一 解析是否为skip宏块，skip块，指的是没有mvd也没有残差的块。

帧间块才有这种。

const SPS *sps = h->ps.sps;int mb_xy;int mb_type, partition_count, cbp = 0;int dct8x8_allowed = h->ps.pps->transform_8x8_mode; const int decode_chroma = sps->chroma_format_idc == 1 || sps->chroma_format_idc == 2;const int pixel_shift = h->pixel_shift;mb_xy = sl->mb_xy = sl->mb_x + sl->mb_y*h->mb_stride;ff_tlog(h->avctx, "pic:%d mb:%d/%d\n", h->poc.frame_num, sl->mb_x, sl->mb_y);if (sl->slice_type_nos != AV_PICTURE_TYPE_I) {int skip;/* a skipped mb needs the aff flag from the following mb */if (FRAME_MBAFF(h) && (sl->mb_y & 1) == 1 && sl->prev_mb_skipped)skip = sl->next_mb_skipped;elseskip = decode_cabac_mb_skip(h, sl, sl->mb_x, sl->mb_y );/* read skip flags */if( skip ) {if (FRAME_MBAFF(h) && (sl->mb_y & 1) == 0) {h->cur_pic.mb_type[mb_xy] = MB_TYPE_SKIP;sl->next_mb_skipped = decode_cabac_mb_skip(h, sl, sl->mb_x, sl->mb_y+1 );if(!sl->next_mb_skipped)sl->mb_mbaff = sl->mb_field_decoding_flag = decode_cabac_field_decoding_flag(h, sl);}decode_mb_skip(h, sl);h->cbp_table[mb_xy] = 0;h->chroma_pred_mode_table[mb_xy] = 0;sl->last_qscale_diff = 0;return 0;}}//解析是否为skip块

然后填充相邻块信息，因为解码当前块需要用到周围块的相关信息，比如需要计算mvp等。

二 解析宏块类型mb_type,B帧中的块类型解析，B块会多一些分析，比如是否为Direct块（有残差，没有MVD）

fill_decode_neighbors(h, sl, -(MB_FIELD(sl))); //填充周围块的相关信息if (sl->slice_type_nos == AV_PICTURE_TYPE_B) { //如果是B帧int ctx = 0;av_assert2(sl->slice_type_nos == AV_PICTURE_TYPE_B);if (!IS_DIRECT(sl->left_type[LTOP] - 1))ctx++;if (!IS_DIRECT(sl->top_type - 1))ctx++;if( !get_cabac_noinline( &sl->cabac, &sl->cabac_state[27+ctx] ) ){mb_type= 0; /* B_Direct_16x16 */} else if ( !get_cabac_noinline( &sl->cabac, &sl->cabac_state[27+3] ) ) {mb_type= 1 + get_cabac_noinline( &sl->cabac, &sl->cabac_state[27+5] ); /* B_L[01]_16x16 */} else {int bits;bits = get_cabac_noinline( &sl->cabac, &sl->cabac_state[27+4] ) << 3;bits+= get_cabac_noinline( &sl->cabac, &sl->cabac_state[27+5] ) << 2;bits+= get_cabac_noinline( &sl->cabac, &sl->cabac_state[27+5] ) << 1;bits+= get_cabac_noinline( &sl->cabac, &sl->cabac_state[27+5] );if( bits < 8 ){ //解析宏块类型mb_type= bits + 3; /* B_Bi_16x16 through B_L1_L0_16x8 */}else if( bits == 13 ){mb_type = decode_cabac_intra_mb_type(sl, 32, 0);goto decode_intra_mb;}else if( bits == 14 ){mb_type= 11; /* B_L1_L0_8x16 */}else if( bits == 15 ){mb_type= 22; /* B_8x8 */}else{bits= ( bits<<1 ) + get_cabac_noinline( &sl->cabac, &sl->cabac_state[27+5] );mb_type= bits - 4; /* B_L0_Bi_* through B_Bi_Bi_* */}}partition_count = ff_h264_b_mb_type_info[mb_type].partition_count;mb_type         = ff_h264_b_mb_type_info[mb_type].type;} else if (sl->slice_type_nos == AV_PICTURE_TYPE_P) {

三 P帧中块类型的解析

else if (sl->slice_type_nos == AV_PICTURE_TYPE_P) {if( get_cabac_noinline( &sl->cabac, &sl->cabac_state[14] ) == 0 ) {/* P-type */if( get_cabac_noinline( &sl->cabac, &sl->cabac_state[15] ) == 0 ) {/* P_L0_D16x16, P_8x8 */mb_type= 3 * get_cabac_noinline( &sl->cabac, &sl->cabac_state[16] );} else {/* P_L0_D8x16, P_L0_D16x8 */mb_type= 2 - get_cabac_noinline( &sl->cabac, &sl->cabac_state[17] );}partition_count = ff_h264_p_mb_type_info[mb_type].partition_count;mb_type         = ff_h264_p_mb_type_info[mb_type].type;} else {mb_type = decode_cabac_intra_mb_type(sl, 17, 0);goto decode_intra_mb;}} 

四 帧内块解析

else {mb_type = decode_cabac_intra_mb_type(sl, 3, 1);if (sl->slice_type == AV_PICTURE_TYPE_SI && mb_type)mb_type--;av_assert2(sl->slice_type_nos == AV_PICTURE_TYPE_I);
decode_intra_mb:partition_count = 0;cbp                      = ff_h264_i_mb_type_info[mb_type].cbp;sl->intra16x16_pred_mode = ff_h264_i_mb_type_info[mb_type].pred_mode;mb_type                  = ff_h264_i_mb_type_info[mb_type].type;}

五 如果是帧内块 (IPB帧都会有)

if( IS_INTRA( mb_type ) ) { //如果是帧内块int i, pred_mode;if( IS_INTRA4x4( mb_type ) ) { //如果是4x4的帧内块if (dct8x8_allowed /*如果8x8是允许的*/ && get_cabac_noinline(&sl->cabac, &sl->cabac_state[399 + sl->neighbor_transform_size])) {mb_type |= MB_TYPE_8x8DCT; //宏块类型8x8 DCTfor( i = 0; i < 16; i+=4 ) {int pred = pred_intra_mode(h, sl, i); //预测模式 int mode = decode_cabac_mb_intra4x4_pred_mode(sl, pred); //解码cabac intra 4x4 预测模式fill_rectangle(&sl->intra4x4_pred_mode_cache[scan8[i]], 2, 2, 8, mode, 1);}} else {for( i = 0; i < 16; i++ ) {int pred = pred_intra_mode(h, sl, i);sl->intra4x4_pred_mode_cache[scan8[i]] = decode_cabac_mb_intra4x4_pred_mode(sl, pred);ff_tlog(h->avctx, "i4x4 pred=%d mode=%d\n", pred,sl->intra4x4_pred_mode_cache[scan8[i]]);}}//会写当前块的预测模式write_back_intra_pred_mode(h, sl);if (ff_h264_check_intra4x4_pred_mode(sl->intra4x4_pred_mode_cache, h->avctx,sl->top_samples_available, sl->left_samples_available) < 0 )return -1;} else { //其它就是 16x16的模式sl->intra16x16_pred_mode = ff_h264_check_intra_pred_mode(h->avctx, sl->top_samples_available,sl->left_samples_available, sl->intra16x16_pred_mode, 0);if (sl->intra16x16_pred_mode < 0) return -1;}if(decode_chroma){ //如果有色度信息 , 解码色度宏块类型h->chroma_pred_mode_table[mb_xy] =pred_mode                        = decode_cabac_mb_chroma_pre_mode(h, sl);//获取色度预测模式pred_mode = ff_h264_check_intra_pred_mode(h->avctx, sl->top_samples_available,sl->left_samples_available, pred_mode, 1 );if( pred_mode < 0 ) return -1;sl->chroma_pred_mode = pred_mode;} else {sl->chroma_pred_mode = DC_128_PRED8x8;}}

六 如果是4个8x8的帧间块

else if( partition_count == 4 ) { //4个8x8的partitions， int i, j, sub_partition_count[4], list, ref[2][4];if (sl->slice_type_nos == AV_PICTURE_TYPE_B ) { //如果是B帧for( i = 0; i < 4; i++ ) { //解码4个子块的类型sl->sub_mb_type[i] = decode_cabac_b_mb_sub_type(sl);sub_partition_count[i] = ff_h264_b_sub_mb_type_info[sl->sub_mb_type[i]].partition_count;sl->sub_mb_type[i]     = ff_h264_b_sub_mb_type_info[sl->sub_mb_type[i]].type;}if (IS_DIRECT(sl->sub_mb_type[0] | sl->sub_mb_type[1] |sl->sub_mb_type[2] | sl->sub_mb_type[3])) { //如果其中有一个是direct块ff_h264_pred_direct_motion(h, sl, &mb_type);sl->ref_cache[0][scan8[4]] =sl->ref_cache[1][scan8[4]] =sl->ref_cache[0][scan8[12]] =sl->ref_cache[1][scan8[12]] = PART_NOT_AVAILABLE;for( i = 0; i < 4; i++ )fill_rectangle(&sl->direct_cache[scan8[4*i]], 2, 2, 8, (sl->sub_mb_type[i] >> 1) & 0xFF, 1);}} else { //非B slicefor( i = 0; i < 4; i++ ) {sl->sub_mb_type[i] = decode_cabac_p_mb_sub_type(sl); //子块类型sub_partition_count[i] = ff_h264_p_sub_mb_type_info[sl->sub_mb_type[i]].partition_count;//子块partitionssl->sub_mb_type[i]     = ff_h264_p_sub_mb_type_info[sl->sub_mb_type[i]].type;}}for( list = 0; list < sl->list_count; list++ ) {for( i = 0; i < 4; i++ ) {if(IS_DIRECT(sl->sub_mb_type[i])) continue;if(IS_DIR(sl->sub_mb_type[i], 0, list)){unsigned rc = sl->ref_count[list] << MB_MBAFF(sl);if (rc > 1) {ref[list][i] = decode_cabac_mb_ref(sl, list, 4 * i); // 返回的是参考帧个数if (ref[list][i] >= rc) {av_log(h->avctx, AV_LOG_ERROR, "Reference %d >= %d\n", ref[list][i], rc);return -1;}}elseref[list][i] = 0; //没有参考帧，全都是帧内块？} else {ref[list][i] = -1;  //IDR没有参考帧}sl->ref_cache[list][scan8[4 * i] + 1] =sl->ref_cache[list][scan8[4 * i] + 8] = sl->ref_cache[list][scan8[4 * i] + 9] = ref[list][i];}}if(dct8x8_allowed) 正常都是4x4的dct，也有8x8的，更高的profile中dct8x8_allowed = get_dct8x8_allowed(h, sl);

七 接下来解析mvd，并且计算得到mv

 //下面解码MVfor (list = 0; list < sl->list_count; list++) {for(i=0; i<4; i++){sl->ref_cache[list][scan8[4 * i]] = sl->ref_cache[list][scan8[4 * i] + 1]; //ref delta pocif(IS_DIRECT(sl->sub_mb_type[i])){ //如果是Direct块, 没有mvd，只有残差fill_rectangle(sl->mvd_cache[list][scan8[4*i]], 2, 2, 8, 0, 2);continue;}if(IS_DIR(sl->sub_mb_type[i], 0, list) && !IS_DIRECT(sl->sub_mb_type[i])){const int sub_mb_type= sl->sub_mb_type[i]; //子块类型const int block_width= (sub_mb_type & (MB_TYPE_16x16|MB_TYPE_16x8)) ? 2 : 1;for(j=0; jmv_cache[list][ scan8[index] ];uint8_t (* mvd_cache)[2]= &sl->mvd_cache[list][ scan8[index] ];pred_motion(h, sl, index, block_width, list, sl->ref_cache[list][ scan8[index] ], &mx, &my); //计算出来mvpDECODE_CABAC_MB_MVD(sl, list, index) //cabac解析mvx mvy ff_tlog(h->avctx, "final mv:%d %d\n", mx, my); //mv也给取出来了，mv = mvp + mvd ，在上面这个宏里面计算的。if(IS_SUB_8X8(sub_mb_type)){mv_cache[ 1 ][0]=mv_cache[ 8 ][0]= mv_cache[ 9 ][0]= mx;mv_cache[ 1 ][1]=mv_cache[ 8 ][1]= mv_cache[ 9 ][1]= my;mvd_cache[ 1 ][0]=mvd_cache[ 8 ][0]= mvd_cache[ 9 ][0]= mpx; //这些解析到的都是mvdmvd_cache[ 1 ][1]=mvd_cache[ 8 ][1]= mvd_cache[ 9 ][1]= mpy;}else if(IS_SUB_8X4(sub_mb_type)){mv_cache[ 1 ][0]= mx;mv_cache[ 1 ][1]= my;mvd_cache[ 1 ][0]=  mpx;mvd_cache[ 1 ][1]= mpy;}else if(IS_SUB_4X8(sub_mb_type)){mv_cache[ 8 ][0]= mx;mv_cache[ 8 ][1]= my;mvd_cache[ 8 ][0]= mpx;mvd_cache[ 8 ][1]= mpy;}mv_cache[ 0 ][0]= mx;mv_cache[ 0 ][1]= my;mvd_cache[ 0 ][0]= mpx;mvd_cache[ 0 ][1]= mpy;}}else{fill_rectangle(sl->mv_cache [list][ scan8[4*i] ], 2, 2, 8, 0, 4); //填充当前块的mv刀缓存中fill_rectangle(sl->mvd_cache[list][ scan8[4*i] ], 2, 2, 8, 0, 2); //填充当前块的mvd刀缓存中}}}

八 如果是direct块，没有mvd，直接mvp就是mv

else if( IS_DIRECT(mb_type) ) { //如果是direct块，B块才有，没有mvd，只有残差。ff_h264_pred_direct_motion(h, sl, &mb_type); //直接mvp就是mvfill_rectangle(sl->mvd_cache[0][scan8[0]], 4, 4, 8, 0, 2);fill_rectangle(sl->mvd_cache[1][scan8[0]], 4, 4, 8, 0, 2);dct8x8_allowed &= sps->direct_8x8_inference_flag;

九 其它正常的预测方式 解析运动向量mx和my

else { //其它的情况 int list, i;if(IS_16X16(mb_type)){ //如果是16x16的块for (list = 0; list < sl->list_count; list++) {if(IS_DIR(mb_type, 0, list)){int ref;unsigned rc = sl->ref_count[list] << MB_MBAFF(sl);if (rc > 1) {ref= decode_cabac_mb_ref(sl, list, 0);if (ref >= rc) {av_log(h->avctx, AV_LOG_ERROR, "Reference %d >= %d\n", ref, rc);return -1;}}elseref=0;fill_rectangle(&sl->ref_cache[list][ scan8[0] ], 4, 4, 8, ref, 1);} //解析参考帧列表}for (list = 0; list < sl->list_count; list++) {if(IS_DIR(mb_type, 0, list)){int mx,my,mpx,mpy;pred_motion(h, sl, 0, 4, list, sl->ref_cache[list][ scan8[0] ], &mx, &my); //计算mvp DECODE_CABAC_MB_MVD(sl, list, 0) //从码流中解析mvd 并且加到mvp上 得到mvff_tlog(h->avctx, "final mv:%d %d\n", mx, my);fill_rectangle(sl->mvd_cache[list][ scan8[0] ], 4, 4, 8, pack8to16(mpx,mpy), 2); //填充mvd 缓存fill_rectangle(sl->mv_cache[list][ scan8[0] ], 4, 4, 8, pack16to32(mx,my), 4); //填充mv缓存。}}}

十 解析参考帧

else { //其它的情况 int list, i;if(IS_16X16(mb_type)){ //如果是16x16的块for (list = 0; list < sl->list_count; list++) {if(IS_DIR(mb_type, 0, list)){int ref;unsigned rc = sl->ref_count[list] << MB_MBAFF(sl);if (rc > 1) {ref= decode_cabac_mb_ref(sl, list, 0);if (ref >= rc) {av_log(h->avctx, AV_LOG_ERROR, "Reference %d >= %d\n", ref, rc);return -1;}}elseref=0;fill_rectangle(&sl->ref_cache[list][ scan8[0] ], 4, 4, 8, ref, 1);} //解析参考帧列表}for (list = 0; list < sl->list_count; list++) {if(IS_DIR(mb_type, 0, list)){int mx,my,mpx,mpy;pred_motion(h, sl, 0, 4, list, sl->ref_cache[list][ scan8[0] ], &mx, &my); //计算mvp DECODE_CABAC_MB_MVD(sl, list, 0) //从码流中解析mvd 并且加到mvp上 得到mvff_tlog(h->avctx, "final mv:%d %d\n", mx, my);fill_rectangle(sl->mvd_cache[list][ scan8[0] ], 4, 4, 8, pack8to16(mpx,mpy), 2); //填充mvd 缓存fill_rectangle(sl->mv_cache[list][ scan8[0] ], 4, 4, 8, pack16to32(mx,my), 4); //填充mv缓存。}}}else if(IS_16X8(mb_type)){ //如果是16x8的块，做法类似for (list = 0; list < sl->list_count; list++) {for(i=0; i<2; i++){ //两个16x8的块 if(IS_DIR(mb_type, i, list)){int ref;unsigned rc = sl->ref_count[list] << MB_MBAFF(sl);if (rc > 1) {ref= decode_cabac_mb_ref(sl, list, 8 * i); //解析得到参考帧if (ref >= rc) {av_log(h->avctx, AV_LOG_ERROR, "Reference %d >= %d\n", ref, rc);return -1;}}elseref=0;fill_rectangle(&sl->ref_cache[list][ scan8[0] + 16*i ], 4, 2, 8, ref, 1); //填充参考帧缓存}elsefill_rectangle(&sl->ref_cache[list][ scan8[0] + 16*i ], 4, 2, 8, (LIST_NOT_USED&0xFF), 1);}}for (list = 0; list < sl->list_count; list++) { //循环解析，mvd，计算mvp 最终得到mv 并存入缓存for(i=0; i<2; i++){if(IS_DIR(mb_type, i, list)){int mx,my,mpx,mpy;pred_16x8_motion(h, sl, 8*i, list, sl->ref_cache[list][scan8[0] + 16*i], &mx, &my);DECODE_CABAC_MB_MVD(sl, list, 8*i)ff_tlog(h->avctx, "final mv:%d %d\n", mx, my);fill_rectangle(sl->mvd_cache[list][ scan8[0] + 16*i ], 4, 2, 8, pack8to16(mpx,mpy), 2);fill_rectangle(sl->mv_cache[list][ scan8[0] + 16*i ], 4, 2, 8, pack16to32(mx,my), 4);}else{fill_rectangle(sl->mvd_cache[list][ scan8[0] + 16*i ], 4, 2, 8, 0, 2);fill_rectangle(sl->mv_cache[list][ scan8[0] + 16*i ], 4, 2, 8, 0, 4);}}}}else{av_assert2(IS_8X16(mb_type));for (list = 0; list < sl->list_count; list++) {for(i=0; i<2; i++){if(IS_DIR(mb_type, i, list)){ //FIXME optimizeint ref;unsigned rc = sl->ref_count[list] << MB_MBAFF(sl);if (rc > 1) {ref = decode_cabac_mb_ref(sl, list, 4 * i);if (ref >= rc) {av_log(h->avctx, AV_LOG_ERROR, "Reference %d >= %d\n", ref, rc);return -1;}}elseref=0;fill_rectangle(&sl->ref_cache[list][ scan8[0] + 2*i ], 2, 4, 8, ref, 1);}elsefill_rectangle(&sl->ref_cache[list][ scan8[0] + 2*i ], 2, 4, 8, (LIST_NOT_USED&0xFF), 1);}}for (list = 0; list < sl->list_count; list++) {for(i=0; i<2; i++){if(IS_DIR(mb_type, i, list)){int mx,my,mpx,mpy;pred_8x16_motion(h, sl, i*4, list, sl->ref_cache[list][ scan8[0] + 2*i ], &mx, &my);DECODE_CABAC_MB_MVD(sl, list, 4*i)ff_tlog(h->avctx, "final mv:%d %d\n", mx, my);fill_rectangle(sl->mvd_cache[list][ scan8[0] + 2*i ], 2, 4, 8, pack8to16(mpx,mpy), 2);fill_rectangle(sl->mv_cache[list][ scan8[0] + 2*i ], 2, 4, 8, pack16to32(mx,my), 4);}else{fill_rectangle(sl->mvd_cache[list][ scan8[0] + 2*i ], 2, 4, 8, 0, 2);fill_rectangle(sl->mv_cache[list][ scan8[0] + 2*i ], 2, 4, 8, 0, 4);}}}}}

十一 解析cbp code block parttern,如果全是0，就是都没有残差

//下面cabac解析cbp，codec block parttern, 如果全是0，说明没有残差。一个bit代表一个子块if( !IS_INTRA16x16( mb_type ) ) {cbp  = decode_cabac_mb_cbp_luma(sl);if(decode_chroma)cbp |= decode_cabac_mb_cbp_chroma(sl) << 4;} else {if (!decode_chroma && cbp>15) {av_log(h->avctx, AV_LOG_ERROR, "gray chroma\n");return AVERROR_INVALIDDATA;}}h->cbp_table[mb_xy] = sl->cbp = cbp;if( dct8x8_allowed && (cbp&15) && !IS_INTRA( mb_type ) ) {mb_type |= MB_TYPE_8x8DCT * get_cabac_noinline(&sl->cabac, &sl->cabac_state[399 + sl->neighbor_transform_size]);}

十二 解析qp值和色度qp

if( cbp || IS_INTRA16x16( mb_type ) ) { //cpb不为0，有块有残差，需要有QP解码 code block partternconst uint8_t *scan, *scan8x8;const uint32_t *qmul;// decode_cabac_mb_dqp 解码宏块的qp值if(get_cabac_noinline( &sl->cabac, &sl->cabac_state[60 + (sl->last_qscale_diff != 0)])){int val = 1;int ctx= 2;const int max_qp = 51 + 6*(sps->bit_depth_luma-8);while( get_cabac_noinline( &sl->cabac, &sl->cabac_state[60 + ctx] ) ) { //val为解析刀的delta qpctx= 3;val++;if(val > 2*max_qp){ //prevent infinite loopav_log(h->avctx, AV_LOG_ERROR, "cabac decode of qscale diff failed at %d %d\n", sl->mb_x, sl->mb_y);return -1;}}if( val&0x01 )val=   (val + 1)>>1 ;elseval= -((val + 1)>>1);sl->last_qscale_diff = val; //最终缓存的delta qp存储刀last_qscale_diffsl->qscale += val; //当前块的qscale = 之前的qscale + diff值。 就是当前的qp值if (((unsigned)sl->qscale) > max_qp){if (sl->qscale < 0) sl->qscale += max_qp + 1;else                sl->qscale -= max_qp + 1;}//pps里面有对应亮度qp和色度qp的对应关系，可以通过查表获取sl->chroma_qp[0] = get_chroma_qp(h->ps.pps, 0, sl->qscale); sl->chroma_qp[1] = get_chroma_qp(h->ps.pps, 1, sl->qscale);}else

十三 扫描dct系数数组

if(IS_INTERLACED(mb_type)){scan8x8 = sl->qscale ? h->field_scan8x8 : h->field_scan8x8_q0; //得到系数数组scan    = sl->qscale ? h->field_scan : h->field_scan_q0;}else{scan8x8 = sl->qscale ? h->zigzag_scan8x8 : h->zigzag_scan8x8_q0;scan    = sl->qscale ? h->zigzag_scan : h->zigzag_scan_q0;}

十四 解码dct系数，反量化，反dct

decode_cabac_luma_residual(h, sl, scan, scan8x8, pixel_shift, mb_type, cbp, 0); //解析cabac 的亮度残差if (CHROMA444(h)) {decode_cabac_luma_residual(h, sl, scan, scan8x8, pixel_shift, mb_type, cbp, 1);decode_cabac_luma_residual(h, sl, scan, scan8x8, pixel_shift, mb_type, cbp, 2);} else if (CHROMA422(h)) {if( cbp&0x30 ){int c;for (c = 0; c < 2; c++)decode_cabac_residual_dc_422(h, sl, sl->mb + ((256 + 16*16*c) << pixel_shift), 3,CHROMA_DC_BLOCK_INDEX + c,ff_h264_chroma422_dc_scan, 8);}if( cbp&0x20 ) {int c, i, i8x8;for( c = 0; c < 2; c++ ) {int16_t *mb = sl->mb + (16*(16 + 16*c) << pixel_shift);qmul = h->ps.pps->dequant4_coeff[c+1+(IS_INTRA( mb_type ) ? 0:3)][sl->chroma_qp[c]]; //反量化for (i8x8 = 0; i8x8 < 2; i8x8++) {for (i = 0; i < 4; i++) {const int index = 16 + 16 * c + 8*i8x8 + i;decode_cabac_residual_nondc(h, sl, mb, 4, index, scan + 1, qmul, 15);mb += 16<{0,6,10},{1,7,11},{2,8,12},{5,9,13}};const uint32_t *qmul;int i8x8, i4x4;int qscale = p == 0 ? sl->qscale : sl->chroma_qp[p - 1]; //选择亮度还是色度的qp值if( IS_INTRA16x16( mb_type ) ) {AV_ZERO128(sl->mb_luma_dc[p]+0);AV_ZERO128(sl->mb_luma_dc[p]+8);AV_ZERO128(sl->mb_luma_dc[p]+16);AV_ZERO128(sl->mb_luma_dc[p]+24);//p是通道编号，decode_cabac_residual_dc(h, sl, sl->mb_luma_dc[p], ctx_cat[0][p], LUMA_DC_BLOCK_INDEX+p, scan, 16); //解析dc系数, 最后一个参数最多16个参数if( cbp&15 ) {qmul = h->ps.pps->dequant4_coeff[p][qscale]; //反量化步长for( i4x4 = 0; i4x4 < 16; i4x4++ ) {const int index = 16*p + i4x4; //4x4的AC系数decode_cabac_residual_nondc(h, sl, sl->mb + (16*index << pixel_shift), ctx_cat[1][p], index, scan + 1, qmul, 15); //已经使用了一个dc系数的位置了，//还能最多解析15个AC系数。}} else {fill_rectangle(&sl->non_zero_count_cache[scan8[16*p]], 4, 4, 8, 0, 1);}

h->cur_pic.qscale_table[mb_xy] = sl->qscale; //当前块的qp存到缓存数组中

以上就是cabac解码h264宏块的粗略过程。