CODEWITHSUNDEEP
c++opencv
Ilan Reuven
Ilan Reuven

Reputation: 61

OpenCV RGB to gray

I am doing a project for video monitoring.
I can't see the conversion from RGB to gray; I get a black window for the gray.
Could you please help me with the problem? (code attached)
Also, how can I get the difference between current frame and previous frame?
Thanks a lot. Ilan

#include "stdafx.h"
#include <stdio.h>  // For printf
#include <cv.h>  
#include <cxcore.h>  
#include <highgui.h>      

int main()  
{  


    int key = 0; 



    CvCapture* capture = cvCaptureFromAVI( "macroblock.mpg" );  
    IplImage* frame = cvQueryFrame( capture );
    IplImage* gray = cvCreateImage(cvGetSize(frame),IPL_DEPTH_8U,1);
    cvCvtColor(frame, gray, CV_RGB2GRAY);

        if ( !capture ) 

    {  
        fprintf( stderr, "Cannot open AVI!\n" );  
        return 1;  
        }
          int fps = ( int )cvGetCaptureProperty( capture, CV_CAP_PROP_FPS );

           cvNamedWindow( "video", CV_WINDOW_AUTOSIZE ); 
         cvNamedWindow( "grayvideo", CV_WINDOW_AUTOSIZE ); 

        while( key != 'x' )  
    {  
        frame = cvQueryFrame( capture );




    if(key==27 )break;
     cvShowImage( "video",frame );
     cvShowImage( "grayvideo",gray );


           key = cvWaitKey( 1000 / fps );  
    }  
           cvDestroyWindow( "video" );  
    cvReleaseCapture( &capture );  

    return 0;  
}

Upvotes: 1

Views: 27035

Answers (3)

sietschie
sietschie

Reputation: 7543

You need to convert each frame from color to gray scale. It does not get converted automatically if you do it once in the beginning. So you need to add 

cvCvtColor(frame, gray, CV_BGR2GRAY);

into your while-loop after your call to cvQueryFrame.

Upvotes: 3

Dawit
Dawit

Reputation: 1

use one of below opencv functions based on you need:

subtract(img_current,img_prev, img_diff);
absdiff(img_current, img_prev, img_diff);

Upvotes: 0

karlphillip
karlphillip

Reputation: 93410

There were a few problems with your code.

  • It was a mess and poorly idented;
  • When you need to check the return of a function, you should do it right after you call them. Ex: cvCaptureFromAVI();
  • As cvQueryFrame() might fail, you need to check it's return as well;
  • You forgot to execute the RGB -> GRAY conversion: cvCvtColor(frame, gray, CV_RGB2GRAY); before displaying the gray frame;
  • You also forgot to release the resources for the grayvideo window;

Isn't it easier to read/understand a code when it looks like this?

int main()  
{      
    CvCapture* capture = cvCaptureFromAVI( "macroblock.mpg" );  
    if ( !capture ) 
    {  
        fprintf( stderr, "Cannot open AVI!\n" );  
        return 1;  
    }

    int fps = ( int )cvGetCaptureProperty( capture, CV_CAP_PROP_FPS );
    printf("FPS: %d\n", fps);

    cvNamedWindow( "video", CV_WINDOW_AUTOSIZE ); 
    cvNamedWindow( "grayvideo", CV_WINDOW_AUTOSIZE ); 

    int key = 0; 
    IplImage* gray = NULL;
    IplImage* prev_frame = NULL;

    while( key != 'x' )  
    {  
        frame = cvQueryFrame( capture );
        if (!frame) 
        {
            // print error and abort the loop
            break;
        }

        cvShowImage( "video", frame );

        if (!gray) // allocate space for the GRAY frame only once
        {
            gray = cvCreateImage(cvGetSize(frame), frame->depth,1);
        }
        cvCvtColor(frame, gray, CV_RGB2GRAY); // convert RGB frame to GRAY
        cvShowImage( "grayvideo", gray );

        if (!prev_frame) // allocate space for the GRAY frame only once
        {
            prev_frame = cvCreateImage(cvGetSize(frame), frame->depth,1);
            cvCopy( frame, prev_frame, 0 );
        }

        // perform process to compute the "difference" of the current 
        // and previous frames:
        // <add your code here>

        // then, update prev_frame now so in the next iteration it holds the previous frame
        cvCopy( frame, prev_frame, 0 );

        key = cvWaitKey( 1000 / fps );  
        if ( key==27 ) // ESC was pressed
            break;
    }  

    cvDestroyWindow( "video" );  
    cvDestroyWindow( "grayvideo" );  

    cvReleaseCapture( &capture );  

    if (gray)
        cvReleaseImage( &gray );

    if (prev_frame)
        cvReleaseImage( &prev_frame );

    return 0;  
}

Upvotes: 1

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