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Tag: Computer Vision

OpenCV: adding two images

This is a very simple example of how to open two images and display them added.

I got two pictures at project Commons from Wikimedia that were highlighted on Featured Pictures. I did a crop on both to have the same size, as I’m trying to make this example as simple as possible.

The first one is a photo of our Milky Way, taken at Paranal Observatory by Stéphane Guisard.


The second one is a California surfer inside wave, taken by Mila Zinkova.


In this simple OpenCV code below, we open the images, create a new one to display the result and use cvAdd to add them. We do not save the result or handle more than the ordinary case of two images with the same size.


int main( int argc, char **argv ){
    IplImage *surfer, *milkyway, *result;
    int key = 0;
    CvSize size;

    /* load images, check, get size (both should have the same) */
    surfer = cvLoadImage("surfer.jpg", CV_LOAD_IMAGE_COLOR);
    milkyway = cvLoadImage("milkyway.jpg", CV_LOAD_IMAGE_COLOR);
        printf("Could not open one or more images.");
        exit -1;
    size = cvGetSize(surfer);

    /* create a empty image, same size, depth and channels of others */
    result = cvCreateImage(size, surfer->depth, surfer->nChannels);

    /* result = surfer + milkyway (NULL mask)*/
    cvAdd(surfer, milkyway, result, NULL);
    /* create a window, display the result, wait for a key */
    cvNamedWindow("example", CV_WINDOW_AUTOSIZE);
    cvShowImage("example", result);

    /* free memory and get out */
    return 0;

/* gcc add.c -o add `pkg-config opencv --libs --cflags` */

Compile it (on a well configured OpenCV development environment) and run it:

gcc add.c -o add `pkg-config opencv –libs –cflags`

The result got pretty cool, a milky way surfer.

surfer in the milk way

OpenCV: Edge Detection

This is a simple example of how pass edge detection in a video using OpenCV. It uses the built-in OpenCV Canny edge detector algorithm.


int main(int argc, char *argv[]) {
    int delay = 0, key=0, i=0;
    char *window_name;
    CvCapture *video = NULL;
    IplImage  *frame = NULL;
    IplImage  *grey  = NULL;
    IplImage  *edges = NULL;

    /* check for video file passed by command line */
    if (argc>1) {
        video = cvCaptureFromFile(argv[1]);
    } else {
        printf("Usage: %s VIDEO_FILE\n", argv[0]);
        return 1;

    /* check file was correctly opened */
    if (!video) {
        printf("Unable to open \"%s\"\n", argv[1]);
        return 1;

    /* create a video window with same name of the video file, auto sized */
    window_name = argv[1];
    cvNamedWindow(window_name, CV_WINDOW_AUTOSIZE);

    /* Get the first frame and create a edges image with the same size */
    frame = cvQueryFrame(video);
    grey  = cvCreateImage(cvGetSize(frame), IPL_DEPTH_8U, 1);
    edges = cvCreateImage(cvGetSize(frame), IPL_DEPTH_8U, 1);

    /* calculate the delay between each frame and display video's FPS */
    printf("%2.2f FPS\n", cvGetCaptureProperty(video, CV_CAP_PROP_FPS));
    delay = (int) (1000/cvGetCaptureProperty(video, CV_CAP_PROP_FPS));

    while (frame) {
	/* Edges on the input gray image (needs to be grayscale) using the Canny algorithm.
           Uses two threshold and a aperture parameter for Sobel operator. */
        cvCvtColor(frame, grey, CV_BGR2GRAY);
        cvCanny( grey, edges, 1.0, 1.0, 3);

	/* show loaded frame */
        cvShowImage(window_name, edges);
	/* load and check next frame*/
        frame = cvQueryFrame(video);
	if(!frame) {
		printf("error loading frame.\n");
		return 1;

	/* wait delay and check for the quit key */
        key = cvWaitKey(delay);
        if(key=='q') break;

To compile it in a well configured OpenCV development environment:

gcc edgeplayer.c -o edgeplayer `pkg-config opencv –libs –cflags`

To run it call edgeplayer and the name of the video:

./edgeplayer rick.avi

The result is something similar to this:

rick roll edge