Question

Basically I need to capture the video from videocamera do some processing on frames and for each frame show a rectangle of detection.

Example: http://www.youtube.com/watch?v=aYd2kAN0Y20

How would you superimpose this rectangle on the output of videocamera (usb)? (c++)

Answer 1

I would use OpenCV, an open source imaging library to get input from a webcam/video file.
Here is a tutorial on how to install it:

http://opensourcecollection.blogspot.com.es/2011/04/how-to-setup-opencv-22-in-codeblocks.html

Then I would use this code:

CvCapture *capture = cvCreateCameraCapture(-1);
IplImage* frame = cvQueryFrame(capture);

To get the image, frame from the CvCapture, capture.

In this case, capture is taken directly from a video camera, but you can also create it from a video file with:
CvCapture *capture = cvCreateFileCapture("filename.avi");

Then, I would draw on the image with functions defined here: http://opencv.willowgarage.com/documentation/drawing_functions.html

By the way, the shape in the Youtube video is not a rectangle. It’s a parallelogram.

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If you want to do it live, then you can basically put this in a loop, getting a frame, processing it, drawing on it, and then outputting the image, like this:

You would include this before your loop:
cvNamedWindow("Capture", CV_WINDOW_AUTOSIZE);

And then, in your loop, you would say this:
cvShowImage("Capture", frame);

After the processing.

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EDIT To do this in C++, open your webcam like this:

VideoCapture cap(0); // open the default camera
if(!cap.isOpened())  // check if we succeeded
    return -1;

To initialize it from a file, instead of putting in the camera index, put the file path.

Get a frame from the camera like this:

 Mat frame;
 cap >> frame; // get a new frame from camera

Then you can find drawing functions here:
http://opencv.willowgarage.com/documentation/cpp/core_drawing_functions.html

Cheers!