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Chapter 1 ■ Practical Aspects of a Vision System 11

robotics, games, biometrics, and places where computers are easily available
and very high quality is not essential.
There are a great many types of webcam, and the details of how they work
are not relevant to this discussion. If a webcam is properly installed, then
OpenCV should be able to detect it, and the capture functions should be able
to acquire images from it. The scheme used by OpenCV is to first declare and
initialize a camera, using a handle created by the system. Assuming that this
is successful, images can be captured through the handle.
Initializing a camera uses the cvCaptureFromCAM function:

CvCapture *camera = 0;
camera = cvCaptureFromCAM( CV_CAP_ANY );
if( !camera ) error ...

The type CvCapture is internal, and represents the handle used to capture
images. The function cvCaptureFromCam initializes capturing a video from a
camera, which is specified using the single parameter. CV_CAP_ANY will allow
any connected camera to be used, but the system will choose which one. If
0 is returned, then no camera was seen, and image capture is not possible;
otherwise, the camera’s handle is returned and is needed to grab images.
A frame (image) can be captured using the cvQueryFrame function:

IplImage *frame = 0;
frame = cvQueryFrame( camera );
The image returned is an IplImage pointer, which can be used immediately.
When the program is complete, it is always a good idea to free any resources
allocated. In this case, that means releasing the camera, as follows:

cvReleaseCapture( &camera );

It is now possible to write a program that drives the webcam. Let’s have
the images displayed in a window so that the live video can be seen. When a
key is pressed, the program will save the current image in a JPEG file named
VideoFramexx.jpg,where xx is a number that increases each time.

// Capture.c - image capture from a webcam
#include “stdafx.h“
#include “stdio.h“
#include “string.h“
#include “cv.h“
#include “highgui.h“

int main(int argc, char ** argv)
{
CvCapture *camera = 0;