.00 size: 62
x264 [info]: frame B:6 Avg QP:28.00 size: 38
x264 [info]: consecutive B-frames: 11.1% 0.0% 0.0% 88.9%
x264 [info]: mb I I16..4: 3.0% 41.4% 55.6%
x264 [info]: mb P I16..4: 0.0% 0.0% 0.0% P16..4: 3.0% 1.3% 1.7% 0.0% 0
.0% skip:93.9%
x264 [info]: mb B I16..4: 0.0% 0.0% 0.0% B16..8: 4.4% 0.2% 0.3% direct:
0.7% skip:94.4% L0:56.0% L1:40.5% BI: 3.4%
x264 [info]: 8x8 transform intra:41.4% inter:25.9%
x264 [info]: coded y,uvDC,uvAC intra: 83.6% 81.8% 68.7% inter: 1.1% 0.1% 0.0%
x264 [info]: i16 v,h,dc,p: 100% 0% 0% 0%
x264 [info]: i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 25% 30% 15% 4% 4% 4% 7% 5% 6%
x264 [info]: i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 35% 18% 8% 5% 5% 10% 5% 8% 6%
x264 [info]: ref P L0: 88.9% 0.0% 11.1%
x264 [info]: kb/s:92.08
encoded 10 frames, 27.70 fps, 92.08 kb/s
5.
--frames 10 --qp 26 --pass 3 -o test.264 F:\.....\bin\akiyo_qcif.yuv 176x144
x264 [info]: 176x144 @ 25.00 fps
x264 [info]: using cpu capabilities: MMX2 SSE2 Cache64 Slow_mod4_stack
x264 [info]: profile High, level 1.1
x264 [info]: frame I:1 Avg QP:23.00 size: 4189
x264 [info]: frame P:3 Avg QP:26.00 size: 62
x264 [info]: frame B:6 Avg QP:28.00 size: 38
x264 [info]: consecutive B-frames: 11.1% 0.0% 0.0% 88.9%
x264 [info]: mb I I16..4: 3.0% 41.4% 55.6%
x264 [info]: mb P I16..4: 0.0% 0.0% 0.0% P16..4: 3.0% 1.3% 1.7% 0.0% 0
.0% skip:93.9%
x264 [info]: mb B I16..4: 0.0% 0.0% 0.0% B16..8: 4.4% 0.2% 0.3% direct:
0.7% skip:94.4% L0:56.0% L1:40.5% BI: 3.4%
x264 [info]: 8x8 transform intra:41.4% inter:25.9%
x264 [info]: coded y,uvDC,uvAC intra: 83.6% 81.8% 68.7% inter: 1.1% 0.1% 0.0%
x264 [info]: i16 v,h,dc,p: 100% 0% 0% 0%
x264 [info]: i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 25% 30% 15% 4% 4% 4% 7% 5% 6%
x264 [info]: i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 35% 18% 8% 5% 5% 10% 5% 8% 6%
x264 [info]: ref P L0: 88.9% 0.0% 11.1%
x264 [info]: kb/s:92.08
encoded 10 frames, 25.64 fps, 92.08 kb/s
對于3,4,5是關于Pass的實驗比較:
多次壓縮碼率控制
1:第一次壓縮,建立統計檔案
2:按建立的統計檔案壓縮并輸出,不覆寫統計檔案,
3:按建立的統計檔案壓縮,優化統計檔案
在想得到建好的效果的時候采用pass 2就可以了。
6.
--frames 10 --qp 26 --bitrate 64 -o test.264 F:\.....\bin\akiyo_qcif.yuv 176x144
x264 [info]: 176x144 @ 25.00 fps
x264 [info]: using cpu capabilities: MMX2 SSE2 Cache64 Slow_mod4_stack
x264 [info]: profile High, level 1.1
x264 [info]: frame I:1 Avg QP:38.31 size: 1461
x264 [info]: frame P:3 Avg QP:42.00 size: 18
x264 [info]: frame B:6 Avg QP:45.00 size: 14
x264 [info]: consecutive B-frames: 11.1% 0.0% 0.0% 88.9%
x264 [info]: mb I I16..4: 15.2% 68.7% 16.2%
x264 [info]: mb P I16..4: 0.0% 0.0% 0.0% P16..4: 2.0% 0.0% 0.3% 0.0% 0
.0% skip:97.6%
x264 [info]: mb B I16..4: 0.0% 0.0% 0.0% B16..8: 0.2% 0.0% 0.0% direct:
0.0% skip:99.8% L0: 0.0% L1:100.0% BI: 0.0%
x264 [info]: final ratefactor: 31.50
x264 [info]: 8x8 transform intra:68.7%
x264 [info]: coded y,uvDC,uvAC intra: 48.0% 61.6% 32.3% inter: 0.0% 0.0% 0.0%
x264 [info]: i16 v,h,dc,p: 33% 47% 7% 13%
x264 [info]: i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 35% 17% 20% 3% 4% 7% 3% 7% 5%
x264 [info]: i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 50% 14% 8% 5% 5% 5% 3% 6% 4%
x264 [info]: kb/s:31.94
encoded 10 frames, 31.25 fps, 31.94 kb/s
7.
--frames 250 --qp 26 --bitrate 64 -o test.264 F:\.....\bin\akiyo_qcif.yuv 176x144
x264 [info]: 176x144 @ 25.00 fps
x264 [info]: using cpu capabilities: MMX2 SSE2 Cache64 Slow_mod4_stack
x264 [info]: profile High, level 1.1
x264 [info]: frame I:1 Avg QP:34.62 size: 1779
x264 [info]: frame P:92 Avg QP:19.81 size: 569
x264 [info]: frame B:157 Avg QP:26.76 size: 53
x264 [info]: consecutive B-frames: 15.7% 0.0% 2.4% 81.9%
x264 [info]: mb I I16..4: 14.1% 61.6% 24.2%
x264 [info]: mb P I16..4: 0.0% 0.0% 0.0% P16..4: 25.8% 9.4% 9.9% 0.0% 0
.0% skip:54.8%
x264 [info]: mb B I16..4: 0.0% 0.0% 0.0% B16..8: 13.9% 0.7% 1.4% direct:
1.1% skip:83.0% L0:16.6% L1:72.1% BI:11.3%
x264 [info]: final ratefactor: 18.97
x264 [info]: 8x8 transform intra:61.5% inter:40.4%
x264 [info]: coded y,uvDC,uvAC intra: 61.3% 65.4% 34.6% inter: 8.6% 6.8% 2.8%
x264 [info]: i16 v,h,dc,p: 57% 43% 0% 0%
x264 [info]: i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 32% 22% 18% 4% 2% 7% 3% 7% 4%
x264 [info]: i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 45% 10% 10% 5% 6% 7% 6% 6% 5%
x264 [info]: ref P L0: 87.6% 7.6% 4.8%
x264 [info]: ref B L0: 95.0% 5.0%
x264 [info]: kb/s:49.92
encoded 250 frames, 16.74 fps, 49.92 kb/s
6,7是針對不同的編碼幀數來進行比較的,在編碼幀數越多,帶寬利用的效果就越好。
6,7是在設定了目标碼率64kp/s時,采用的是ABR的RC模型,在設定了目标碼率能夠根據目标碼率的大小改變QP大小,能夠控制碼率。