openh264--基礎庫

原創

2020-02-20 22:56

依次介紹bit數組，

bit數組

/*
* Bit-stream auxiliary reading / writing
*/
typedef struct TagBitStringAux {
uint8_t* pStartBuf;   // buffer to start position
uint8_t* pEndBuf;     // buffer + length
int32_t iBits;       // count bits of overall bitstreaming input

intX_t   iIndex;      //only for cavlc usage
uint8_t* pCurBuf;     // current reading position
uint32_t uiCurBits;
int32_t iLeftBits;   // count number of available bits left ([1, 8]),
// need pointer to next byte start position in case 0 bit left then 8 instead
} SBitStringAux, *PBitStringAux;

#define WRITE_BE_32(ptr, val) do { \
        (ptr)[0] = (val) >> 24; \
        (ptr)[1] = (val) >> 16; \
        (ptr)[2] = (val) >> 8; \
        (ptr)[3] = (val) >> 0; \
    } while (0)

InitBits 初始化，準備一塊內存。

static inline int32_t InitBits (SBitStringAux* pBs, const uint8_t* kpBuf, const int32_t kiSize) {
uint8_t* ptr = (uint8_t*)kpBuf;

pBs->pStartBuf = ptr; //開始指針
pBs->pCurBuf   = ptr; //當前指針
pBs->pEndBuf   = ptr + kiSize; //結束指針
pBs->iLeftBits = 32;   //可存儲32個字節
pBs->uiCurBits = 0; //當前數值

return kiSize;
}

BsWriteBits 寫入iLen長的值kuiValue

static inline int32_t BsWriteBits (PBitStringAux pBitString, int32_t iLen, const uint32_t kuiValue) {
if (iLen < pBitString->iLeftBits) {
    pBitString->uiCurBits = (pBitString->uiCurBits << iLen) | kuiValue;   //直接寫入
    pBitString->iLeftBits -= iLen;
} else {
    iLen -= pBitString->iLeftBits;
    pBitString->uiCurBits = (pBitString->uiCurBits << pBitString->iLeftBits) | (kuiValue >> iLen);

    //剩餘空間不足，只能先寫iLeftBits個值到uiCurBits ，接着把32個bit放到pCurBuf中，然後再寫未寫入的值。
    WRITE_BE_32 (pBitString->pCurBuf, pBitString->uiCurBits);
    pBitString->pCurBuf += 4;
    pBitString->uiCurBits = kuiValue & ((1 << iLen) - 1);
    pBitString->iLeftBits = 32 - iLen;

接着寫剩下的值到uiCurBits 。

}
return 0;
}

BsWriteOneBit：寫入一個Bit。

static inline int32_t BsWriteOneBit (PBitStringAux pBitString, const uint32_t kuiValue) {
BsWriteBits (pBitString, 1, kuiValue);
return 0;
}

BsFlush：對32位剩下的BIT填0，湊夠32位，然後寫入pCurBuf，uiCurBits改爲0， iLeftBits改爲32。

static inline int32_t BsFlush (PBitStringAux pBitString) {
WRITE_BE_32 (pBitString->pCurBuf, pBitString->uiCurBits << pBitString->iLeftBits);
pBitString->pCurBuf += 4 - pBitString->iLeftBits / 8;
pBitString->iLeftBits = 32;
pBitString->uiCurBits = 0;
return 0;
}

K=0的UE編碼，對於 k =0時：CodeNum=3。編碼如下：
二進制表示爲11，去掉k=0位後加1得100；
所以M=2；所以編碼後結果爲[MZeros][1][Info] = [MZeros][1 Info] = 00100

/*
* Write unsigned exp golomb codes
*/

static inline int32_t BsWriteUE (PBitStringAux pBitString, const uint32_t kuiValue) {
uint32_t iTmpValue = kuiValue + 1;
if (256 > kuiValue) {
    BsWriteBits (pBitString, g_kuiGolombUELength[kuiValue], kuiValue + 1);
} else {
    uint32_t n = 0;
    if (iTmpValue & 0xffff0000) {
      iTmpValue >>= 16;
      n += 16;
    }
    if (iTmpValue & 0xff00) {
      iTmpValue >>= 8;
      n += 8;
    }

//n += (g_kuiGolombUELength[iTmpValue] >> 1);

n += (g_kuiGolombUELength[iTmpValue - 1] >> 1);
BsWriteBits (pBitString, (n << 1) + 1, kuiValue + 1);
}
return 0;
}

K=0的SE編碼

/*
* Write signed exp golomb codes
*/
static inline int32_t BsWriteSE (PBitStringAux pBitString, const int32_t kiValue) {
uint32_t iTmpValue;
if (0 == kiValue) {
    BsWriteOneBit (pBitString, 1);
} else if (0 < kiValue) {
    iTmpValue = (kiValue << 1) - 1;
    BsWriteUE (pBitString, iTmpValue);
} else {
    iTmpValue = ((-kiValue) << 1);
    BsWriteUE (pBitString, iTmpValue);
}
return 0;
}