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.

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.

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;

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

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)

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