Playing a video in OpenCV - c++

Question

I am a beginner to OpenCV and I wish to play a video in OpenCV. I've made a code but it's displaying a single image only.
I am using OpenCV 2.1 and Visual Studio 2008.
I would really appreciate it if someone guided me where am I going wrong.
Here is my pasted code:
#include "stdafx.h"
#include "cv.h"
#include "highgui.h"
int main()
{
CvCapture* capture = cvCaptureFromAVI("C:/OpenCV2.1/samples/c/tree.avi");
IplImage* img = 0;
if(!cvGrabFrame(capture)){ // capture a frame
printf("Could not grab a frame\n\7");
exit(0);}
cvQueryFrame(capture); // this call is necessary to get correct
// capture properties
int frameH = (int) cvGetCaptureProperty(capture, CV_CAP_PROP_FRAME_HEIGHT);
int frameW = (int) cvGetCaptureProperty(capture, CV_CAP_PROP_FRAME_WIDTH);
int fps = (int) cvGetCaptureProperty(capture, CV_CAP_PROP_FPS);
int numFrames = (int) cvGetCaptureProperty(capture, CV_CAP_PROP_FRAME_COUNT);
///numFrames=total number of frames
printf("Number of rows %d\n",frameH);
printf("Number of columns %d\n",frameW,"\n");
printf("frames per second %d\n",fps,"\n");
printf("Number of frames %d\n",numFrames,"\n");
for(int i=0;i<numFrames;i++)
{
IplImage* img = 0;
img=cvRetrieveFrame(capture);
cvNamedWindow( "img" );
cvShowImage("img", img);
}
cvWaitKey(0);
cvDestroyWindow( "img" );
cvReleaseImage( &img );
cvReleaseCapture(&capture);
return 0;
}

Answer 1

You have to use cvQueryFrame instead of cvRetrieveFrame. Also as pointed out by #Chipmunk, you have to add a delay after cvShowImage.
#include "stdafx.h"
#include "cv.h"
#include "highgui.h"
cvNamedWindow( "img" );
for(int i=0;i<numFrames;i++)
{
IplImage* img = cvQueryFrame(capture);
cvShowImage("img", img);
cvWaitKey(10);
}
Here is the complete method to play a video using OpenCV:
int main()
{
CvCapture* capture = cvCreateFileCapture("C:/OpenCV2.1/samples/c/tree.avi");
IplImage* frame = NULL;
if(!capture)
{
printf("Video Not Opened\n");
return -1;
}
int width = (int)cvGetCaptureProperty(capture,CV_CAP_PROP_FRAME_WIDTH);
int height = (int)cvGetCaptureProperty(capture,CV_CAP_PROP_FRAME_HEIGHT);
double fps = cvGetCaptureProperty(capture, CV_CAP_PROP_FPS);
int frame_count = (int)cvGetCaptureProperty(capture, CV_CAP_PROP_FRAME_COUNT);
printf("Video Size = %d x %d\n",width,height);
printf("FPS = %f\nTotal Frames = %d\n",fps,frame_count);
while(1)
{
frame = cvQueryFrame(capture);
if(!frame)
{
printf("Capture Finished\n");
break;
}
cvShowImage("video",frame);
cvWaitKey(10);
}
cvReleaseCapture(&capture);
return 0;
}

Answer 2

After showing a image on the window, there has to be a delay or a wait before the next image can be show, I think you can guess why that is. Okay so for that delay we use cvWaitKey() .
And thats what I have added in the code in the loop.
cvNamedWindow( "img" );
for(int i=0;i<numFrames;i++)
{
IplImage* img = 0;
img=cvRetrieveFrame(capture);
cvShowImage("img", img);
cvWaitKey(10);
}