(x,y) bug in MouseCallback when image is larger than your screen (Bug #1523)

thanks for the report and sample. We will deal with the bug as soon as we can; for now I have to lower priority of the bug, since highgui is not meant to be very sophisticated tool. As a possible workaround, you can create the highgui window using namedWindow(title, 0), which will mean variable scale and so the window will fit the display.

I could not reproduce your problem on OSX. Even with 3000 rows those circles appear exactly where I click.

- Sebastian Haase

PS: thanks for posting a complete sample - that always helps a lot !

I couldn't reproduce it either (opencv 2.4.5, Ubuntu 12.04)

I'm able to reproduce this bug with Windows 7 using OpenCV 2.4.8 compiled with Visual Studio 2012. I do not have Qt enabled. Win32 UI and OpenGL support are enabled (although OpenGL is not used in affected window). It's just created with cv::namedWindow() and updated with cv::imshow() without any special flags.

It seems like mouse coordinates given to mouse callback are scaled with (window width / screen width, window height / screen height) as they are correct in top left corner of the window and using an image twice the height of native desktop seems to double the coordinate values for mouse callback.

Just moving the window outside visible area does not trigger the bug. The window must be larger than native desktop resolution for bug to trigger.

The problem is caused by event handler in window_w32.cpp that scales the image coordinates unconditionally for the case where CV_WINDOW_AUTOSIZE is not set. It also works correctly as long as actual window size and underlying image size is the same.

The functionality is wrong for the case where image is not drawn using rescaling (CV_WINDOW_AUTOSIZE in use) and created window does not have the correct size. In this case coordinates need to be original coordinates because part of the image is just clipped from right side or bottom.

By replacing function call in window_w32.cpp near line 1484

window->on_mouse(event, pt.x*size.cx / MAX(rect.right - rect.left, 1),
                        pt.y*size.cy / MAX(rect.bottom - rect.top, 1), flags,
                 window->on_mouse_param);

with the following code, all the cases seem to work on Windows as well:

if (window->flags & CV_WINDOW_AUTOSIZE)
{
    // As user can't change window size, do not scale window coordinates. Underlying windowing system
    // may prevent full window from being displayed and in this case coordinates should not be scaled.
    window->on_mouse(event, pt.x, pt.y, flags, window->on_mouse_param);
} else
{
    // Full window is displayed using different size. Scale coordinates to match underlying positions.
    window->on_mouse(event, pt.x*size.cx / MAX(rect.right - rect.left, 1),
                            pt.y*size.cy / MAX(rect.bottom - rect.top, 1), flags,
                     window->on_mouse_param);
}

Sorry for not providing an actual patch against the latest version.