深入剖析H.264帧内预测(I宏块)

工具: foreman_qcif.yuv, JM8.6, H.264visa. 下面仅讨论第一帧第一个宏块MB（16 * 16）的亮度Y分量.

     

步骤1：用JM8.6编码foreman_qcif.yuv的第一帧, 得到test.264,用H.264visa打开test.264, 然后用H.264visa打开foreman_qcif.yuv. 从H.264visa中的结果可以看出，第一帧的大部分宏块都是4*4预测的，只有5个宏块采用的是16*16预测的., 这5个宏块的特点也都很明显：非常平滑.

步骤2：给出几个重要的矩阵，然后分析这样一个关系：

               orgMB - predictMB = encoderResMB  ------>  max(0, min(255, decoderResMB + predictMB) )= preFilterMB

                                  ( 编 码 端)                                                                 ( 解  码  端)

 

 

原始的foreman_qcif.yuv在H.264visa中的结果是：(orgMB)

====================== 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,|
+----------------+----------------+----------------+----------------+

 

 预测值在H.264visa中的结果为：（predictMB）

====================== 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,|
+----------------+----------------+----------------+----------------+

 

编码端的残差值即为上述两个矩阵相减（orgMB - predictMB），解码端的残差值在H.264visa中为：（decoderResMB）

====================== 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,|
+------------------------+------------------------+------------------------+------------------------+

 

解码端的滤波前的值在H.264visa中为：（preFilterMB）

====================== Y Data ======================
+----------------+----------------+----------------+----------------+
| 50,216,255,238,|255,252,255,255,|247,255,252,251,|255,255,255,243,|
| 47,191,195,205,|235,206,216,180,|199,204,186,239,|198,212,206,180,|
| 45,190,176,167,|192,173,183,137,|191,183,141,181,|144,177,171,134,|
| 48,215,221,160,|174,184,194,169,|200,206,176,166,|172,194,187,157,|
+----------------+----------------+----------------+----------------+
| 44,183,212,183,|204,224,220,201,|219,219,219,219,|224,224,224,224,|
| 26,139,183,180,|216,228,225,214,|227,227,227,227,|230,230,230,230,|
| 30,123,199,216,|209,217,217,216,|219,219,219,219,|217,217,217,217,|
| 28,127,218,229,|212,218,229,229,|227,227,227,227,|222,222,222,222,|
+----------------+----------------+----------------+----------------+
| 33,130,216,224,|224,224,224,224,|223,223,225,224,|219,218,219,222,|
| 33,130,216,224,|224,224,224,224,|221,220,217,214,|212,219,228,229,|
| 33,130,216,224,|224,224,224,224,|221,219,214,212,|212,226,227,215,|
| 33,130,216,224,|224,224,224,224,|226,211,180,165,|188,200,189,165,|
+----------------+----------------+----------------+----------------+
| 29,126,216,228,|232,228,233,220,|204,182,158,108,|152,153,147,150,|
| 35,137,227,234,|228,197,182,187,|166,149,150,146,|130,144,163,180,|
| 33,136,208,196,|168,151,133,133,|141,140,128,132,|149,169,198,207,|
| 27,125,177,150,|141,153,140,116,|122,159,171,176,|209,209,211,211,|
+----------------+----------------+----------------+----------------+

 

          先来看看第一宏块信息:

 

        （1）分析第一个4*4块. 最初（第一个MB的第一个4*4）块的预测值是什么呢？从H.264visa中可以看出，predictMB[i][j] = 128（0<= i, j <= 3）, 而从JM8.6的源码中也可以看出，事实的确如此。源码如下：

 if (block_available_up && block_available_left)//上有左有
  {   
    // no edge

	 //P_...为重建帧的值，直流预测是 sum(P_...)/n
	 //加上4是为了实现四舍五入
    s0 = (P_A + P_B + P_C + P_D + P_I + P_J + P_K + P_L + 4)/(2*BLOCK_SIZE);
  }
  else if (!block_available_up && block_available_left)//上空左有
  {
    // upper edge
    s0 = (P_I + P_J + P_K + P_L + 2)/BLOCK_SIZE;             
  }
  else if (block_available_up && !block_available_left)//上有左空
  {
    // left edge
    s0 = (P_A + P_B + P_C + P_D + 2)/BLOCK_SIZE;             
  }
  else //if (!block_available_up && !block_available_left)
  {
    // top left corner, nothing to predict from
    
	 //上空左空，也就是没有像素可供预测，此时设置预测值为128（很好）
	 //这个相当于是编解码器的一个约定，如果把128改为1280，那么编码效果特差
	 s0 = 128;                           
  }

        
        通过H.264visa发现，orgMB[0][0] = 43（foreman的第一个像素的亮度是43）. 那么在编码端得到的残差便是：encoderResMB[0][0] = 43 - 128 = -85, 用H.264visa可以看出，解码端得到的残差是decoderResMB[0][0]为-78（有损是必然的），在解码端得到的滤波前的值为preFilterMB[0][0] = -78 + 128 = 50, 这就是解码端的滤波前的值（50和原始的43有一定出入是必然的）.以上分析的是i = j =0时候的情况，其实0<= i, j <= 3时的分析方法完全一致.

      

        (2) 分析第二个4*4块，从H.264visa看出，这个4*4块的预测方式是Horizontal的. 第一个4*4块的滤波前的值为preFilterMB[0][3] = 238,  preFilterMB[1][3] = 205,   preFilterMB[2][3] = 167,  preFilterMB[3][3] = 160.  既然第二个4*4采用的是水平预测方式，那么看下该4*4块的预测值到底是不是这四个数字呢？观察发现 predictMB[0][j] = 238,  predictMB[1][j] = 205,  predictMB[2][j] = 167,  predictMB[3][j] = 160(其中4<=j<=7). 嗯，数据高度一致. 剩下的分析方法与(1)中完全一致.

 

         (3) 帧内预测的参考值是参考块的滤波前的值，而不是重建值，这一点与帧间预测不同，因为帧类预测的参考块还没有来得及滤波呢. 这一点也可以用H.264visa进行验证.