H264碼流中 分離和解析 nal

H.264碼流解析的步驟就是首先從碼流中搜索0x000001和0x00000001，

然後再分離出NALU；分析NALU的各個字段。

找到00 00 01 或 00 00 00 01

 if (3 != fread (Buf, 1, 3, h264bitstream)){  
        free(Buf);  
        return 0;  
    }  
    info2 = FindStartCode2 (Buf);  
    if(info2 != 1) {  
        if(1 != fread(Buf+3, 1, 1, h264bitstream)){  
            free(Buf);  
            return 0;  
        }  
        info3 = FindStartCode3 (Buf);  
        if (info3 != 1){   
            free(Buf);  
            return -1;  
        }  
        else {  
            pos = 4;  
            nalu->startcodeprefix_len = 4;  
        }  
    }  
    else{  
        nalu->startcodeprefix_len = 3;  
        pos = 3;  
    }  
    StartCodeFound = 0;  
    info2 = 0;  
    info3 = 0;  

分離NALU程序：

 static int FindStartCode2 (unsigned char *Buf){  
    if(Buf[0]!=0 || Buf[1]!=0 || Buf[2] !=1) return 0; //0x000001?  
    else return 1;  
}  
  
static int FindStartCode3 (unsigned char *Buf){  
    if(Buf[0]!=0 || Buf[1]!=0 || Buf[2] !=0 || Buf[3] !=1) return 0;//0x00000001?  
    else return 1;  
}  

while (!StartCodeFound){  
        if (feof (h264bitstream)){  
            nalu->len = (pos-1)-nalu->startcodeprefix_len;  
            memcpy (nalu->buf, &Buf[nalu->startcodeprefix_len], nalu->len);       
            nalu->forbidden_bit = nalu->buf[0] & 0x80; //1 bit  
            nalu->nal_reference_idc = nalu->buf[0] & 0x60; // 2 bit  
            nalu->nal_unit_type = (nalu->buf[0]) & 0x1f;// 5 bit  
            free(Buf);  
            return pos-1;  
        }  
        Buf[pos++] = fgetc (h264bitstream);  
        info3 = FindStartCode3(&Buf[pos-4]);  
        if(info3 != 1)  
            info2 = FindStartCode2(&Buf[pos-3]);  
        StartCodeFound = (info2 == 1 || info3 == 1);  
    }  

解析：

 rewind = (info3 == 1)? -4 : -3;  
fseek (h264bitstream, rewind, SEEK_CUR)
 nalu->len = (pos+rewind)-nalu->startcodeprefix_len;  
    memcpy (nalu->buf, &Buf[nalu->startcodeprefix_len], nalu->len);//  
    nalu->forbidden_bit = nalu->buf[0] & 0x80; //1 bit  
    nalu->nal_reference_idc = nalu->buf[0] & 0x60; // 2 bit  
    nalu->nal_unit_type = (nalu->buf[0]) & 0x1f;// 5 bit