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 plain copy

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;
}
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 plain copy

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;
}
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.
为了简便起见，上面的逻辑关系图没有考虑"防止竞争"机制.