H.264中NALU、RBSP、SODB的关系 (弄清码流结构)
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http://blog.csdn.net/stpeace/article/details/8221945
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.
- void SODBtoRBSP(Bitstream *currStream)
- {
- currStream->byte_buf <<= 1;
- currStream->byte_buf |= 1;
- currStream->bits_to_go--;
- currStream->byte_buf <<= currStream->bits_to_go;
- currStream->streamBuffer[currStream->byte_pos++] = currStream->byte_buf;
- currStream->bits_to_go = 8;
- currStream->byte_buf = 0;
- }
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.
- typedef struct
- {
- int startcodeprefix_len; //! 4 for parameter sets and first slice in picture, 3 for everything else (suggested)
- unsigned len; //! Length of the NAL unit (Excluding the start code, which does not belong to the NALU)
- unsigned max_size; //! Nal Unit Buffer size
- int nal_unit_type; //! NALU_TYPE_xxxx
- int nal_reference_idc; //! NALU_PRIORITY_xxxx
- int forbidden_bit; //! should be always FALSE
- byte *buf; //! conjtains the first byte followed by the EBSP
- } NALU_t;
- int RBSPtoNALU (char *rbsp, NALU_t *nalu, int rbsp_size, int nal_unit_type, int nal_reference_idc,
- int min_num_bytes, int UseAnnexbLongStartcode)
- {
- int len;
- // 断言,以后要学会用assert进行断言,很重要滴.
- assert (nalu != NULL);
- assert (nal_reference_idc <=3 && nal_reference_idc >=0);
- assert (nal_unit_type > 0 && nal_unit_type <= 10);
- assert (rbsp_size < MAXRBSPSIZE);
- // 下面这个是必须的,所以不需要通过参数传进来
- nalu->forbidden_bit = 0;
- // 下面两个通过参数传进来
- nalu->nal_reference_idc = nal_reference_idc;
- nalu->nal_unit_type = nal_unit_type;
- // 判断是否在Start Code Prefix前面加Ox00
- nalu->startcodeprefix_len = UseAnnexbLongStartcode?4:3;
- // 对nalu->buf[i]进行赋值
- nalu->buf[0] =
- nalu->forbidden_bit << 7 |
- nalu->nal_reference_idc << 5 |
- nalu->nal_unit_type;
- memcpy (&nalu->buf[1], rbsp, rbsp_size);
- // printf ("First Byte %x\n", nalu->buf[0]);
- // 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]);
- len = 1 + RBSPtoEBSP (&nalu->buf[1], 0, rbsp_size, min_num_bytes);
- // 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]);
- // printf ("len %d\n\n", len);
- nalu->len = len;
- return len;
- }
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)
000000000000000000000000000000010110011101000010000000000001111011110001011000010110001001100010000000000000000000000000
- H.264中NALU、RBSP、SODB的关系 (弄清码流结构)
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