我想在C中制作一个快速程序，它将打开一个视频，将每个帧保存为ppm，并转储运动矢量。我能找到的所有教程都来自差不多十年前，并且调用了已弃用或不存在的函数。
是否有任何良好的在线资源，网站，视频或教科书涵盖了做这类事情的现代方法？
答案 0 :(得分：7)
我已经使用ffmpeg和libav好几年了，也发现没有像样的最新API级教程。有时我只需要深入了解源代码以了解发生了什么以及如何使用它。此外，阅读CLI程序的源代码（使用底层库）也可以提供信息。
事实上，ffmpeg建议你只阅读源代码，无论是真实的OSS项目还是他们的例子。 Libav提供了Doxygen文档，但没有逐步的教程。 IMO这是典型的开源项目：源代码可能很好，但通常缺乏文档。
退一步后，您可以考虑OpenCV，其中有详细记录（books存在！）并且具有更简单的API。最后，ppm是一种简单的格式，您可以编写50行函数来自己保存像素，我找到了一个运动提取程序here。
答案 1 :(得分：2)
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <fstream>
#include <math.h>
#define __STDC_CONSTANT_MACROS
//FFMPEG LIBRARIES
extern "C"
{
#include "libavcodec/avcodec.h"
#include "libswscale/swscale.h"
#include "libavutil/opt.h"
#include "libavutil/common.h"
#include "libavutil/channel_layout.h"
#include "libavutil/imgutils.h"
#include "libavutil/mathematics.h"
#include "libavutil/samplefmt.h"
#include "libavutil/time.h"
#include "libavformat/avformat.h"
#include "libavformat/avio.h"
#include "libavfilter/avfilter.h"
#include "libavdevice/avdevice.h"
#include "libavfilter/avfiltergraph.h"
#include "libavfilter/buffersink.h"
}
using namespace std;
void SaveMyFrame(AVFrame *sAVFrame , int swidth, int sheight, int iFrame)
{
FILE *pfile;
char szFilename[32];
int y;
sprintf(szFilename , "frame%d.ppm" , iFrame);
pfile = fopen(szFilename , "wb");
if(pfile == NULL)
{
cout<<"\n\ncould'nt open file";
return;
}
fprintf(pfile , "P6\n%d %d\n255\n" , swidth , sheight );
for( y=0; y<sheight; y++)
{
fwrite(sAVFrame->data[0]+y*sAVFrame->linesize[0] , 1 , swidth*3 , pfile );
}
fclose(pfile);
}
int CaptureScene(int VideoStreamIndx ,
AVFormatContext *bAVFormatContext ,
AVCodecContext *bAVCodecContext,
AVCodec *bAVCodec )
{
AVPacket bAVPacket;
AVFrame *bAVFrame = NULL;
bAVFrame = av_frame_alloc();
AVFrame *bAVFrameRGB = NULL;
bAVFrameRGB = av_frame_alloc();
if(bAVFrame == NULL)
{
cout<<"\n\nframe alloc failed";
}
if(bAVFrameRGB == NULL)
{
cout<<"\n\nframe alloc RGB failed";
}
int numBytes;
uint8_t *buffer = NULL;
numBytes = av_image_get_buffer_size(AV_PIX_FMT_RGB24 , bAVCodecContext->width,bAVCodecContext->height, 32); // avpicture_get_size deprecated
buffer=(uint8_t *)av_malloc(numBytes*sizeof(uint8_t));
avpicture_fill((AVPicture *)bAVFrameRGB , buffer , AV_PIX_FMT_RGB24 , bAVCodecContext->width , bAVCodecContext->height);
int framefinish;
struct SwsContext *sws_ctx = NULL;
sws_ctx = sws_getContext( bAVCodecContext->width,
bAVCodecContext->height,
bAVCodecContext->pix_fmt,
bAVCodecContext->width,
bAVCodecContext->height,
AV_PIX_FMT_RGB24,
SWS_BILINEAR,
NULL,NULL,NULL);
int i =0;
while(av_read_frame(bAVFormatContext,&bAVPacket) >=0)
{
if(bAVPacket.stream_index == VideoStreamIndx)
{
avcodec_decode_video2(bAVCodecContext , bAVFrame , &framefinish , &bAVPacket);
if(framefinish)
{
// convert image from native format to RGB
sws_scale(sws_ctx , (uint8_t const* const *)bAVFrame->data ,
bAVFrame->linesize , 0, bAVCodecContext->height,
bAVFrameRGB->data , bAVFrameRGB->linesize);
// save frame to disk
if(++i <= 100)SaveMyFrame(bAVFrameRGB , bAVCodecContext->width , bAVCodecContext->height , i );
}
}
}
av_free(bAVFrame);
av_free(bAVFrameRGB);
}
int main()
{
avdevice_register_all();
avcodec_register_all();
av_register_all();
char *dev_name = "/dev/video0";
int VideoStreamIndx = -1;
AVCodecContext *pAVCodecContext = NULL;
AVCodec *pAVCodec = NULL;
AVInputFormat *inputFormat =av_find_input_format("v4l2");
AVDictionary *options = NULL;
av_dict_set(&options, "framerate", "20", 0);
AVFormatContext *pAVFormatContext = NULL;
if(avformat_open_input(&pAVFormatContext, dev_name, inputFormat, NULL) != 0)
{
cout<<"\nError : could'nt open video source\n\n";
return -1;
}
if( avformat_find_stream_info( pAVFormatContext , NULL) < 0)
{
cout<<"Error : streams not found";
return -1;
}
av_dump_format(pAVFormatContext , 0 , "/dev/video1" , 0 );
for(int i=0; i<pAVFormatContext->nb_streams ;i++ )
{
if( pAVFormatContext->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO ) // if video stream found then get the index.
{
VideoStreamIndx = i;
break;
}
}
if((VideoStreamIndx) == -1)
{
cout<<"Error : video streams not found";
return -1;
}
pAVCodecContext = pAVFormatContext->streams[VideoStreamIndx]->codec;
pAVCodec = avcodec_find_decoder( pAVCodecContext->codec_id );
if(pAVCodec == NULL)
{
fprintf(stderr,"Unsupported codec !");
return -1;
}
int value = avcodec_open2(pAVCodecContext , pAVCodec , NULL);
if( value < 0)
{
cout<<"Error : Could not open codec";
return -1;
}
int Vwidth , Vheight , videoFPS , videoBaseTime , duration_2 ;
int sframe , length , Fheight;
/*
To fetch/display some media information programatically
*/
//int64_t duration_1 = pAVFormatContext->duration;
videoFPS = av_q2d(pAVFormatContext->streams[VideoStreamIndx]->r_frame_rate);
videoBaseTime = av_q2d(pAVFormatContext->streams[VideoStreamIndx]->time_base);
Vwidth = pAVFormatContext->streams[VideoStreamIndx]->codec->width;
Vheight = pAVFormatContext->streams[VideoStreamIndx]->codec->height;
//duration_2 = (unsigned long)pAVFormatContext->streams[VideoStreamIndx]->duration*(videoFPS*videoBaseTime);
cout<<"Video FPS :"<<videoFPS;
cout<<"\n\n width : "<<Vwidth;
cout<<"\n\n height : "<<Vheight;
cout<<"\n\n time base"<<videoBaseTime;
//cout<<"\n\nduration (1): "<<duration_1;
//cout<<"\n\nduration (2): "<<duration_2;
CaptureScene( VideoStreamIndx , pAVFormatContext , pAVCodecContext , pAVCodec );
avcodec_close(pAVCodecContext);
avformat_close_input(&pAVFormatContext);
return 0;
}
此C ++代码将帮助您从基于USB（UVC）的相机获取.ppm图像。它以.ppm格式保存。 使用v4l2 for linux和dshow for windows。 而不是相机帧馈送，视频也可以作为输入。 这段代码用最新的ffmpeg库编写。
libavutil 55. 24.100 / 55. 24.100
libavcodec 57. 43.100 / 57. 43.100
libavformat 57. 37.100 / 57. 37.100
libavdevice 57. 0.101 / 57. 0.101
libavfilter 6. 46.100 / 6. 46.100
libswscale 4. 1.100 / 4. 1.100
libswresample 2. 0.101 / 2. 0.101
libpostproc 54. 0.100 / 54. 0.100