h264 nalu结构
2016-11-24 17:40
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NALU:Coded H.264 data is stored or transmitted as a series of packets known as NetworkAbstraction LayerUnits. (NALU单元)
000000000000000000000000000000010110011101000010000000000001111011110001011000010110001001
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; [cpp] view plain copy 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)
000000000000000000000000000000010110011101000010000000000001111011110001011000010110001001
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