H.264中NALU、RBSP、SODB的关系 (弄清码流结构)

      NALU：Coded H.264 data is stored or transmitted as a series of
packets known as NetworkAbstraction
LayerUnits. （NALU单元）

      RBSP ：A NALU contains a Raw
Byte Sequence
Payload, a sequence of bytes containingsyntax elements.（原始数据字节流）

      SODB：String
Of Data Bits （原始数据比特流, 长度不一定是8的倍数，故需要补齐）

 

      逻辑关系：

 

                                                 
SODB  + RBSP trailing bits    =  RBSP

                                                      
NAL header(1 byte)      +      RBSP   = NALU

     Start Code Prefix(3 bytes)  +   NALU  +  Start Code Prefix(3 bytes)  +   NALU  
+ ...+  = H.264BitsStream

 

     说明：

    1.
SODB即编码形成的真实码流，为了使一个RBSP为整字节数，需要加trailing bits, 具体加的方法可以看JM8.6中的SODBtoRBSP函数.

[cpp] view plaincopy

1. void SODBtoRBSP(Bitstream *currStream)  
2. {  
3.   currStream->byte_buf <<= 1;  
4.   currStream->byte_buf |= 1;  
5.   currStream->bits_to_go--;  
6.   currStream->byte_buf <<= currStream->bits_to_go;  
7.   currStream->streamBuffer[currStream->byte_pos++] = currStream->byte_buf;  
8.   currStream->bits_to_go = 8;  
9.   currStream->byte_buf = 0;  
10. }  

 

      2. NALU header为一个字节，这8个比特分别对应forbidden_zero_bit,
nal_ref_idc, nal_unit_type. NALU的body其实就是RBSP. 由RBSP转NALU是由RBSPtoNALU函数来实现的.

[cpp] view plaincopy

1. typedef struct   
2. {  
3.   int startcodeprefix_len;      //! 4 for parameter sets and first slice in picture, 3 for everything else (suggested)  
4.   unsigned len;                 //! Length of the NAL unit (Excluding the start code, which does not belong to the NALU)  
5.   unsigned max_size;            //! Nal Unit Buffer size  
6.   int nal_unit_type;            //! NALU_TYPE_xxxx  
7.   int nal_reference_idc;        //! NALU_PRIORITY_xxxx  
8.   int forbidden_bit;            //! should be always FALSE  
9.   byte *buf;        //! conjtains the first byte followed by the EBSP  
10. } NALU_t;  

[cpp] view plaincopy

1. int RBSPtoNALU (char *rbsp, NALU_t *nalu, int rbsp_size, int nal_unit_type, int nal_reference_idc,   
2.                 int min_num_bytes, int UseAnnexbLongStartcode)  
3. {  
4.   int len;  
5.   
6.   // 断言，以后要学会用assert进行断言，很重要滴.  
7.   assert (nalu != NULL);  
8.   assert (nal_reference_idc <=3 && nal_reference_idc >=0);  
9.   assert (nal_unit_type > 0 && nal_unit_type <= 10);  
10.   assert (rbsp_size < MAXRBSPSIZE);  
11.   
12.   // 下面这个是必须的，所以不需要通过参数传进来  
13.   nalu->forbidden_bit = 0;  
14.   // 下面两个通过参数传进来  
15.   nalu->nal_reference_idc = nal_reference_idc;  
16.   nalu->nal_unit_type = nal_unit_type;  
17.   
18.   // 判断是否在Start Code Prefix前面加Ox00  
19.   nalu->startcodeprefix_len = UseAnnexbLongStartcode?4:3;  
20.     
21.   // 对nalu->buf[i]进行赋值  
22.   nalu->buf[0] =  
23.     nalu->forbidden_bit << 7      |  
24.     nalu->nal_reference_idc << 5  |  
25.     nalu->nal_unit_type;  
26.   memcpy (&nalu->buf[1], rbsp, rbsp_size);  
27.   
28. // printf ("First Byte %x\n", nalu->buf[0]);  
29. // printf ("RBSPtoNALU: Before: NALU len %d\t RBSP %x %x %x %x\n", rbsp_size, (unsigned) nalu->buf[1], (unsigned) nalu->buf[2], (unsigned) nalu->buf[3], (unsigned) nalu->buf[4]);  
30.   
31.   len = 1 + RBSPtoEBSP (&nalu->buf[1], 0, rbsp_size, min_num_bytes);  
32.   
33. // printf ("RBSPtoNALU: After : NALU len %d\t EBSP %x %x %x %x\n", rbsp_size, (unsigned) nalu->buf[1], (unsigned) nalu->buf[2], (unsigned) nalu->buf[3], (unsigned) nalu->buf[4]);  
34. // printf ("len %d\n\n", len);  
35.   nalu->len = len;  
36.   
37.   return len;  
38. }  

 

      3. Start Code
Prefix为3个字节. 但是，为了寻址方便，要求数据流在长度上对齐，因此H.264建议在Start Code Prefix前面加若干个0.    

      4. 为了简便起见，上面的逻辑关系图没有考虑"防止竞争"机制.

 

     编码foreman_part_qcif.yuv的第一帧，码流如下:
(对照trace_enc.txt分析即可，由于码流太多，篇幅有限，故不一一分析)

00000000000000000000000000000001011001110100001000000000000111101111000101100001011000100110001000000000000000000000000000000001011010001100100010100001010000111000100000000000000000000000000000000001011001011000100010000100000000100110001101100001011111000000000000011101011011101111110000000000011100111110000011000001011101110011000011110010000101000011101101100101101001000100100101110101001111100001011000000110100110000000011111111111100111110010111110010110000000001010100110101001011101001110011001101110000010010101010100011010000001011001000101100101110010000100101011001110001000000100111111000000010000100000101100100100110001101010110000100100111100000111000000001001000001110100100011011111100100010001110110101001111101010000101001101111010000111010110101111110011001010011001001111000101100000011100100100000110000000010111010001101101001001111100101110011100000010000010100011101010000100010100011100101111111110001110001110111011101110111111001001110010111100000011011010011011011100000001011010100111101000010110010110010000101001011110110111101111100000101101010101100000011000000100101011010110111000100000000011100100110011011100011001001101010101001001111010000011011111001110101101010111011100111001001000001110010110001011101000010100010110100110101111111111110000000010000001000010010010011001000111111101111100011001001111101000111111110111011111011110011111111101110111101010011000010001111100101000111111111111000110001101001000111110101100000110100011000111101101110101100101011101010010111000010110010011001010000001011001011010010011011100011000010010001111000001101100011100110000101001001101100111010101100110011110000011101000100010111111110111011101110000011110111011111011110101111010100000001000000001001100000101100011111111100010011101101001111111101000000100011010000101000111011100010110000001100111000111111110100010011001011111111111100111110110010001001111001010001010000100111101010111101011111111011001010111101000001111000101011001101110011110111101110111110111011111100111111110101000000010010000010011000101100111001011110000011000100111010001010110111111111110010001101111010010000000111110010011111000001000001001100000110111110000110001101001000010001110101001001011111100011001111000101101010011111000001110111001011000011110000101100111110110000101001111000001011111000110111110111110001010111111111100001011010101110010111110111110111000111100110111001111101101001100011111001110000110100111011111110101001111001100111010111101010011110011011100000000001110111101101011011000010011100100110001101100111010101001100110111001110110100000011001101100101000010011110000010001110000100110111100100000101010110010110111110110000100011111100110101101011110001000111110101100100101101111011000110001101111001101001101001111111100111010101111100111111100000010100010101100001101101010011010111001001011011111001100111101111010010110110110111011001010010010101101101100101101001000011011011110110111101010010001010001101101100111010101101001110010101000110010111001100111100111100111010010100111011100001110101001010000011100101110000010111011001110111110010110010001100000000000100001101100001100101110101111011011101010010000011000101011001101100110011101110010111110001010111010111100011001000010001011111111000011111000100111000101110111100011100010111100111101001001111001010011001111011100000000010100000100010100100001010110000011111011010011111001001110001001110000100010000000111100100101111101001100010111101100000010100000001001010001000100111011001110100000100110101010001110100101111111111010111100011001111100000100011001110011011010101011110000000110001001110110010111001001110011001111100111111001111010010101011101111100100001101100011101001011111011000001011011101010100010111001001101011001110010011010011110101110010000011111010000000001001100101010100011000001000000101101101101010011110010011001011000100010100000100011110000000100110111101000111100001010010011101011001101011011101100010111101000111101100111110100011111001011011101100101101001010011000001000010011001111000111111110010110010101010010011110101101011110110000010100000011011110011100001000001111101110110011010110001100100110110110000110010000000100001110110111100100110101000101101100000111111011011111011111101101111101001111111001101000011011011000000001000100110110100100001101001011011000011010001111111000101001111101101111011100000000100001010110110000001011000111111110100010011000110000110001011100000010000110001011111100100000100010111110010111111000011101100100111001100101011010111101111011101110111011111101010101010011100000001000000001011000010110001011111110001000000001010001011000101100101100110011001100001111100010110100100011110110000011111111111100010101001111001111101000011010001001101001001100110011001111001100100110010101110000000110111100111011101000001111000101011010101110100001010110101110010110111011101111000001110001111111111100101110111100100111101010001110011111010001101111000001100001101111001001000000011111001001111100010111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