H.264 中的指數哥倫布編碼

          H.264的一個重要特色就是採用UVLC（Universal Variable Length coding, 統一可變長編碼）編碼，提高了編碼效率。

          UVLC編碼中有一種編碼方式就是指數哥倫布編碼。程式實現該編碼時需要計算編碼長度len及其value，下面分別對無符號和有符號兩種指數哥倫布編碼進行分析。

          設編碼值為code_val。

    static const int i_size0_255[256] =
    {
        1,1,2,2,3,3,3,3,4,4,4,4,4,4,4,4,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
        6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
        7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
        7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
        8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
        8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
        8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
        8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8
    };

    void bs_write( bs_t *s, int bit_len, int value); 

1. ue(v)

        1) value = code_val+1        2)len = 2*i_size0_255[value]+1編程實現如下：

void ue_v( bs_t *s, unsigned int code_val )
{
    int i_size = 0;
    if( val == 0 )
    {
        bs_write( s, 1, 1); 
    }
    else
    {
        unsigned int tmp = ++code_val;        if( tmp >= 0x00010000 )
        {
            i_size += 16;
            tmp >>= 16;
        }
        if( tmp >= 0x100 )
        {
            i_size += 8;
            tmp >>= 8;
        }
        i_size += i_size0_255[tmp];        bs_write( s, 2 * i_size - 1, code_val);
    }
}

2. se(v)

        1) 若code_val>0:    value = 2*code_val-1            若code_val>0:     value = -2*val       2)len = 2*i_size0_255[value]+1編程實現如下：

static void se_v( bs_t *s, int code_val )
{
    ue_v( s, code_val <= 0 ? -code_val  * 2 : code_val * 2 - 1);
}