H264解析

H264的结构：

H264是由一个个NALU组成，NALU之间通过分隔符进行分割，一般为0x000001(3Byte)或者0x00000001(4Byte)，一般在NALU开始的时候使用0x00000001(4Byte)。

NALU	NALU	NALU	NALU	NALU	NALU	……
SPS	PPS	SEI	I Frame	P Frame	B Frame	……

NALU详解：

NALU分为多种：SPS、PPS、SEI、IDR、P Frame、B Frame，其中：

SPS：Sequence Paramater Set序列参数集，其nal_unit_type为7， SPS中保存了一组视频编码序列的全局参数，序列中每一帧编码后的数据所依赖的参数保存于图像参数集中，一般SPS和PPS位于整个码流的起始位置，但是在某些特殊情况下，在码流中间也会出现这2种结构，如：解码器需要在码流中间开始解码，编码器在编码的过程中改变了码流的参数(如分辨率)。

 

PPS：Picture Paramater Set图像参数集，其nal_unit_type为8。

 

SEI：主要起补充和增强的作用，其nal_unit_type为6。

 

I Frame：关键帧，其nal_unit_type为5，slice_type为7，解码时只需要本帧数据就可以完成。

 

P Frame：其nal_unit_type为1，slice_type为5，表示本帧与之前的一个关键帧的差别，解码时需要用之前缓存的画面叠加上本帧定义的差别，生成画面。

 

B Frame：其nal_unit_type为1，slice_type为6，双向差别帧，解码时需要用到之前缓存的画面，同时还需要之后的画面，通过前后画面与本帧数据的叠加获得最终的画面。

 解析代码参考雷神写的即可https://blog.youkuaiyun.com/leixiaohua1020/article/details/50534369

 

#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;

/*
* 判断是否为0x000001
*/
static int FindStartCode2 (unsigned char *Buf){
	if(Buf[0]!=0 || Buf[1]!=0 || Buf[2] !=1) return 0; //0x000001?
	else return 1;
}
/*
* 判断是否为0x00000001
*/
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;

    // 找到下一个分界标记位
	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; // 回退Bytes若为0x00000001则需要回退4Bytes，若为0x000001则需要回退3位
 
	if (0 != fseek (h264bitstream, rewind, SEEK_CUR)){ // 回退到上一个NALU的末尾
		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;
}