vision – Silveira Neto

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.

#include 
#include 
#include 

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:

Here’s a simple video player that also performs facial detection thought the Open Computer Vision Library.

Here’s a code developed using codes from nashruddin.com and samples from OpenCV, including the haar classifier xml. More detailed explanation on the theory about how the OpenCV face detection algorithm works can be found here.

The code:

#include 
#include 
#include 

CvHaarClassifierCascade *cascade;
CvMemStorage *storage;

int main(int argc, char *argv[]) {
    CvCapture *video = NULL;
    IplImage *frame = NULL;
    int delay = 0, key, i=0;
    char *window_name = "Video";
    char *cascadefile = "haarcascade_frontalface_alt.xml";

    /* 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;
    }

    /* load the classifier */
    cascade = ( CvHaarClassifierCascade* )cvLoad( cascadefile, 0, 0, 0 );
    if(!cascade){
        printf("Error loading the classifier.");
	return 1;
    }

    /* setup the memory buffer for the face detector */
    storage = cvCreateMemStorage( 0 );
    if(!storage){
        printf("Error creating the memory storage.");
	return 1;
    }

    /* create a video window, auto size */
    cvNamedWindow(window_name, CV_WINDOW_AUTOSIZE);

    /* get a frame. Necessary for use the cvGetCaptureProperty */
    frame = cvQueryFrame(video);

    /* 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) {
	/* show loaded frame */
        cvShowImage(window_name, frame);
	
	/* wait delay and check for the quit key */
        key = cvWaitKey(delay);
        if(key=='q') break;
	/* load and check next frame*/
        frame = cvQueryFrame(video);
	if(!frame) {
		printf("error loading frame.\n");
		return 1;
	}

        /* detect faces */
        CvSeq *faces = cvHaarDetectObjects(
            frame, /* image to detect objects in */
            cascade, /* haar classifier cascade */
            storage, /* resultant sequence of the object candidate rectangles */
            1.1, /* increse window by 10% between the subsequent scans*/
            3, /* 3 neighbors makes up an object */
            0 /* flags CV_HAAR_DO_CANNY_PRUNNING */,
            cvSize( 40, 40 ) 
        );

        /* for each face found, draw a red box */
        for( i = 0 ; i < ( faces ? faces->total : 0 ) ; i++ ) {
             CvRect *r = ( CvRect* )cvGetSeqElem( faces, i );
             cvRectangle( frame,
                  cvPoint( r->x, r->y ),
                  cvPoint( r->x + r->width, r->y + r->height ),
                  CV_RGB( 255, 0, 0 ), 1, 8, 0 );
        }
    }
}

Yeah, I know the code needs a few adjustments. Â¬Â¬

To compile it in a well configured OpenCV development environment:

gcc faceplayer.c -o faceplayer `pkg-config opencv ‑‑libs ‑‑cflags`

To run it you have to put in the same directory of the binary the XML classifier (haarcascade_frontalface_alt.xml) that comes with OpenCV sources at OpenCV-2.0.0/data/haarcascades/. And so:

./faceplayer video.avi

The results I got so far is that it works well for faces but sometimes its also detects more than faces. And here a video of it working live.

A example of good result:

rick roll face detection

A example of bad result:

rick roll face detection bad result

Maybe with some adjustments it could performs even better. But was really easy to create it using OpenCV.

Tag: vision

OpenCV: Edge Detection

Simple Face Detection Player