熵编码（Entropy Encoding）部分的源代码在整个x264中的位置如下图所示。
x264_cavlc_mb_header_i()：写入I宏块MB Header数据。包含帧内预测模式等。
x264_cavlc_mb_header_p()：写入P宏块MB Header数据。包含MVD、参考帧序号等。
x264_cavlc_mb_header_b()：写入B宏块MB Header数据。包含MVD、参考帧序号等。
x264_cavlc_qp_delta()：写入QP。
x264_cavlc_block_residual()：写入残差数据。
/*****************************************************************************
* x264_macroblock_write:
*
* 注释和处理：雷霄骅
* http://blog.csdn.net/leixiaohua1020
* leixiaohua1020@126.com
*****************************************************************************/
void x264_macroblock_write_cavlc( x264_t *h )
{
bs_t *s = &h->out.bs;
const int i_mb_type = h->mb.i_type;
int plane_count = CHROMA444 ? 3 : 1;
int chroma = !CHROMA444;
#if RDO_SKIP_BS
s->i_bits_encoded = 0;
#else
const int i_mb_pos_start = bs_pos( s );
int i_mb_pos_tex;
#endif
if( SLICE_MBAFF
&& (!(h->mb.i_mb_y & 1) || IS_SKIP(h->mb.type[h->mb.i_mb_xy - h->mb.i_mb_stride])) )
{
bs_write1( s, MB_INTERLACED );
#if !RDO_SKIP_BS
h->mb.field_decoding_flag = MB_INTERLACED;
#endif
}
#if !RDO_SKIP_BS
if( i_mb_type == I_PCM )
{
static const uint8_t i_offsets[3] = {5,23,0};
uint8_t *p_start = s->p_start;
bs_write_ue( s, i_offsets[h->sh.i_type] + 25 );
i_mb_pos_tex = bs_pos( s );
h->stat.frame.i_mv_bits += i_mb_pos_tex - i_mb_pos_start;
bs_align_0( s );
for( int p = 0; p < plane_count; p++ )
for( int i = 0; i < 256; i++ )
bs_write( s, BIT_DEPTH, h->mb.pic.p_fenc[p][i] );
if( chroma )
for( int ch = 1; ch < 3; ch++ )
for( int i = 0; i < 16>>CHROMA_V_SHIFT; i++ )
for( int j = 0; j < 8; j++ )
bs_write( s, BIT_DEPTH, h->mb.pic.p_fenc[ch][i*FENC_STRIDE+j] );
bs_init( s, s->p, s->p_end - s->p );
s->p_start = p_start;
h->stat.frame.i_tex_bits += bs_pos(s) - i_mb_pos_tex;
return;
}
#endif
if( h->sh.i_type == SLICE_TYPE_P )
x264_cavlc_mb_header_p( h, i_mb_type, chroma );//写入P宏块MB Header数据-CAVLC
else if( h->sh.i_type == SLICE_TYPE_B )
x264_cavlc_mb_header_b( h, i_mb_type, chroma );//写入B宏块MB Header数据-CAVLC
else //if( h->sh.i_type == SLICE_TYPE_I )
x264_cavlc_mb_header_i( h, i_mb_type, 0, chroma );//写入I宏块MB Header数据-CAVLC
#if !RDO_SKIP_BS
i_mb_pos_tex = bs_pos( s );
h->stat.frame.i_mv_bits += i_mb_pos_tex - i_mb_pos_start;
#endif
/* Coded block pattern */
if( i_mb_type != I_16x16 )
bs_write_ue( s, cbp_to_golomb[chroma][IS_INTRA(i_mb_type)][(h->mb.i_cbp_chroma << 4)|h->mb.i_cbp_luma] );
/* transform size 8x8 flag */
if( x264_mb_transform_8x8_allowed( h ) && h->mb.i_cbp_luma )
bs_write1( s, h->mb.b_transform_8x8 );
if( i_mb_type == I_16x16 )
{
x264_cavlc_qp_delta( h );
/* DC Luma */
for( int p = 0; p < plane_count; p++ )
{
x264_cavlc_block_residual( h, DCT_LUMA_DC, LUMA_DC+p, h->dct.luma16x16_dc[p] );
/* AC Luma */
if( h->mb.i_cbp_luma )
for( int i = p*16; i < p*16+16; i++ )
x264_cavlc_block_residual( h, DCT_LUMA_AC, i, h->dct.luma4x4[i]+1 );
}
}
else if( h->mb.i_cbp_luma | h->mb.i_cbp_chroma )
{
x264_cavlc_qp_delta( h );
//残差数据
x264_cavlc_macroblock_luma_residual( h, plane_count );
}
if( h->mb.i_cbp_chroma )
{
/* Chroma DC residual present */
x264_cavlc_block_residual( h, DCT_CHROMA_DC, CHROMA_DC+0, h->dct.chroma_dc[0] );
x264_cavlc_block_residual( h, DCT_CHROMA_DC, CHROMA_DC+1, h->dct.chroma_dc[1] );
if( h->mb.i_cbp_chroma == 2 ) /* Chroma AC residual present */
{
int step = 8 << CHROMA_V_SHIFT;
for( int i = 16; i < 3*16; i += step )
for( int j = i; j < i+4; j++ )
x264_cavlc_block_residual( h, DCT_CHROMA_AC, j, h->dct.luma4x4[j]+1 );
}
}
#if !RDO_SKIP_BS
h->stat.frame.i_tex_bits += bs_pos(s) - i_mb_pos_tex;
#endif
}
（1）根据Slice类型的不同，调用不同的函数输出宏块头（MB Header）：下文将会分别分析其中涉及到的几个函数。a)对于P Slice，调用x264_cavlc_mb_header_p()b)对于B Slice，调用x264_cavlc_mb_header_b()c)对于I Slice，调用x264_cavlc_mb_header_i()（2）调用x264_cavlc_qp_delta()输出宏块QP值
（3）调用x264_cavlc_block_residual()输出CAVLC编码的残差数据
//写入I宏块Header数据-CAVLC
static void x264_cavlc_mb_header_i( x264_t *h, int i_mb_type, int i_mb_i_offset, int chroma )
{
bs_t *s = &h->out.bs;
if( i_mb_type == I_16x16 )
{
bs_write_ue( s, i_mb_i_offset + 1 + x264_mb_pred_mode16x16_fix[h->mb.i_intra16x16_pred_mode] +
h->mb.i_cbp_chroma * 4 + ( h->mb.i_cbp_luma == 0 ? 0 : 12 ) );
}
else //if( i_mb_type == I_4x4 || i_mb_type == I_8x8 )
{
int di = i_mb_type == I_8x8 ? 4 : 1;
bs_write_ue( s, i_mb_i_offset + 0 );
if( h->pps->b_transform_8x8_mode )
bs_write1( s, h->mb.b_transform_8x8 );
/* Prediction: Luma */
for( int i = 0; i < 16; i += di )
{
//写入Intra4x4宏块的帧内预测模式
//获得帧内模式的预测值（通过左边和上边的块）
int i_pred = x264_mb_predict_intra4x4_mode( h, i );
//获得当前帧内模式
int i_mode = x264_mb_pred_mode4x4_fix( h->mb.cache.intra4x4_pred_mode[x264_scan8[i]] );
if( i_pred == i_mode )
bs_write1( s, 1 ); //如果当前模式正好等于预测值/* b_prev_intra4x4_pred_mode */
else
bs_write( s, 4, i_mode - (i_mode > i_pred) );//否则传送差值（差值=当前模式-预测模式）
}
}
if( chroma )
bs_write_ue( s, x264_mb_chroma_pred_mode_fix[h->mb.i_chroma_pred_mode] );
}
从源代码可以看出，x264_cavlc_mb_header_i()在宏块为Intra16x16和Intra4x4的时候做了不同的处理。在Intra4x4帧内编码的宏块中，每个4x4的子块都有自己的帧内预测方式。H.264码流中并不是直接保存了每个子块的帧内预测方式（不利于压缩）。而是优先通过有周围块的信息推测当前块的帧内预测模式。具体的方法就是获取到左边块和上边块的预测模式，然后取它们的最小值作为当前块的预测模式。X264中有关这一部分的实现位于x264_mb_predict_intra4x4_mode()函数中。
//获得Intra4x4帧内模式的预测值
static ALWAYS_INLINE int x264_mb_predict_intra4x4_mode( x264_t *h, int idx )
{
//左边4x4块的帧内预测模式
const int ma = h->mb.cache.intra4x4_pred_mode[x264_scan8[idx] - 1];
//上边4x4块的帧内预测模式
const int mb = h->mb.cache.intra4x4_pred_mode[x264_scan8[idx] - 8];
//取左边和上边的最小值，作为预测值
const int m = X264_MIN( x264_mb_pred_mode4x4_fix(ma),
x264_mb_pred_mode4x4_fix(mb) );
if( m < 0 )
return I_PRED_4x4_DC;
return m;
}
//写入P宏块Header数据-CAVLC
static ALWAYS_INLINE void x264_cavlc_mb_header_p( x264_t *h, int i_mb_type, int chroma )
{
bs_t *s = &h->out.bs;
if( i_mb_type == P_L0 )
{
if( h->mb.i_partition == D_16x16 )
{
bs_write1( s, 1 );
//写入参考帧序号
if( h->mb.pic.i_fref[0] > 1 )
bs_write_te( s, h->mb.pic.i_fref[0] - 1, h->mb.cache.ref[0][x264_scan8[0]] );
/*
* 向码流中写入MVD
*
* 运动矢量缓存mv[]
* 第3个参数是mv数据的起始点（scan8[]序号），在这里是mv[scan8[0]]
*
* 写入了Y
* |
* --+--------------
* | 0 0 0 0 0 0 0 0
* | 0 0 0 0 Y 1 1 1
* | 0 0 0 0 1 1 1 1
* | 0 0 0 0 1 1 1 1
* | 0 0 0 0 1 1 1 1
*/
x264_cavlc_mvd( h, 0, 0, 4 );
}
else if( h->mb.i_partition == D_16x8 )
{
bs_write_ue( s, 1 );
/*
* 向码流中写入参考帧序号、MVD
* 写入了Y
* |
* --+--------------
* | 0 0 0 0 0 0 0 0
* | 0 0 0 0 Y 1 1 1
* | 0 0 0 0 1 1 1 1
* | 0 0 0 0 Y 2 2 2
* | 0 0 0 0 2 2 2 2
*/
if( h->mb.pic.i_fref[0] > 1 )
{
bs_write_te( s, h->mb.pic.i_fref[0] - 1, h->mb.cache.ref[0][x264_scan8[0]] );
bs_write_te( s, h->mb.pic.i_fref[0] - 1, h->mb.cache.ref[0][x264_scan8[8]] );
}
x264_cavlc_mvd( h, 0, 0, 4 );
x264_cavlc_mvd( h, 0, 8, 4 );
}
else if( h->mb.i_partition == D_8x16 )
{
bs_write_ue( s, 2 );
/*
* 向码流中写入参考帧序号、MVD
* 写入了Y
* |
* --+--------------
* | 0 0 0 0 0 0 0 0
* | 0 0 0 0 Y 1 Y 2
* | 0 0 0 0 1 1 2 2
* | 0 0 0 0 1 1 2 2
* | 0 0 0 0 1 1 2 2
*/
if( h->mb.pic.i_fref[0] > 1 )
{
bs_write_te( s, h->mb.pic.i_fref[0] - 1, h->mb.cache.ref[0][x264_scan8[0]] );
bs_write_te( s, h->mb.pic.i_fref[0] - 1, h->mb.cache.ref[0][x264_scan8[4]] );
}
x264_cavlc_mvd( h, 0, 0, 2 );
x264_cavlc_mvd( h, 0, 4, 2 );
}
}
else if( i_mb_type == P_8x8 )
{
int b_sub_ref;
if( (h->mb.cache.ref[0][x264_scan8[0]] | h->mb.cache.ref[0][x264_scan8[ 4]] |
h->mb.cache.ref[0][x264_scan8[8]] | h->mb.cache.ref[0][x264_scan8[12]]) == 0 )
{
bs_write_ue( s, 4 );
b_sub_ref = 0;
}
else
{
bs_write_ue( s, 3 );
b_sub_ref = 1;
}
/* sub mb type */
if( h->param.analyse.inter & X264_ANALYSE_PSUB8x8 )
for( int i = 0; i < 4; i++ )
bs_write_ue( s, subpartition_p_to_golomb[ h->mb.i_sub_partition[i] ] );
else
bs_write( s, 4, 0xf );
/* ref0 */
//参考帧序号
if( b_sub_ref )
{
bs_write_te( s, h->mb.pic.i_fref[0] - 1, h->mb.cache.ref[0][x264_scan8[0]] );
bs_write_te( s, h->mb.pic.i_fref[0] - 1, h->mb.cache.ref[0][x264_scan8[4]] );
bs_write_te( s, h->mb.pic.i_fref[0] - 1, h->mb.cache.ref[0][x264_scan8[8]] );
bs_write_te( s, h->mb.pic.i_fref[0] - 1, h->mb.cache.ref[0][x264_scan8[12]] );
}
//写入8x8块的子块的MVD
for( int i = 0; i < 4; i++ )
x264_cavlc_8x8_mvd( h, i );
}
else //if( IS_INTRA( i_mb_type ) )
x264_cavlc_mb_header_i( h, i_mb_type, 5, chroma );
}
//写入MVD
static void x264_cavlc_mvd( x264_t *h, int i_list, int idx, int width )
{
bs_t *s = &h->out.bs;
ALIGNED_4( int16_t mvp[2] );
//获得预测MV
x264_mb_predict_mv( h, i_list, idx, width, mvp );
//实际存储MVD
//MVD=MV-预测MV
bs_write_se( s, h->mb.cache.mv[i_list][x264_scan8[idx]][0] - mvp[0] );
bs_write_se( s, h->mb.cache.mv[i_list][x264_scan8[idx]][1] - mvp[1] );
}从源代码可以看出，x264_cavlc_mvd()首先调用x264_mb_predict_mv()通过左边，上边和右上宏块的运动矢量推算出预测运动矢量，然后将当前实际运动矢量与预测运动矢量相减后输出。
//获得预测的运动矢量MV（通过取中值）
void x264_mb_predict_mv( x264_t *h, int i_list, int idx, int i_width, int16_t mvp[2] )
{
const int i8 = x264_scan8[idx];
const int i_ref= h->mb.cache.ref[i_list][i8];
int i_refa = h->mb.cache.ref[i_list][i8 - 1];
int16_t *mv_a = h->mb.cache.mv[i_list][i8 - 1];
int i_refb = h->mb.cache.ref[i_list][i8 - 8];
int16_t *mv_b = h->mb.cache.mv[i_list][i8 - 8];
int i_refc = h->mb.cache.ref[i_list][i8 - 8 + i_width];
int16_t *mv_c = h->mb.cache.mv[i_list][i8 - 8 + i_width];
// Partitions not yet reached in scan order are unavailable.
if( (idx&3) >= 2 + (i_width&1) || i_refc == -2 )
{
i_refc = h->mb.cache.ref[i_list][i8 - 8 - 1];
mv_c = h->mb.cache.mv[i_list][i8 - 8 - 1];
if( SLICE_MBAFF
&& h->mb.cache.ref[i_list][x264_scan8[0]-1] != -2
&& MB_INTERLACED != h->mb.field[h->mb.i_mb_left_xy[0]] )
{
if( idx == 2 )
{
mv_c = h->mb.cache.topright_mv[i_list][0];
i_refc = h->mb.cache.topright_ref[i_list][0];
}
else if( idx == 8 )
{
mv_c = h->mb.cache.topright_mv[i_list][1];
i_refc = h->mb.cache.topright_ref[i_list][1];
}
else if( idx == 10 )
{
mv_c = h->mb.cache.topright_mv[i_list][2];
i_refc = h->mb.cache.topright_ref[i_list][2];
}
}
}
if( h->mb.i_partition == D_16x8 )
{
if( idx == 0 )
{
if( i_refb == i_ref )
{
CP32( mvp, mv_b );
return;
}
}
else
{
if( i_refa == i_ref )
{
CP32( mvp, mv_a );
return;
}
}
}
else if( h->mb.i_partition == D_8x16 )
{
if( idx == 0 )
{
if( i_refa == i_ref )
{
CP32( mvp, mv_a );
return;
}
}
else
{
if( i_refc == i_ref )
{
CP32( mvp, mv_c );
return;
}
}
}
int i_count = (i_refa == i_ref) + (i_refb == i_ref) + (i_refc == i_ref);
//如果可参考运动矢量的个数大于1个
if( i_count > 1 )
{
median:
//取中值
//x264_median_mv()内部调用了2次x264_median()，分别求了运动矢量的x分量和y分量的中值
x264_median_mv( mvp, mv_a, mv_b, mv_c );
}
else if( i_count == 1 ) //如果可参考运动矢量的个数只有1个
{
//直接赋值
if( i_refa == i_ref )
CP32( mvp, mv_a );
else if( i_refb == i_ref )
CP32( mvp, mv_b );
else
CP32( mvp, mv_c );
}
else if( i_refb == -2 && i_refc == -2 && i_refa != -2 )
CP32( mvp, mv_a );
else
goto median;
}
//QP
static void x264_cavlc_qp_delta( x264_t *h )
{
bs_t *s = &h->out.bs;
//相减
int i_dqp = h->mb.i_qp - h->mb.i_last_qp;
/* Avoid writing a delta quant if we have an empty i16x16 block, e.g. in a completely
* flat background area. Don‘t do this if it would raise the quantizer, since that could
* cause unexpected deblocking artifacts. */
if( h->mb.i_type == I_16x16 && !(h->mb.i_cbp_luma | h->mb.i_cbp_chroma)
&& !h->mb.cache.non_zero_count[x264_scan8[LUMA_DC]]
&& !h->mb.cache.non_zero_count[x264_scan8[CHROMA_DC+0]]
&& !h->mb.cache.non_zero_count[x264_scan8[CHROMA_DC+1]]
&& h->mb.i_qp > h->mb.i_last_qp )
{
#if !RDO_SKIP_BS
h->mb.i_qp = h->mb.i_last_qp;
#endif
i_dqp = 0;
}
if( i_dqp )
{
if( i_dqp < -(QP_MAX_SPEC+1)/2 )
i_dqp += QP_MAX_SPEC+1;
else if( i_dqp > QP_MAX_SPEC/2 )
i_dqp -= QP_MAX_SPEC+1;
}
bs_write_se( s, i_dqp );
}
static ALWAYS_INLINE void x264_cavlc_macroblock_luma_residual( x264_t *h, int plane_count )
{
if( h->mb.b_transform_8x8 )
{
/* shuffle 8x8 dct coeffs into 4x4 lists */
for( int p = 0; p < plane_count; p++ )
for( int i8 = 0; i8 < 4; i8++ )
if( h->mb.cache.non_zero_count[x264_scan8[p*16+i8*4]] )
h->zigzagf.interleave_8x8_cavlc( h->dct.luma4x4[p*16+i8*4], h->dct.luma8x8[p*4+i8],
&h->mb.cache.non_zero_count[x264_scan8[p*16+i8*4]] );
}
for( int p = 0; p < plane_count; p++ )
FOREACH_BIT( i8, 0, h->mb.i_cbp_luma )
for( int i4 = 0; i4 < 4; i4++ )
x264_cavlc_block_residual( h, DCT_LUMA_4x4, i4+i8*4+p*16, h->dct.luma4x4[i4+i8*4+p*16] );
}
雷霄骅
leixiaohua1020@126.comhttp://blog.csdn.net/leixiaohua1020
x264源代码简单分析：熵编码（Entropy Encoding）部分
原文地址：http://blog.csdn.net/leixiaohua1020/article/details/45944811
