类分析_CABAC__CabacContextModel

 // CabacContextModel头文件：

class H264AVCCOMMONLIB_API CabacContextModel
{
public:
	CabacContextModel();
	~CabacContextModel();

  const UChar getState()           { return m_ucState>>1; }
  const UChar getMps()             { return m_ucState&1;  }
  Void toggleMps()                 { m_ucState ^= 1;      }
  Void setState( UChar ucState )   { m_ucState = (ucState<<1)+getMps(); }

  Void init( Short asCtxInit[], Int iQp )
  {
	  Int iState = ( ( asCtxInit[0] * iQp ) >> 4 ) + asCtxInit[1];
    iState = min (max ( 1, iState), 126 );

    if (iState>=64)
    {
      m_ucState = iState - 64;
      m_ucState += m_ucState + 1;
    }
    else
    {
      m_ucState = 63 - iState;
      m_ucState += m_ucState;
    }
    m_uiCount = 0;
  }

  Void initEqualProbability()
  {
    m_ucState = 0;
    m_uiCount = 0;
  }

  Void  incrementCount()  { m_uiCount++; }

private:
  UChar m_ucState;
  UInt  m_uiCount;

  static  const Double m_afProbability[128];
  static  const Double m_afEntropy    [128];
};

两个私有成员，m_uiCount似乎是上下文的编号，m_ucState最后一位表示MPS符号是什么，前面的位数表示

m_afProbability[128]和m_afEntropy [128] 是常数

 

构造函数和initEqualProbability都把m_ucState和m_uiCount初始化为零。

函数incrementCount：当输入的是MPS符号时，sigma右移一位，表现为m_uiCount自增1
 toggleMps：交换MPS和LPS符号

 

 

类CabacContextModel2DBuffer：

成员变量

 CabacContextModel*  m_pcCContextModel;
  const UInt          m_uiSizeX;
  const UInt          m_uiSizeY;

事实上形成了一个 m_uiSizeX * m_uiSizeY 的CabacContextModel数组，m_uiSizeX 是列数

 初始化：

Void init( Short asCtxInit[], Int iQp )
  {
	  Int iState = ( ( asCtxInit[0] * iQp ) >> 4 ) + asCtxInit[1];
    iState = min (max ( 1, iState), 126 );

    if (iState>=64)
    {
      m_ucState = iState - 64;
      m_ucState += m_ucState + 1;
    }
    else
    {
      m_ucState = 63 - iState;
      m_ucState += m_ucState;
    }
    m_uiCount = 0;
  }

 

函数：

CabacContextModel* get( UInt uiY )                          取得第uiY 的地址

CabacContextModel& get( UInt uiY, UInt uiX )          取得第uiY行第uiX 列的元素

 ErrVal initBuffer( Short* psCtxModel, Int iQp )  初始化

/******************************************************************************************************************************/

CabacTables.h:记录了三张表格

g_aucLPSTable64x4  ：64行4列的表格

g_aucACNextStateMPS64   ：新来的符号是MPS时，状态下标的迁移

g_aucACNextStateLPS64  ： 新来的符号时LPS时，状态下标的迁移

const UChar g_aucACNextStateMPS64[64] =
{
   1, 2, 3, 4, 5, 6, 7, 8, 9,10,
  11,12,13,14,15,16,17,18,19,20,
  21,22,23,24,25,26,27,28,29,30,
  31,32,33,34,35,36,37,38,39,40,
  41,42,43,44,45,46,47,48,49,50,
  51,52,53,54,55,56,57,58,59,60,
  61,62,62,63
};

 

const UChar g_aucACNextStateLPS64[64] =
{
   0, 0, 1, 2, 2, 4, 4, 5, 6, 7,
   8, 9, 9,11,11,12,13,13,15,15,
  16,16,18,18,19,19,21,21,22,22,
  23,24,24,25,26,26,27,27,28,29,
  29,30,30,30,31,32,32,33,33,33,
  34,34,35,35,35,36,36,36,37,37,
  37,38,38,63
};