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要在JM8.6代码中找出某一数据，不能瞎撞，不能用蛮力，而是要根据H.264的流程来找.比如，要在编码端找到像素残差，就可以按照下面的思路来找. （为简便起见，仅以I4x4宏块为例)
提前说一句：有时候对于同一个量，有好几个地方多可以提取，所以思路不要太局限，重要的是知道该量过去从何而来，将来又何去何从. 下面的思路是提取I4x4宏块中残差的思路.(啰嗦一句：如何要提取所有类型宏块的残差，那么仅在下面这个地方提取则是不够的)
前面说过，原始的YUV像素存储在imgY_org中，而编码端要对残差进行变换编码，而encode_one_macroblock就是编码一个宏块的关键函数，所以，如果要在编码端提取残差，就可以尝试在encode_one_macroblock所在的文件搜索imgY_org, 一搜索，还真有，但很多，回想一下，残差是怎么形成的？很显然残差的形成必然涉及到相减，也就是要在代码中找“imgY_org[][] - ...”这样形式的代码. 找找，还真有，但是我们需要的“imgY_org[][] - ...”形式的代码却在Mode_Decision_for_4x4IntraBlocks函数中，没关系的，因为encode_one_macroblock调用了Mode_Decision_for_4x4IntraBlocks函数.
提取残差的代码如下：（其中只有部分代码用于提取残差，以I4x4宏块为例)）
int Mode_Decision_for_4x4IntraBlocks (int b8, int b4, double lambda, int* min_cost){ int ipmode, best_ipmode = 0, i, j, k, x, y, cost, dummy; int c_nz, nonzero = 0, rec4x4[4][4], diff[16]; double rdcost; int block_x = 8*(b8%2)+4*(b4%2); int block_y = 8*(b8/2)+4*(b4/2); int pic_pix_x = img->pix_x+block_x; int pic_pix_y = img->pix_y+block_y; int pic_opix_x = img->opix_x+block_x; int pic_opix_y = img->opix_y+block_y; int pic_block_x = pic_pix_x/4; int pic_block_y = pic_pix_y/4; double min_rdcost = 1e30; int left_available, up_available, all_available; int upMode; int leftMode; int mostProbableMode; static int controlFlag = 0; PixelPos left_block; PixelPos top_block; getLuma4x4Neighbour(img->current_mb_nr, block_x/4, block_y/4, -1, 0, &left_block); getLuma4x4Neighbour(img->current_mb_nr, block_x/4, block_y/4, 0, -1, &top_block); // constrained intra pred if (input->UseConstrainedIntraPred) { left_block.available = left_block.available ? img->intra_block[left_block.mb_addr] : 0; top_block.available = top_block.available ? img->intra_block[top_block.mb_addr] : 0; } upMode = top_block.available ? img->ipredmode[top_block.pos_x ][top_block.pos_y ] : -1; leftMode = left_block.available ? img->ipredmode[left_block.pos_x][left_block.pos_y] : -1; mostProbableMode = (upMode < 0 || leftMode < 0) ? DC_PRED : upMode < leftMode ? upMode : leftMode; *min_cost = (1<<20); //===== INTRA PREDICTION FOR 4x4 BLOCK ===== intrapred_luma (pic_pix_x, pic_pix_y, &left_available, &up_available, &all_available); //===== LOOP OVER ALL 4x4 INTRA PREDICTION MODES ===== for (ipmode=0; ipmode rdopt) { for (k=j=0; j<4; j++) for (i=0; i<4; i++, k++) { diff[k] = imgY_org[pic_opix_y+j][pic_opix_x+i] - img->mprr[ipmode][j][i]; } cost = (ipmode == mostProbableMode) ? 0 : (int)floor(4 * lambda ); cost += SATD (diff, input->hadamard); if (cost < *min_cost) { best_ipmode = ipmode; *min_cost = cost; } } else { // get prediction and prediction error for (j=0; j<4; j++) for (i=0; i<4; i++) { img->mpr[block_x+i][block_y+j] = img->mprr[ipmode][j][i]; img->m7[i][j] = imgY_org[pic_opix_y+j][pic_opix_x+i] - img->mprr[ipmode][j][i]; }//************************************************************** // 下面是提取残差的代码(原始值，预测值和残差都打印出来) // controlFlag是static int 类型的变量，用于表示宏块中的小块 // 比如controlFlag为2，则表示第一宏块的第二个4*4小块 controlFlag++; if(2 == controlFlag) { printf("****************************\n"); for (j = 0; j < 4; j++) { for (i = 0; i < 4; i++) { printf("%d\t",imgY_org[pic_opix_y+j][pic_opix_x+i]); } printf("\n"); } printf("****************************\n"); for (j = 0; j < 4; j++) { for (i = 0; i < 4; i++) { printf("%d\t",img->mprr[ipmode][j][i]); } printf("\n"); } printf("****************************\n"); for (j=0; j<4; j++) { for (i=0; i<4; i++) { printf("%d\t",img->m7[i][j]); } printf("\n"); } } //************************************************************** //===== store the coding state ===== store_coding_state (cs_cm); // get and check rate-distortion cost if ((rdcost = RDCost_for_4x4IntraBlocks (&c_nz, b8, b4, ipmode, lambda, min_rdcost, mostProbableMode)) < min_rdcost) { //--- set coefficients --- for (j=0; j<2; j++) for (i=0; i<18;i++) cofAC4x4[j][i]=img->cofAC[b8][b4][j][i]; //--- set reconstruction --- for (y=0; y<4; y++) for (x=0; x<4; x++) rec4x4[y][x] = enc_picture->imgY[pic_pix_y+y][pic_pix_x+x]; //--- flag if dct-coefficients must be coded --- nonzero = c_nz; //--- set best mode update minimum cost --- min_rdcost = rdcost; best_ipmode = ipmode; } reset_coding_state (cs_cm); } } } //===== set intra mode prediction ===== img->ipredmode[pic_block_x][pic_block_y] = best_ipmode; img->mb_data[img->current_mb_nr].intra_pred_modes[4*b8+b4] = mostProbableMode == best_ipmode ? -1 : best_ipmode < mostProbableMode ? best_ipmode : best_ipmode-1; if (!input->rdopt) { // get prediction and prediction error for (j=0; j<4; j++) for (i=0; i<4; i++) { img->mpr[block_x+i][block_y+j] = img->mprr[best_ipmode][j][i]; img->m7[i][j] = imgY_org[pic_opix_y+j][pic_opix_x+i] - img->mprr[best_ipmode][j][i]; } nonzero = dct_luma (block_x, block_y, &dummy, 1); } else { //===== restore coefficients ===== for (j=0; j<2; j++) for (i=0; i<18;i++) img->cofAC[b8][b4][j][i]=cofAC4x4[j][i]; //===== restore reconstruction and prediction (needed if single coeffs are removed) ===== for (y=0; y<4; y++) for (x=0; x<4; x++) { enc_picture->imgY[pic_pix_y+y][pic_pix_x+x] = rec4x4[y][x]; img->mpr[block_x+x][block_y+y] = img->mprr[best_ipmode][y][x]; } } return nonzero;} 结果为：
****************************
251 254 254 253 228 205 213 185 197 173 185 136 177 189 198 160 **************************** 238 238 238 238 205 205 205 205 167 167 167 167 160 160 160 160 **************************** 13 16 16 15 23 0 8 -20 30 6 18 -31 17 29 38 0
三个矩阵分别对应第一帧第一宏块的第二个4*4块的原始值、预测值和残差，为了验证上述结果的正确性，可以用H.264visa分析一下，结果如下：
原始值为：（注意: H.264visa也可以读取原始的YUV数据）
====================== Y Data ====================== +----------------+----------------+----------------+----------------+ | 43,216,254,249,|251,254,254,253,|251,252,254,254,|254,254,254,253,| | 49,198,193,211,|228,205,213,185,|211,207,186,248,|198,203,208,183,| | 48,194,177,171,|197,173,185,136,|191,195,138,179,|142,176,177,135,| | 46,214,225,169,|177,189,198,160,|203,208,177,165,|173,196,191,156,| +----------------+----------------+----------------+----------------+ | 41,185,208,180,|203,228,226,200,|214,226,225,227,|228,225,224,210,| | 31,130,173,178,|215,230,221,212,|220,229,227,228,|229,227,226,226,| | 29,119,194,216,|211,213,219,222,|225,223,220,219,|218,218,218,218,| | 25,126,219,224,|217,224,227,227,|227,226,225,224,|220,220,221,222,| +----------------+----------------+----------------+----------------+ | 26,131,215,223,|226,225,225,225,|225,226,223,219,|221,221,219,220,| | 30,136,216,226,|223,224,225,225,|224,221,217,221,|222,219,220,226,| | 30,136,216,227,|224,224,225,223,|221,218,221,216,|211,224,224,211,| | 29,135,217,225,|222,221,222,222,|221,209,181,155,|186,210,186,164,| +----------------+----------------+----------------+----------------+ | 29,134,216,224,|226,230,230,227,|206,177,146,113,|149,162,147,150,| | 29,135,219,231,|225,201,190,185,|163,144,153,140,|127,143,165,184,| | 30,139,210,192,|165,142,134,133,|143,141,129,138,|150,178,201,207,| | 30,125,166,145,|144,154,132,111,|118,161,175,180,|204,214,213,209,| +----------------+----------------+----------------+----------------+
预测值为：（注意：H.264visa相当于一个解码器，解码端的残差和编码端的残差必定相等）
====================== Y Data ======================
+----------------+----------------+----------------+----------------+ |128,128,128,128,|238,238,238,238,|255,255,255,255,|245,228,210,192,| |128,128,128,128,|205,205,205,205,|180,180,180,180,|210,192,174,170,| |128,128,128,128,|167,167,167,167,|137,137,137,137,|174,170,166,166,| |128,128,128,128,|160,160,160,160,|169,169,169,169,|166,166,166,166,| +----------------+----------------+----------------+----------------+ |161,161,161,161,|179,189,182,169,|201,201,201,201,|219,219,219,219,| |161,161,161,161,|184,185,175,169,|214,214,214,214,|227,227,227,227,| |161,161,161,161,|189,182,169,169,|216,216,216,216,|219,219,219,219,| |161,161,161,161,|185,175,169,169,|229,229,229,229,|227,227,227,227,| +----------------+----------------+----------------+----------------+ | 28,127,218,229,|224,224,224,224,|227,227,228,227,|224,224,224,224,| | 28,127,218,229,|224,224,224,224,|224,225,227,227,|214,214,214,214,| | 28,127,218,229,|224,224,224,224,|224,224,224,225,|212,212,212,212,| | 28,127,218,229,|224,224,224,224,|224,224,224,224,|165,165,165,165,| +----------------+----------------+----------------+----------------+ | 33,130,216,224,|231,223,215,194,|204,182,160,142,|127,133,139,147,| | 33,130,216,224,|215,194,173,162,|160,142,125,120,|139,147,154,165,| | 33,130,216,224,|173,162,150,150,|125,120,116,116,|154,165,176,176,| | 33,130,216,224,|150,150,150,150,|116,116,116,116,|176,176,176,176,| +----------------+----------------+----------------+----------------+
残差为：(注意：H.264visa相当于一个解码器，而解码端的残差与编码端的残差只是近似，并不相等)
====================== Y Data ======================
+------------------------+------------------------+------------------------+------------------------+ | -78, 88, 132, 110,| 24, 14, 24, 19,| -8, 7, -3, -4,| 16, 31, 48, 51,| | -81, 63, 67, 77,| 30, 1, 11, -25,| 19, 24, 6, 59,| -12, 20, 32, 10,| | -83, 62, 48, 39,| 25, 6, 16, -30,| 54, 46, 4, 44,| -30, 7, 5, -32,| | -80, 87, 93, 32,| 14, 24, 34, 9,| 31, 37, 7, -3,| 6, 28, 21, -9,| +------------------------+------------------------+------------------------+------------------------+ | -117, 22, 51, 22,| 25, 35, 38, 32,| 18, 18, 18, 18,| 5, 5, 5, 5,| | -135, -22, 22, 19,| 32, 43, 50, 45,| 13, 13, 13, 13,| 3, 3, 3, 3,| | -131, -38, 38, 55,| 20, 35, 48, 47,| 3, 3, 3, 3,| -2, -2, -2, -2,| | -133, -34, 57, 68,| 27, 43, 60, 60,| -2, -2, -2, -2,| -5, -5, -5, -5,| +------------------------+------------------------+------------------------+------------------------+ | 5, 3, -2, -5,| 0, 0, 0, 0,| -4, -4, -3, -3,| -5, -6, -5, -2,| | 5, 3, -2, -5,| 0, 0, 0, 0,| -3, -5, -10, -13,| -2, 5, 14, 15,| | 5, 3, -2, -5,| 0, 0, 0, 0,| -3, -5, -10, -13,| 0, 14, 15, 3,| | 5, 3, -2, -5,| 0, 0, 0, 0,| 2, -13, -44, -59,| 23, 35, 24, 0,| +------------------------+------------------------+------------------------+------------------------+ | -4, -4, 0, 4,| 1, 5, 18, 26,| 0, 0, -2, -34,| 25, 20, 8, 3,| | 2, 7, 11, 10,| 13, 3, 9, 25,| 6, 7, 25, 26,| -9, -3, 9, 15,| | 0, 6, -8, -28,| -5, -11, -17, -17,| 16, 20, 12, 16,| -5, 4, 22, 31,| | -6, -5, -39, -74,| -9, 3, -10, -34,| 6, 43, 55, 60,| 33, 33, 35, 35,| +------------------------+------------------------+------------------------+------------------------+
最后，如果上面代码中的if(2 == controlFlag)改为if(1 == controlFlag)，那么结果为：
****************************
43 216 254 249 49 198 193 211 48 194 177 171 46 214 225 169 **************************** 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 128 **************************** -85 88 126 121 -79 70 65 83 -80 66 49 43 -82 86 97 41
与上面的H.264visa的结果进行对比，一切都一目了然，便不再赘述了.
P.S. 后来，我发现，上面的过程是有瑕疵的，因为对于一个宏块来说，Mode_Decision_for_4x4IntraBlocks函数不止被调用16次，所以并不对应16个4*4块，为什么会有这种情况呢？跟踪代码后发现，原来对于一个4*4块而言，要选出最终最合理的帧内预测模式，所要进行的DCT变换次数远不止16次. 所以如果要真正在编码端提取像素残差，就应该对这么多次有所选择, 而JM8.6代码必然要作出了这样的选择，找找就可以了.
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