get handle to current active window in OpenCV - opencv

Question

Are there OpenCV equivalents of the GLUT glutGetWindow()/glutSetWindow() functions, which allows the current active window to be identified and switched from your own codes?
Basically, I'd like to able to identify the current active window from a within a mouse callback function registered with all windows, and have it call another processing function with different parameters for each window.
Any help would be appreciated.

Answer 1

There's no function to do that in OpenCV, however, the signature of cvSetMouseCallback() allows you to register one callback per window.
You will have to register individual callbacks to achieve what you need to do.
Here is the complete list of features supported by the HIGHGUI module.
Another (hardcore) alternative is to dive into the native API of the OS you are working with and search for methods that accomplish this. The problem is that this solution is not cross-platform.

Answer 2

Actually, cvGetWindowHandle(const char* windowname) is available up in opencv/highgui/highgui_c.h. This is available up until openCV 4 when this answer was written.
I suggest that you add
#include <opencv/highgui/highgui_c.h>
and use
cvGetWindowHandle(window_name_.c_str())

Answer 3

Include <opencv / highgui / highgui_c.h> could be a solution, but it really won't let you turn to Opencv4 +.
For those of you who are still using Opencv in MFC DialogBox, there is a different solution
FindWindows returns the Parent Window handle, and MFC works with the child window, so you'll need FindWindow and FindWindowEx.
New source code for MFC and Opencv4+
namedWindow(windowname, WINDOW_AUTOSIZE);
////// This will work on opencv 4.X //////
HWND hParent = (HWND)FindWindow(NULL, windowname.c_str());
HWND hWnd = (HWND)FindWindowEx(hParent, NULL, L"HighGUI class", NULL);
::SetParent(hWnd, GetDlgItem(IDC_PICTURE)->m_hWnd);
::ShowWindow(hParent, SW_HIDE);
CWnd* pWnd = new CWnd();
pWnd->CWnd::Attach(hParent);
Maybe you're still in troubles because string to LPCWSTR conversion fails, and hParent returns NULL. There is many ways to convert string to LPCWSTR, but because you are using MFC, try
namedWindow(windowname, WINDOW_AUTOSIZE);
////// This will work on opencv 4.X //////
CString CstrWindowname = windowname.data();
HWND hParent = (HWND)FindWindow(NULL, CstrWindowname);
HWND hWnd = (HWND)FindWindowEx(hParent, NULL, L"HighGUI class", NULL);
::SetParent(hWnd, GetDlgItem(IDC_PICTURE)->m_hWnd);
::ShowWindow(hParent, SW_HIDE);
CWnd* pWnd = new CWnd();
pWnd->CWnd::Attach(hParent);
The new code should replace this old code
namedWindow(windowname, WINDOW_AUTOSIZE);
///// OLD version. Used on opencv 3.X on MFC Dialog Box /////
HWND hWnd = (HWND) cvGetWindowHandle(windowname.c_str());
HWND hParent = ::GetParent(hWnd);
::SetParent(hWnd, GetDlgItem(IDC_PICTURE)->m_hWnd);
::ShowWindow(hParent, SW_HIDE);
CWnd* pWnd = new CWnd();
pWnd->CWnd::Attach(hParent);
Try,

Answer 4

Well, there is no OpenCV API for retreiving focused window, but OS GUI Shell usually provides. Using this approach would be better because mouse callbacks can't detect ALT-TAB and programmatic focusing.
Here's some example code on python for windows that gets the job done:
import ctypes
import cv2
user32 = ctypes.windll.user32
def exists_cv_window(title):
# seems to work on python-opencv version 4.6.0
return cv2.getWindowProperty(title, cv2.WND_PROP_VISIBLE) != 0.0
def get_active_cv_window():
focused_window_handle = user32.GetForegroundWindow()
length = user32.GetWindowTextLengthW(focused_window_handle)
buffer = bytes([0]) * 2 * length
buff_pointer = ctypes.c_char_p(buffer)
user32.GetWindowTextW(window_handle, buff_pointer, length)
active_window_title = buffer.decode('utf-16')
if exists_cv_window(active_window_title):
return active_window_title
# example use case for the function
def main():
im1 = cv2.imread('cookie.png')
im2 = cv2.imread('cat.png')
cv2.imshow('figure 1', im1)
cv2.imshow('figure 2', im2)
while True:
key = cv2.waitKey(10)
if key == 23: # CTRL + W
title = get_active_cv_window()
if title is not None:
cv2.destroyWindow(title)
# in the example above the ability to target active window allows applying
# CTRL + W shortcut to a specific figure
It's a shame this is not part of OpenCV