Video reading in opencv - opencv

Question

I am new to opencv .I am trying to read video mp4 file but my code always says cannot open video device or file. I have placed video file in code folder .
VideoCapture capture;
capture.open("a.mp4"); // Open file
if (!capture.isOpened())
{
cout << "Cannot open video device or file!" << endl;
getch();
return -1;
}
Mat frame;
namedWindow("video", CV_WINDOW_AUTOSIZE);
while(true)
{
capture >> frame;
if (frame.empty())
break;
imshow("video", frame);
if (waitKey(30) == 'q')
break;
}

Answer 1

I am posting this answer in case someone came across this question as I did.
I was facing almost the same problem, trying to open a video file. My code worked with .avi but failed to open files with .mp4 files.
Turns out openCV needs ffmpeg decoder to be installed on the system. In case of windows OS, you will need to copy opencv_ffmpeg2411.dll file into C:\Windows\System32 directory. The DLL different systems can be found in OpenCV extracted directory.
Hope this helps someone.

Answer 2

this is correct code for video and image reading. I don't know what was the exact error. But using different code from orieally help me do my task in a different way.
// newproject.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include "highgui.h"
#include <stdio.h>
#include <cv.h>
#include <highgui.h>
#include <stdio.h>
#include <conio.h>
#include <opencv2/imgproc/imgproc.hpp> // Gaussian Blur
#include <opencv2/core/core.hpp> // Basic OpenCV structures (cv::Mat, Scalar)
#include <opencv2/highgui/highgui.hpp>
#include <iostream>
#include <conio.h>
using namespace cv;
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
//Image Reading
IplImage* img = cvLoadImage( "b.jpg" );
cvNamedWindow( "Example1", CV_WINDOW_AUTOSIZE );
cvShowImage( "Example1", img );
cvWaitKey(0);
cvReleaseImage( &img );
cvDestroyWindow( "Example1" );
//Video Reading
// cvNamedWindow( "Example2", CV_WINDOW_AUTOSIZE );
// CvCapture* capture = cvCreateFileCapture( "video.avi" );
// IplImage* frame;
/* while(1) {
frame = cvQueryFrame( capture );
if( !frame ) break;
cvShowImage( "Example2", frame );
char c = cvWaitKey(33);
if( c == 27 ) break;//if user enter escape key then exit
}*/
//Summarize by sampling
CvCapture* capture = 0;
capture = cvCreateFileCapture( "video.avi" );
if(!capture){
return -1;
}
IplImage *bgr_frame=cvQueryFrame(capture);//Init the video read
double fps = cvGetCaptureProperty (capture,CV_CAP_PROP_FPS);
CvSize size = cvSize((int)cvGetCaptureProperty( capture, CV_CAP_PROP_FRAME_WIDTH),(int)cvGetCaptureProperty( capture, CV_CAP_PROP_FRAME_HEIGHT));
//double fps = cvGetCaptureProperty (capture,CV_CAP_PROP_FPS);
CvVideoWriter *writer = cvCreateVideoWriter("Ayesha",CV_FOURCC('M','J','P','G'),fps,size);
int x=0;
while(1){
x++;
bgr_frame = cvQueryFrame( capture );
if(!bgr_frame)
break;
if(x==19||x==21){
cvShowImage("Example2",bgr_frame);
cvWriteFrame( writer, bgr_frame );
}
if(x==20){
cvShowImage("Example2",bgr_frame);
cvWriteFrame( writer, bgr_frame );
x=0;
}
char c=cvWaitKey(30);
if (c==27) break;
}
cvReleaseVideoWriter( &writer );
cvReleaseCapture( &capture );
cvDestroyWindow( "Example2" );
}