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CAVLC编码过程详解编码过程:
假设有一个4*4数据块 { 0, 3, -1, 0, 0, -1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0 } 数据重排列:0,3,0,1,-1,-1,0,1,0…… 1) 初始值设定: 非零系数的数目(TotalCoeffs) = 5; 拖尾系数的数目(TrailingOnes)= 3; 最后一个非零系数前零的数目(Total_zeros) = 3; 变量NC=1; (说明:NC值的确定:色度的直流系数NC=-1;其他系数类型NC值是根据当前块左边4*4块的非零系数数目(NA)当前块上面4*4块的非零系数数目(NB)求得的,见毕厚杰书P120表6.10) suffixLength = 0; i = TotalCoeffs = 5; 2) 编码coeff_token: 查标准(BS ISO/IEC 14496-10:2003)Table 9-5,可得: If (TotalCoeffs == 5 && TrailingOnes == 3 && 0 <= NC < 2) coeff_token = 0000 100; Code = 0000 100; 3) 编码所有TrailingOnes的符号: 逆序编码,三个拖尾系数的符号依次是+(0),-(1),-(1); 即: TrailingOne sign[i--] = 0; TrailingOne sign[i--] = 1; TrailingOne sign[i--] = 1; Code = 0000 1000 11; 4) 编码除了拖尾系数以外非零系数幅值Levels: 过程如下: (1)将有符号的Level[ i ]转换成无符号的levelCode; 如果Level[ i ]是正的,levelCode = (Level[ i ]<<1) – 2; 如果Level[ i ]是负的,levelCode = - (Level[ i ]<<1) – 1; (2)计算level_prefix:level_prefix = levelCode / (1<<suffixLength); 查表9-6可得所对应的bit string; (3)计算level_suffix:level_suffix = levelCode % (1<<suffixLength); (4)根据suffixLength的值来确定后缀的长度; (5)suffixLength updata: If ( suffixLength == 0 ) suffixLength++; else if ( levelCode > (3<<suffixLength-1) && suffixLength <6) suffixLength++; 回到例子中,依然按照逆序,Level[i--] = 1;(此时i = 1) levelCode = 0;level_prefix = 0; 查表9-6,可得level_prefix = 0时对应的bit string = 1; 因为suffixLength初始化为0,故该Level没有后缀; 因为suffixLength = 0,故suffixLength++; Code = 0000 1000 111; 编码下一个Level:Level[0] = 3; levelCode = 4;level_prefix = 2;查表得bit string = 001; level_suffix = 0;suffixLength = 1;故码流为0010; Code = 0000 1000 1110 010; i = 0,编码Level结束。 5)编码最后一个非零系数前零的数目(TotalZeros): 查表9-7,当TotalCoeffs = 5,total_zero = 3时,bit string = 111; Code = 0000 1000 1110 0101 11; 6) 对每个非零系数前零的个数(RunBefore)进行编码: i = TotalCoeffs = 5;ZerosLeft = Total_zeros = 3;查表9-10: 依然按照逆序编码 ZerosLeft =3, run_before = 1 run_before[4]=10; ZerosLeft =2, run_before = 0 run_before[3]=1; ZerosLeft =2, run_before = 0 run_before[2]=1; ZerosLeft =2, run_before = 1 run_before[1]=01; ZerosLeft =1, run_before = 1 run_before[0]不需要码流来表示 Code = 0000 1000 1110 0101 1110 1101; 编码完毕。 |