Image Difference with OpenCV

by Carson Reynolds

Connsynn and I are working on a system to track tears. As part of it, we built a very quick image difference program to track motion between frames (to find things like blinks). As there wasn’t a clear example of using cvAbsDiff (which is a bit better than cvSub) we thought it’d be good to post ours. The program, image-diff.cpp follows:

#include <cv.h>
#include <highgui.h>

int main ( int argc, char **argv )
{
  // use first camera attached to computer
  CvCapture *capture;
  capture = cvCaptureFromCAM( 0 );
  assert( capture );

  // image data structures
  IplImage *img1;	
  IplImage *img2;
  IplImage *imggray1;
  IplImage *imggray2;
  IplImage *imggray3;
		
  // get the camera image size
  IplImage *imgsize;
  imgsize = cvQueryFrame( capture );
  if( !imgsize ) return -1;

  // grayscale buffers
  imggray1 = cvCreateImage( cvGetSize( imgsize ), IPL_DEPTH_8U, 1);
  imggray2 = cvCreateImage( cvGetSize( imgsize ), IPL_DEPTH_8U, 1);
  imggray3 = cvCreateImage( cvGetSize( imgsize ), IPL_DEPTH_8U, 1);
  
  int key = 0;
  while ( key != 'q' ) {
    // load image one
    img1 = cvQueryFrame( capture );	
 
   // convert rgb to grayscale
    cvCvtColor( img1, imggray1, CV_RGB2GRAY );
    
    // quit if user press 'q' and wait a bit between images
    key = cvWaitKey( 500 );
    
    // load image two
    img2 = cvQueryFrame( capture );

    // convert rgb to grayscale
    cvCvtColor( img2, imggray2, CV_RGB2GRAY );
    
    // compute difference
    cvAbsDiff( imggray1, imggray2, imggray3 );
    
    // display difference
    cvNamedWindow( "video", 1 );
    cvShowImage( "video", imggray3 );
  }

  // release camera and clean up resources when "q" is pressed
  cvReleaseCapture( &capture );
  cvDestroyWindow( "video" );
  return 0;
}

Here is a Makefile which uses g++ and assumes you have OpenCV installed. For it to work properly, the file above needs to be saved as image-diff.cpp in the same directory as the Makefile. It was tested using OS X and Macports, but will probably also work on Ubuntu, Linux, or anything with g++, pkg-config and OpenCV.

NAME = image-diff
CC = g++
CFLAGS = -Wall -g -O0 `pkg-config --cflags opencv`
LIBS = `pkg-config --libs opencv`

all:
	$(CC) $(CFLAGS) $(LIBS) $(NAME).cpp -o $(NAME)

UPDATE: 12/12/2011 added simplified Makefile, fixed image color conversion ordering bug

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