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  • h.264 scanning process for transform coefficients

    宏块在经过变换、量化后,得到大小为4x4或者8x8的矩阵,矩阵中的数据被称为transform coefficient levels。这些level在后面会被用于熵编码,因此我们需要把矩阵按照一定顺序进行扫描,得到数字序列。

    扫描顺序在帧与场会有所不同

    4x4块矩阵的扫描顺序如下

    image               image

    Zig-zag scan(Frame)                          Field scan           

    8x8块矩阵的扫描顺序如下

    image            image

     

    Zig-zag scan(Frame)                                  Field scan           

     

    在实际的代码处理上(JM),对变换系数的扫描过程是包含在量化过程中的,因为对矩阵内的元素的量化也是逐个进行的,因此就可以按照对变换系数扫描的顺序取出矩阵内的元素进行量化,后续就能直接对这些transform coefficient level序列进行熵编码。

    int quant_4x4_normal(Macroblock *currMB, int **tblock, struct quant_methods *q_method)
    { 
      VideoParameters *p_Vid = currMB->p_Vid;
      QuantParameters *p_Quant = p_Vid->p_Quant;
      Slice *currSlice = currMB->p_Slice;
      Boolean is_cavlc = (Boolean) (currSlice->symbol_mode == CAVLC);
      
      int   block_x = q_method->block_x;
      int  qp = q_method->qp;
      int*  ACL = &q_method->ACLevel[0];
      int*  ACR = &q_method->ACRun[0];  
      LevelQuantParams **q_params_4x4 = q_method->q_params;
      const byte (*pos_scan)[2] = q_method->pos_scan;
      const byte *c_cost = q_method->c_cost;
      int *coeff_cost = q_method->coeff_cost;
      
      
      LevelQuantParams *q_params = NULL;
      int i,j, coeff_ctr;
      
      int *m7;
      int scaled_coeff;
      
      int   level, run = 0;
      int   nonzero = FALSE;
      int   qp_per = p_Quant->qp_per_matrix[qp];
      int   q_bits = Q_BITS + qp_per;
      const byte *p_scan = &pos_scan[0][0];
    
      // Quantization
      // 4x4 block matrix has 16 coefficients
      for (coeff_ctr = 0; coeff_ctr < 16; ++coeff_ctr)
      {
        //scanning positions (Zig-zag scan or Field scan)
        i = *p_scan++;  // horizontal position
        j = *p_scan++;  // vertical position
        
        //block_x,block_y here is the position of a block on a Macroblock with the unit of pixel
        m7 = &tblock[j][block_x + i];
    
        if (*m7 != 0)
        {
          q_params = &q_params_4x4[j][i];
          scaled_coeff = iabs (*m7) * q_params->ScaleComp;
          level = (scaled_coeff + q_params->OffsetComp) >> q_bits;
    
          if (level != 0)
          {
            if (is_cavlc)
              level = imin(level, CAVLC_LEVEL_LIMIT);
    
            *coeff_cost += (level > 1) ? MAX_VALUE : c_cost[run];
    
            level  = isignab(level, *m7);
            *m7     = rshift_rnd_sf(((level * q_params->InvScaleComp) << qp_per), 4);
            // inverse scale can be alternative performed as follows to ensure 16bit
            // arithmetic is satisfied.
            // *m7 = (qp_per<4) ? rshift_rnd_sf((level*q_params->InvScaleComp),4-qp_per) : (level*q_params->InvScaleComp)<<(qp_per-4);
            *ACL++ = level;
            *ACR++ = run;
            // reset zero level counter
            run    = 0;
            nonzero = TRUE;
          }
          else
          {
            *m7 = 0;
            ++run;
          }
        }
        else
        {
          ++run;
        }
      }
    
      *ACL = 0;
    
      return nonzero;
    }
    
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  • 原文地址:https://www.cnblogs.com/TaigaCon/p/5245108.html
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