使用響應事件、回撥函式完成利用OpenCV畫矩形的功能。

CvRect box;
bool drawing_box = false;

void draw_box(IplImage* img, CvRect rect)
{
	cvRectangle(
		img,
		cvPoint(box.x, box.y),
		cvPoint(box.x + box.width, box.y + box.height),
		CvScalar(0xff, 0xff, 0xff)
		);
}

void my_mouse_callback(int event, int x, int y, int flags, void *param)
{
	IplImage *image = (IplImage*)param;
	
	switch (event)
	{
		case CV_EVENT_MOUSEMOVE:
			{
				if (drawing_box)
				{
					box.width = x - box.x;
					box.height = y - box.y;
				}
			}
			break;
		case CV_EVENT_LBUTTONDOWN:
			{
				drawing_box = true;
				box = CvRect(x,y,0,0);
			}
			break;
		case CV_EVENT_LBUTTONUP:
			{
				drawing_box = false;
				if (box.width < 0)
				{
					box.x += box.width;
					box.width *= -1;
				}
				if (box.height < 0)
				{
					box.y += box.height;
					box.height *= -1;
				}
				draw_box(image, box);
			}
			break;
		default:
			break;
	}
}

int main()
{
	box = cvRect(-1, -1, 0, 0);
	IplImage* image = cvCreateImage(cvSize(200,200), IPL_DEPTH_8U, 3);
	cvZero(image);
	IplImage* temp = cvCloneImage(image);

	cvNamedWindow("Box Example");
	cvSetMouseCallback("Box Example", my_mouse_callback, (void*)image);

	while (1)
	{
		cvCopy(image, temp);
		if (drawing_box)
		{
			draw_box(temp, box);
		}
		cvShowImage("Box Example", temp);
		if (cvWaitKey(15) == 27)
		{
			break;
		}
	}

	cvReleaseImage(&image);
	cvReleaseImage(&temp);
	cvDestroyWindow("Box Example");

	return 0;
}