H.264原始码流（又称为“裸流”）是由一个一个的NALU组成的。他们的结构如下图所示。
其中每个NALU之间通过startcode（起始码）进行分隔，起始码分成两种：0x000001（3Byte）或者0x00000001（4Byte）。如果NALU对应的Slice为一帧的开始就用0x00000001，否则就用0x000001。
H.264码流解析的步骤就是首先从码流中搜索0x000001和0x00000001，分离出NALU；然后再分析NALU的各个字段。本文的程序即实现了上述的两个步骤。
整个程序位于simplest_h264_parser()函数中，如下所示
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef enum {
NALU_TYPE_SLICE = 1,
NALU_TYPE_DPA = 2,
NALU_TYPE_DPB = 3,
NALU_TYPE_DPC = 4,
NALU_TYPE_IDR = 5,
NALU_TYPE_SEI = 6,
NALU_TYPE_SPS = 7,
NALU_TYPE_PPS = 8,
NALU_TYPE_AUD = 9,
NALU_TYPE_EOSEQ = 10,
NALU_TYPE_EOSTREAM = 11,
NALU_TYPE_FILL = 12,
} NaluType;
typedef enum {
NALU_PRIORITY_DISPOSABLE = 0,
NALU_PRIRITY_LOW = 1,
NALU_PRIORITY_HIGH = 2,
NALU_PRIORITY_HIGHEST = 3
} NaluPriority;
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 forbidden_bit; //! should be always FALSE
int nal_reference_idc; //! NALU_PRIORITY_xxxx
int nal_unit_type; //! NALU_TYPE_xxxx
char *buf; //! contains the first byte followed by the EBSP
} NALU_t;
FILE *h264bitstream = NULL; //!< the bit stream file
int info2=0, info3=0;
static int FindStartCode2 (unsigned char *Buf){
if(Buf[0]!=0 || Buf[1]!=0 || Buf[2] !=1) return 0; //0x000001?
else return 1;
}
static int FindStartCode3 (unsigned char *Buf){
if(Buf[0]!=0 || Buf[1]!=0 || Buf[2] !=0 || Buf[3] !=1) return 0;//0x00000001?
else return 1;
}
int GetAnnexbNALU (NALU_t *nalu){
int pos = 0;
int StartCodeFound, rewind;
unsigned char *Buf;
if ((Buf = (unsigned char*)calloc (nalu->max_size , sizeof(char))) == NULL)
printf ("GetAnnexbNALU: Could not allocate Buf memory\n");
nalu->startcodeprefix_len=3;
if (3 != fread (Buf, 1, 3, h264bitstream)){
free(Buf);
return 0;
}
info2 = FindStartCode2 (Buf);
if(info2 != 1) {
if(1 != fread(Buf+3, 1, 1, h264bitstream)){
free(Buf);
return 0;
}
info3 = FindStartCode3 (Buf);
if (info3 != 1){
free(Buf);
return -1;
}
else {
pos = 4;
nalu->startcodeprefix_len = 4;
}
}
else{
nalu->startcodeprefix_len = 3;
pos = 3;
}
StartCodeFound = 0;
info2 = 0;
info3 = 0;
while (!StartCodeFound){
if (feof (h264bitstream)){
nalu->len = (pos-1)-nalu->startcodeprefix_len;
memcpy (nalu->buf, &Buf[nalu->startcodeprefix_len], nalu->len);
nalu->forbidden_bit = nalu->buf[0] & 0x80; //1 bit
nalu->nal_reference_idc = nalu->buf[0] & 0x60; // 2 bit
nalu->nal_unit_type = (nalu->buf[0]) & 0x1f;// 5 bit
free(Buf);
return pos-1;
}
Buf[pos++] = fgetc (h264bitstream);
info3 = FindStartCode3(&Buf[pos-4]);
if(info3 != 1)
info2 = FindStartCode2(&Buf[pos-3]);
StartCodeFound = (info2 == 1 || info3 == 1);
}
// Here, we have found another start code (and read length of startcode bytes more than we should
// have. Hence, go back in the file
rewind = (info3 == 1)? -4 : -3;
if (0 != fseek (h264bitstream, rewind, SEEK_CUR)){
free(Buf);
printf("GetAnnexbNALU: Cannot fseek in the bit stream file");
}
// Here the Start code, the complete NALU, and the next start code is in the Buf.
// The size of Buf is pos, pos+rewind are the number of bytes excluding the next
// start code, and (pos+rewind)-startcodeprefix_len is the size of the NALU excluding the start code
nalu->len = (pos+rewind)-nalu->startcodeprefix_len;
memcpy (nalu->buf, &Buf[nalu->startcodeprefix_len], nalu->len);//
nalu->forbidden_bit = nalu->buf[0] & 0x80; //1 bit
nalu->nal_reference_idc = nalu->buf[0] & 0x60; // 2 bit
nalu->nal_unit_type = (nalu->buf[0]) & 0x1f;// 5 bit
free(Buf);
return (pos+rewind);
}
/**
* Analysis H.264 Bitstream
* @param url Location of input H.264 bitstream file.
*/
int simplest_h264_parser(char *url){
NALU_t *n;
int buffersize=100000;
//FILE *myout=fopen("output_log.txt","wb+");
FILE *myout=stdout;
h264bitstream=fopen(url, "rb+");
if (h264bitstream==NULL){
printf("Open file error\n");
return 0;
}
n = (NALU_t*)calloc (1, sizeof (NALU_t));
if (n == NULL){
printf("Alloc NALU Error\n");
return 0;
}
n->max_size=buffersize;
n->buf = (char*)calloc (buffersize, sizeof (char));
if (n->buf == NULL){
free (n);
printf ("AllocNALU: n->buf");
return 0;
}
int data_offset=0;
int nal_num=0;
printf("-----+-------- NALU Table ------+---------+\n");
printf(" NUM | POS | IDC | TYPE | LEN |\n");
printf("-----+---------+--------+-------+---------+\n");
while(!feof(h264bitstream))
{
int data_lenth;
data_lenth=GetAnnexbNALU(n);
char type_str[20]={0};
switch(n->nal_unit_type){
case NALU_TYPE_SLICE:sprintf(type_str,"SLICE");break;
case NALU_TYPE_DPA:sprintf(type_str,"DPA");break;
case NALU_TYPE_DPB:sprintf(type_str,"DPB");break;
case NALU_TYPE_DPC:sprintf(type_str,"DPC");break;
case NALU_TYPE_IDR:sprintf(type_str,"IDR");break;
case NALU_TYPE_SEI:sprintf(type_str,"SEI");break;
case NALU_TYPE_SPS:sprintf(type_str,"SPS");break;
case NALU_TYPE_PPS:sprintf(type_str,"PPS");break;
case NALU_TYPE_AUD:sprintf(type_str,"AUD");break;
case NALU_TYPE_EOSEQ:sprintf(type_str,"EOSEQ");break;
case NALU_TYPE_EOSTREAM:sprintf(type_str,"EOSTREAM");break;
case NALU_TYPE_FILL:sprintf(type_str,"FILL");break;
}
char idc_str[20]={0};
switch(n->nal_reference_idc>>5){
case NALU_PRIORITY_DISPOSABLE:sprintf(idc_str,"DISPOS");break;
case NALU_PRIRITY_LOW:sprintf(idc_str,"LOW");break;
case NALU_PRIORITY_HIGH:sprintf(idc_str,"HIGH");break;
case NALU_PRIORITY_HIGHEST:sprintf(idc_str,"HIGHEST");break;
}
fprintf(myout,"%5d| %8d| %7s| %6s| %8d|\n",nal_num,data_offset,idc_str,type_str,n->len);
data_offset=data_offset+data_lenth;
nal_num++;
}
//Free
if (n){
if (n->buf){
free(n->buf);
n->buf=NULL;
}
free (n);
}
return 0;
}
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