本文介绍了MP4文件格式的组成，包括封装格式、视频和音频参数，以及如何解析MP4文件。通过理解MP4的box结构，如ftyp、mdat、moov等，解析视频帧数据，并详细阐述了获取特定时间点图片数据的过程。此外，还提供了简单的视频解析代码示例。
安装mediaInfo、QTAtomViewer.exe、MP4info.exe等软件
视频一些参数
封装格式：封装视频的容器
编码格式：视频中图像帧的压缩算法
分辨率：图像的长和宽。
帧率：一秒播放的图片数，如15fps、30fps，体现在画面的流畅度。
码率：压缩后每秒显示的图片数据量，通俗地说，就是流量。如298kbps，体现在视频文件的大小和画质。
压缩比：分辨率*帧率/码率，简单来说就是压缩前每秒的数据量比上压缩后每秒的数据量。
音频一些参数
声道：录音时录音的位置不同产生不同的声道，有单声道、立体声（左右声道）、四声道环绕、5.1声道等。放歌时切换到左声道，两个音箱都播放左声道，切换到右声道时，两个音箱都播放右声道，切换到立体音时，左音箱播放左声道，右音箱播放右声道。可用暴风影音体验一下。
音轨： ①电视上看到录音时歌手都是在录音棚里戴着耳机唱歌，其实这就使用了多音轨技术，先把伴奏录好（音轨1），歌手再根据伴奏唱歌（音轨2）。②在K房使用的也是多音轨（原唱和伴唱）③有些老歌经常是粤语版和国语版一起唱，这也是多音轨技术
采样率：录制声音时对声音的采样频率，一般为44.1KHz
MP4文件组成
MP4由一个个的box（以前叫atom）组成，如下图，每个box包含不同的信息（再如下图），在程序中用结构体表示各个box，形象地说MP4文件就是由各个结构体组织起来的。所以，首先要了解这些box的组成和意义。
主要概念：sample、chunk、track
sample，在这里实际上就是一帧或连续几帧
chunk，一个或几个sample组成的单元
track，视频或音频序列，sample的集合
MP4主要由三个box组成，ftyp、mdat和moov。ftyp指示一些头部信息，通过ftyp box可以判断文件类型。mdat box存放的是媒体数据，就是播放的音频实实在在的元数据，比如我们需要取到第一帧的图片数据就存放在这个box里面。这么多数据，怎么知道这些数据的准确位置呢？这就要靠moov box来解析了。一般情况下，一个moov box包含一个mvhd和若干个track box。①mvhd box中存放一些create/modify time（基准时间是1904-1-1 0:00 AM）等信息，duration/time scale就是视频的长度（单位为s）。②track box一般包含tkhd和mdia两个box，tkhd中存放有视频长度和宽度等信息，，，，，，，最重要的是mdia -> minf -> stbl box下面的6个box，分别是stsd、stts、stss、stsc、stsz、stco，比如要查找第N秒的图片数据，步骤如下：
1、确定第N秒在媒体时间坐标系统中的位置，即N*time_scale
2、根据stts（time to sample）确定sample序号
3、根据stsc（sample to chunk）确定对应该sample的chunk
4、根据stco（chunk offset）中提取该chunk的偏移量
5、根据stsz（sample size）找到该sample在chunk内的偏移量和sample的大小。
例如：以test.mp4为例说明，用QTAtomViewer.exe查看此文件可知time_scale为90000，即一秒分为90000个刻度，现在假设要查看第一秒的数据，步骤如下：
1、一秒在媒体时间坐标系统中的位置，即1*90000 = 90000；
2、由stts，sample duration = 6000， 90000/6000 = 15， 即sample序号为15
3、由stsc，因为samples Per chunk = 1， 所以chunk序号也为15
4、由stco，序号为15的chunk的offset = 62719
5、由stsz，sample 在 chunk内的偏移为0，且大小为3554。
也就是说，程序到62719的地址开始，读取3554byte的数据，就是第一秒的图片数据。如果要查看第一帧（第0秒）的数据，同样按照上面的步骤可定位到地址28，读取5280byte的数据
现在知道了MP4文件的组成，也知道了怎么去获取第N帧的数据，那么无论程序多么复杂，它都是围绕这个主题来做文章了
最后附上一小段简单的视频解析的代码
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define BOX_FTYP 0x66747970
#define MDAT_BOX 0x6D646174
#define MOOV_BOX 0x6D6F6F76
#define MVHD_BOX 0x6D766864
#define TRAK_BOX 0x7472616B
#define IODS_BOX 0x696F6473
#define UDTA_BOX 0x75647461
#define FREE_BOX 0x66726565
#define TKHD_BOX 0x746B6864
#define MDIA_BOX 0x6D646961
#define MDHD_BOX 0x6D646864
#define HDLR_BOX 0x68646C72
#define MINF_BOX 0x6D696E66
#define DINF_BOX 0x64696E66
#define TREF_BOX 0x74726566
#define STBL_BOX 0x7374626C
#define STSD_BOX 0x73747364
#define MP4S_BOX 0x6D703473
#define ESDS_BOX 0x65736473
#define STTS_BOX 0x73747473
#define STSC_BOX 0x73747363
#define STSZ_BOX 0x7374737A
#define STCO_BOX 0x7374636F
#define STSS_BOX 0x73747373
#define CTTS_BOX 0x63747473
#define EDTS_BOX 0x65647473
#define VMHD_BOX 0x766D6864
#define HMHD_BOX 0x686D6864
#define SMHD_BOX 0x736D6864
#define NMHD_BOX 0xffffffff
#define ODSM 0x6F64736D
#define SDSM 0x7364736D
#define VIDE 0x76696465
#define HINT 0x68696E74
#define SOUN 0x736F756E
typedef struct BOX_HEAD
{
unsigned int size;
unsigned int type;
}BOX_HEAD;
typedef struct FTYP_BOX
{
unsigned int ftyp_size;
unsigned int ftyp_type;
}FTYP_BOX;
typedef struct MOOV_HEADER_BOX
{
unsigned int create_time;
unsigned int modification_time;
unsigned int time_scale;
unsigned int duration;
unsigned int preferred_rate;
unsigned short preferred_volume;
char reserved[10];
char matrix_structure[36];
char pre_defined[24];
unsigned int next_track_id;
}MOOV_HEADER_BOX;
typedef struct Track_head_box
{
unsigned int create_time;
unsigned int modification_time;
unsigned int track_id;
unsigned int reserved1;
unsigned int duration;
unsigned int reserved2[2];
unsigned short layer;
unsigned short alternate_group;
unsigned short volume;
unsigned short reserved3;
char matrix[36];
unsigned int width;
unsigned int height;
}TRACK_HEAD_BOX;
typedef struct Mdhd_head_box
{
unsigned int create_time;
unsigned int modification_time;
unsigned int time_scale;
unsigned int duration;
unsigned short language;
unsigned short quality;
}MDHD_HEAD_BOX;
typedef struct Ret_Boxhead_Fileptr
{
BOX_HEAD box_head;
FILE *p;
}Ret_Boxhead_Fileptr;
struct MP4_INFO
{
unsigned int create_time;
unsigned int modification_time;
}mp4_info;
unsigned int big_to_small_endian_32bit(unsigned int const big)
{
unsigned int data = big;
unsigned int ret = 0;
ret += (data>>0 & 0xff) << 24;
ret += (data>>8 & 0xff) << 16;
ret += (data>>16 & 0xff) << 8;
ret += (data>>24 & 0xff) << 0;
return ret;
}
__int64 big_to_small_endian_64bit(__int64 const big)
{
__int64 data = big;
__int64 ret = 0;
ret += (data>>0 & 0xff) << 56;
ret += (data>>8 & 0xff) << 48;
ret += (data>>16 & 0xff) << 40;
ret += (data>>24 & 0xff) << 32;
ret += (data>>32 & 0xff) << 24;
ret += (data>>40 & 0xff) << 16;
ret += (data>>48 & 0xff) << 8;
ret += (data>>56 & 0xff) << 0;
return ret;
}
void print_box_type(unsigned int box_type, unsigned int num)
{
for(unsigned int i = 0; i < num; i++)
{
printf("\t");
}
switch(big_to_small_endian_32bit(box_type))
{
case BOX_FTYP:
{
printf("%s\n", "ftyp box");
}
break;
case MOOV_BOX:
{
printf("%s\n", "moov box");
}
break;
case MDAT_BOX:
{
printf("%s\n", "mdat box");
}
break;
case MVHD_BOX:
{
printf("%s\n", "mvhd box");
}
break;
case IODS_BOX:
{
printf("%s\n", "iods box");
}
break;
case UDTA_BOX:
{
printf("%s\n", "udta box");
}
break;
case TRAK_BOX:
{
printf("%s\n", "trak box");
}
break;
case TKHD_BOX:
{
printf("%s\n", "tkhd box");
}
break;
case MDIA_BOX:
{
printf("%s\n", "mdia box");
}
break;
case MDHD_BOX:
{
printf("%s\n", "mdhd box");
}
break;
case HDLR_BOX:
{
printf("%s\n", "hdlr box");
}
break;
case MINF_BOX:
{
printf("%s\n", "minf box");
}
break;
case DINF_BOX:
{
printf("%s\n", "dinf box");
}
break;
case TREF_BOX:
{
printf("%s\n", "tref box");
}
break;
case STBL_BOX:
{
printf("%s\n", "stbl box");
}
break;
case STSD_BOX:
{
printf("%s\n", "stsd box");
}
break;
case MP4S_BOX:
{
printf("%s\n", "mp4s box");
}
break;
case ESDS_BOX:
{
printf("%s\n", "esds box");
}
break;
case STTS_BOX:
{
printf("%s\n", "stts box");
}
break;
case STSC_BOX:
{
printf("%s\n", "stsc box");
}
break;
case STSZ_BOX:
{
printf("%s\n", "stsz box");
}
break;
case STCO_BOX:
{
printf("%s\n", "stco box");
}
break;
case STSS_BOX:
{
printf("%s\n", "stss box");
}
break;
case CTTS_BOX:
{
printf("%s\n", "ctts box");
}
break;
case EDTS_BOX:
{
printf("%s\n", "edts box");
}
break;
case VMHD_BOX:
{
printf("%s\n", "vmhd box");
}
break;
case SMHD_BOX:
{
printf("%s\n", "smhd box");
}
break;
case HMHD_BOX:
{
printf("%s\n", "hmhd box");
}
break;
case NMHD_BOX:
{
printf("%s\n", "nmhd box");
}
break;
}
}
enum timeType
{
CREATE_TIME,
MODIFY_TIME
};
void create_and_modify_time(unsigned int time, enum timeType type)
{
int month_days[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
int leap_month_days[12] = {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
unsigned long local_time = (long)big_to_small_endian_32bit(time);
unsigned int local_year = 1904+local_time/3600/24/365;//此处当做是每年365天，下面再微调
int leap_years_count;//闰年数量
//平年，不精确计算
if(local_year % 4)
{
leap_years_count = (local_year - 1904)/4 + 1;
}
//闰年
else
{
leap_years_count = (local_year - 1904)/4;
}
unsigned int day_count;//由于上面把每年当作365天，此处计算到年初时过去的实际天数
day_count = leap_years_count*366 + (local_year-1904-leap_years_count)*365;
int leave_day_count = local_time/3600/24 - day_count;//算出剩下的天数
int i=0;
int sum = 31;
int local_month = 0;
int local_day = 0;
/*下面程序算出一年中的哪一个月和哪一天*/
//平年
if(local_year % 4)
{
while(leave_day_count > sum)
{
local_day = leave_day_count - sum + 1;//因为从零算起，所以天数要加1
sum += month_days[++i];
}
}
//闰年
else
{
while(leave_day_count > sum)
{
local_day = leave_day_count - sum + 1;
sum += leap_month_days[++i];
}
}
local_month = i+1;
if(local_month == 1)//如果是一月份
{
local_day = leave_day_count + 1;
}
/*上面程序算出一年中的哪一个月和哪一天*/
int out_of_day = leave_day_count + day_count;
int leave_second = local_time - out_of_day * 3600 *24;//计算剩下的秒数
if(type == CREATE_TIME)
{
printf("编码时间：%d-%d-%d %d:%d:%d\n", local_year, local_month, local_day,
leave_second/3600, leave_second%3600/60, leave_second%60);
}
else if(type == MODIFY_TIME)
{
printf("修订时间：%d-%d-%d %d:%d:%d\n", local_year, local_month, local_day,
leave_second/3600, leave_second%3600/60, leave_second%60);
}
}
void mainmenu()
{
printf("------------------------function menu-------------------------\n");
printf("1.编码时间与修订时间\n");
printf("2.影片长度\n");
printf("3.影片声音采样频率、视频帧率、码率（比特率）、分辨率\n");
printf("4.视频关键帧列表\n");
printf("q.输入q退出\n");
printf("------------------------function menu-------------------------\n");
}
char get_user_input()
{
char select;
/*这里scanf必须有\n或者空格，看下面的解释
scanf("%c",&c) 与 scanf(" %c",&c)，后者只是在%前多了个空格，似乎没有什么区别，
但使用起来区别是很大的。scanf()作单字符输入时规定只接收一个字符，但它却把回车符
也作为字符对待的。这就造成程序中只有一个输入字符的scanf()语句时，问题还不大，
但如果后面还跟着第二个scanf()字符输入语句，这个scanf()就把前面输入的驾车符当作
输入字符了。这就在输入逻辑上造成了混乱，达不到人们预期的愿望。有了这个空格，
因为scanf()是跳过空格读字符的，就回避了这个问题。实践证明，这个空格放在%c后面也不能达到目的。
应当说，这也是比较巧妙的应用！
*/
scanf(" %c", &select);
switch(select)
{
case '1':
{
create_and_modify_time(mp4_info.create_time, CREATE_TIME);
create_and_modify_time(mp4_info.modification_time, MODIFY_TIME);
}
break;
case '2':
{
}
break;
case '3':
{
}
break;
case '4':
{
}
break;
case '5':
{
}
break;
}
return select;
}
Ret_Boxhead_Fileptr moov_box_parse(FILE *p)
{
BOX_HEAD box_head;
MOOV_HEADER_BOX mvhd_box;
char version;
char *flag = (char *)malloc(3);
bool stbl_flag = false;
fread(&box_head, sizeof(BOX_HEAD), 1, p);
print_box_type(box_head.type, 1);//mvhd box
fread(&version, sizeof(char), 1, p);
fread(flag, 3, 1, p);
fread(&mvhd_box, sizeof(MOOV_HEADER_BOX), 1, p);
mp4_info.create_time = mvhd_box.create_time;
mp4_info.modification_time = mvhd_box.modification_time;
fread(&box_head, sizeof(BOX_HEAD), 1, p);
while(big_to_small_endian_32bit(box_head.type) != TRAK_BOX)
{
print_box_type(box_head.type, 1);
fseek(p, big_to_small_endian_32bit(box_head.size)-8, SEEK_CUR);
fread(&box_head, sizeof(BOX_HEAD), 1, p);
}
print_box_type(box_head.type, 1);//trak box
while(big_to_small_endian_32bit(box_head.type) != MDAT_BOX &&
big_to_small_endian_32bit(box_head.type) != FREE_BOX)
{
if(0 == fread(&box_head, sizeof(BOX_HEAD), 1, p))
{
break;
}
while(big_to_small_endian_32bit(box_head.type) != TRAK_BOX &&
big_to_small_endian_32bit(box_head.type) != MDAT_BOX &&
big_to_small_endian_32bit(box_head.type) != FREE_BOX )
{
switch(big_to_small_endian_32bit(box_head.type))
{
case TKHD_BOX:
{
print_box_type(box_head.type, 2);//tkhd box
char tkhd_version;
char *tkhd_flag = (char *)malloc(3);
TRACK_HEAD_BOX tkhd_box;
fread(&tkhd_version, 1, 1, p);
fread(tkhd_flag, 3, 1, p);
fread(&tkhd_box, sizeof(TRACK_HEAD_BOX), 1, p);
free(tkhd_flag);
}
break;
case MDIA_BOX:
{
char mdhd_version;
char *mdhd_flag = (char *)malloc(3);
unsigned int minf_head_box_length;
unsigned int handler_type;
MDHD_HEAD_BOX mdhd_box;
print_box_type(box_head.type, 2);//mdia box
fread(&box_head, sizeof(BOX_HEAD), 1, p);
print_box_type(box_head.type, 3);//mdhd box
fread(&mdhd_version, 1, 1, p);
fread(mdhd_flag, 3, 1, p);
fread(&mdhd_box, sizeof(MDHD_HEAD_BOX), 1, p);
fread(&box_head, sizeof(BOX_HEAD), 1, p);
print_box_type(box_head.type, 3);//hdlr box
fseek(p, 8, SEEK_CUR);
fread(&handler_type, 4, 1, p);
fseek(p, big_to_small_endian_32bit(box_head.size)-20, SEEK_CUR);
fread(&box_head, sizeof(BOX_HEAD), 1, p);
print_box_type(box_head.type, 3);//minf box
/* fread(&minf_head_box_length, 4, 1, p);//minf head box
fseek(p, big_to_small_endian_32bit(minf_head_box_length)-4, SEEK_CUR);
fread(&box_head, sizeof(BOX_HEAD), 1, p);
while(big_to_small_endian_32bit(box_head.type) != STBL_BOX)
{
print_box_type(box_head.type, 4);
fseek(p, big_to_small_endian_32bit(box_head.size)-8, SEEK_CUR);
fread(&box_head, sizeof(BOX_HEAD), 1, p);
}
print_box_type(box_head.type, 4);//stbl box*/
switch(big_to_small_endian_32bit(handler_type))
{
case SOUN:
case VIDE:
case HINT:
{
unsigned int now_pos = ftell(p);
unsigned int now_box_size = big_to_small_endian_32bit(box_head.size);
while((ftell(p)-now_pos+8) != now_box_size)
{
fread(&box_head, sizeof(BOX_HEAD), 1, p);
fseek(p, big_to_small_endian_32bit(box_head.size)-8, SEEK_CUR);
switch(big_to_small_endian_32bit(box_head.type))
{
case VMHD_BOX:
case DINF_BOX:
case HMHD_BOX:
case SMHD_BOX:
{
print_box_type(box_head.type, 4);
}
break;
case STBL_BOX:
{
print_box_type(box_head.type, 4);
stbl_flag = true;
fseek(p, 8-big_to_small_endian_32bit(box_head.size), SEEK_CUR);
unsigned int stbl_pos = ftell(p);
unsigned int stbl_box_size = big_to_small_endian_32bit(box_head.size);
while((ftell(p)-stbl_pos+8) != stbl_box_size)
{
fread(&box_head, sizeof(BOX_HEAD), 1, p);
fseek(p, big_to_small_endian_32bit(box_head.size)-8, SEEK_CUR);
switch(big_to_small_endian_32bit(box_head.type))
{
case STSD_BOX:
{
print_box_type(box_head.type, 5);
}
break;
case STTS_BOX:
{
print_box_type(box_head.type, 5);
}
break;
case STSC_BOX:
{
print_box_type(box_head.type, 5);
}
break;
case STSZ_BOX:
{
print_box_type(box_head.type, 5);
}
break;
case STCO_BOX:
{
print_box_type(box_head.type, 5);
}
break;
case STSS_BOX:
{
print_box_type(box_head.type, 5);
}
break;
}
}
}
break;
}
}
}
break;
default:
{
fread(&minf_head_box_length, 4, 1, p);//nmhd
print_box_type(NMHD_BOX, 4);
fseek(p, big_to_small_endian_32bit(minf_head_box_length)-4, SEEK_CUR);
fread(&box_head, sizeof(BOX_HEAD), 1, p);
while(big_to_small_endian_32bit(box_head.type) != STBL_BOX)
{
print_box_type(box_head.type, 4);
fseek(p, big_to_small_endian_32bit(box_head.size)-8, SEEK_CUR);
fread(&box_head, sizeof(BOX_HEAD), 1, p);
}
print_box_type(box_head.type, 4);//stbl box
}
break;
}
if(stbl_flag == true)
{
stbl_flag = false;
free(mdhd_flag);
break;
}
unsigned int cur_pos = ftell(p);
unsigned int cur_box_size = big_to_small_endian_32bit(box_head.size);
while((ftell(p)-cur_pos+8) != cur_box_size)
{
fread(&box_head, sizeof(BOX_HEAD), 1, p);
fseek(p, big_to_small_endian_32bit(box_head.size)-8, SEEK_CUR);
switch(big_to_small_endian_32bit(box_head.type))
{
case STSD_BOX:
{
print_box_type(box_head.type, 5);
}
break;
case STTS_BOX:
{
print_box_type(box_head.type, 5);
}
break;
case STSC_BOX:
{
print_box_type(box_head.type, 5);
}
break;
case STSZ_BOX:
{
print_box_type(box_head.type, 5);
}
break;
case STCO_BOX:
{
print_box_type(box_head.type, 5);
}
break;
case STSS_BOX:
{
print_box_type(box_head.type, 5);
}
break;
case CTTS_BOX:
{
print_box_type(box_head.type, 5);
}
break;
}
}
stbl_flag = false;
free(mdhd_flag);
}
break;
default:
print_box_type(box_head.type, 2);
fseek(p, big_to_small_endian_32bit(box_head.size)-8, SEEK_CUR);
break;
}
/*读取到文件末尾跳出*/
if(0 == fread(&box_head, sizeof(BOX_HEAD), 1, p))
{
break;
}
if(big_to_small_endian_32bit(box_head.type) == TRAK_BOX)
{
print_box_type(box_head.type, 1);
}
}
}
free(flag);
Ret_Boxhead_Fileptr ret;
ret.box_head.size = box_head.size;
ret.box_head.type = box_head.type;
ret.p = p;
return ret;
}
FILE* mdat_box_parse(FILE *p, BOX_HEAD box_head)
{
__int64 box_large_size;
if(big_to_small_endian_32bit(box_head.size) != 1)
{
fseek(p, big_to_small_endian_32bit(box_head.size)-8, SEEK_CUR);
}
else
{
fread(&box_large_size, sizeof(__int64), 1, p);
fseek(p, big_to_small_endian_64bit(box_large_size)-sizeof(__int64)-8, SEEK_CUR);
}
return p;
}
int main( void )
{
FILE *p;
FTYP_BOX ftyp_box;
BOX_HEAD box_head;
char filename[256];
printf("please input mp4 filename:");
scanf("%s", filename);
if((p = fopen(filename,"r")) == NULL)
{
printf("open fail\n");
return -1;
}
printf("----------------------MP4 file structure----------------------\n");
fread(&ftyp_box, sizeof(FTYP_BOX), 1, p);
print_box_type(ftyp_box.ftyp_type, 0);//ftyp box
fseek(p, big_to_small_endian_32bit(ftyp_box.ftyp_size), SEEK_SET);
fread(&box_head, sizeof(BOX_HEAD), 1, p);
print_box_type(box_head.type, 0);//moov box OR mdat box
switch(big_to_small_endian_32bit(box_head.type))
{
case MDAT_BOX:
{
p = mdat_box_parse(p, box_head);
fread(&box_head, sizeof(BOX_HEAD), 1, p);
print_box_type(box_head.type, 0);//moov box
moov_box_parse(p);
}
break;
case MOOV_BOX:
{
Ret_Boxhead_Fileptr ret;
ret = moov_box_parse(p);
print_box_type(ret.box_head.type, 0);
mdat_box_parse(ret.p, ret.box_head);
}
break;
default:
break;
}
fclose(p);
printf("----------------------MP4 file structure----------------------\n\n\n");
mainmenu();
while(get_user_input() != 'q')
{
mainmenu();
}
return 0;
}
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