OpenCV stop rendering while mouse moving - c++

Question

I have a simple window that contains simple black image with small solid circle inside it. I have wrote a simple code to be able to drag and drop this circle. I could do it correctly. Inside the mouse_event function:
void on_mouse_event(int event_type, int x, int y, int flags, void*){
if (event_type == cv::EVENT_RBUTTONDOWN){
//Catch the circle
}
else if (event_type == cv::EVENT_MOUSEMOVE){
//Release the point
}
else if (event_type == cv::EVENT_MOUSEMOVE){
//Change circle position according to curser moving
//re draw the circle again
//show the new image
}
}
The main function:
while (true){
//show image code (simple cv::imshow);
if (cv::waitKey(1) == 27){
break;
}
}
The problem is that if I drag the circle and start to move fast, the image will not change till I stop. However, if I go slowly it will change according to the move. What is the reason of this problem?
P.S I am not in doubt of slow hardware at all. I am working on workstation and I am mentoring the processor utilization and just one of its 8 core reach around 50% and the memory is almost free.
I am using Windows 10 if it helps.

Answer 1

you could test the following code.(adapted from opencv_annotation.cpp)
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>
using namespace std;
using namespace cv;
// Function prototypes
void on_mouse(int, int, int, int, void*);
// Public parameters
Mat image(600, 800, CV_8UC3, Scalar(220, 220, 220));
Mat current_view;
int circle_center_x = image.cols / 2, circle_center_y = image.rows / 2, radius = 40;
bool dragging = false;
const string window_name = "OpenCV Mouse Event Demo";
void on_mouse(int event, int x, int y, int, void *)
{
// Action when left button is clicked
if (event == EVENT_LBUTTONDOWN & (abs(circle_center_x - x) < 20) & (abs(circle_center_y - y) < 20))
{
dragging = true;
}
if (event == EVENT_LBUTTONUP)
{
dragging = false;
}
// Action when mouse is moving
if ((event == EVENT_MOUSEMOVE) && dragging)
{
image.copyTo(current_view);
circle_center_x = x;
circle_center_y = y;
circle(current_view, Point(circle_center_x, circle_center_y), radius, Scalar(255, 0, 0), 5);
imshow(window_name, current_view);
}
}
int main(int argc, const char** argv)
{
// Init window interface and couple mouse actions
namedWindow(window_name, WINDOW_AUTOSIZE);
setMouseCallback(window_name, on_mouse);
image.copyTo(current_view);
circle(current_view, Point(circle_center_x, circle_center_y), radius, Scalar(255, 0, 0), 5);
imshow(window_name, current_view);
int key_pressed = 0;
do
{
// Keys for processing
// Based on the universal ASCII code of the keystroke: http://www.asciitable.com/
// <SPACE> = 32 add circle to current image
// <ESC> = 27 exit program
key_pressed = 0xFF & waitKey(0);
if (key_pressed==32)
{
// draw a circle on the image
circle(image, Point(circle_center_x, circle_center_y), radius, Scalar(0, 0, 255), -1);
image.copyTo(current_view);
circle(current_view, Point(circle_center_x, circle_center_y), radius, Scalar(255, 0, 0), 5);
imshow(window_name, current_view);
}
}
// Continue as long as the <ESC> key has not been pressed
while (key_pressed != 27);
// Close down the window
destroyWindow(window_name);
return 0;
}