How could I display a crosshair cursor with x-y coordinates when hovering over a picturebox?

Question

I want to make a crosshair pointer when hovering over a picturebox and store a co-ordinates when mouse left button is pressed over a picturebox.

My code looks like below:

private: System::Void button1_Click(System::Object^  sender, System::EventArgs^  e) {
        cv::VideoCapture cap;
        cap.open(0);

        if (!cap.isOpened()) {
            MessageBox::Show("Failed To Open WebCam");
            _getch();
            return;
        }

        ///query_maximum_resolution(cap, pictureBox1->Width, pictureBox1->Height);
        cap.set(CV_CAP_PROP_FRAME_WIDTH, pictureBox1->Width);
        cap.set(CV_CAP_PROP_FRAME_HEIGHT, pictureBox1->Height);

        Pen^ myPen = gcnew Pen(Brushes::Red);

        while (1)
        {
            cap.read(frame);

            pictureBox1->Image = mat2bmp.Mat2Bimap(frame);
            Graphics^ g = Graphics::FromImage(pictureBox1->Image);
            Point pos = this->PointToClient(System::Windows::Forms::Cursor::Position);
            g->DrawLine(myPen, pos.X, 0, pos.X, pictureBox1->Height);
            g->DrawLine(myPen, 0, pos.Y, pictureBox1->Width, pos.Y);
            pictureBox1->Refresh();
            delete g;
        }
    }

But when I run the code it becomes slower and non responsive. Any idea to make it fast and efficient. Any help would be helpful.

Answer 1

IO happens on the UI thread, which is the main application UI rendering thread. Button click is an event handler and will come on the UI thread. If you have a while loop running inside your UI thread, it will make the application hang. Work done on UI thread should be small or Asynchronous.

Edit 1: Just found out that you have marked winform as one of the tag. If you are using winforms, you have to add a MouseHover event handler to your UI controls. Whenever the mouse reaches in this area, this method will be called ( https://docs.microsoft.com/en-us/dotnet/api/system.windows.forms.control.mousehover?view=netframework-4.7.2 ). In this method just write the above code without the while loop. Something like this.

private: System::Void button1_MouseHover(System::Object^  sender, System::EventArgs^  e) {
    cv::VideoCapture cap;
    cap.open(0);

    if (!cap.isOpened()) {
        MessageBox::Show("Failed To Open WebCam");
        _getch();
        return;
    }

    ///query_maximum_resolution(cap, pictureBox1->Width, pictureBox1->Height);
    cap.set(CV_CAP_PROP_FRAME_WIDTH, pictureBox1->Width);
    cap.set(CV_CAP_PROP_FRAME_HEIGHT, pictureBox1->Height);

    Pen^ myPen = gcnew Pen(Brushes::Red);

    cap.read(frame);

    pictureBox1->Image = mat2bmp.Mat2Bimap(frame);
    Graphics^ g = Graphics::FromImage(pictureBox1->Image);
    Point pos = this->PointToClient(System::Windows::Forms::Cursor::Position);
    g->DrawLine(myPen, pos.X, 0, pos.X, pictureBox1->Height);
    g->DrawLine(myPen, 0, pos.Y, pictureBox1->Width, pos.Y);
    pictureBox1->Refresh();
    delete g;
}

Note: This event also comes in the UI thread. This will come everytime mouse oves in the region of interest. Hence you will not need the while loop. Adding a while loop here will again result in the same issue that you have asked in the question.