Need help to take screenshot of openGL app in other app (C++)

I have openGL application.
I need to take screenshot of this application in my application (c++).
I know HWND of openGl application. If ::GetDC(hwnd); and then ::BitBlt, it works for winForm applications but doesn’t work for openGL…
I take black picture.
May be somebody know how to openGl application window to HBITMAP or other…

In cross-platform OpenGL applications, it’s common to use glReadPixels() for screenshots.

glReadPixels() is affected by glReadBuffer(); the initial value is GL_BACK for double-buffered contexts. To read a frame which has already been rendered, you need to change it to GL_FRONT (the contents of the back buffer become undefined after a buffer swap, so reading from the back buffer only makes sense if you do it after rendering but before the buffer swap).

It is also affected by the GL_PACK_* settings for glPixelStore(). The default value for GL_PACK_ALIGNMENT is 4; change it to 1 unless the number of bytes per row is guaranteed to be a multiple of 4.

However: I don’t know if or how glReadPixels() interacts with the “Aero” compositing in later versions of Windows (that may be related to getting a black screenshot for OpenGL programs). In the worst case, you may need to redraw the frame, reading the back buffer prior to the swap.

this is part of my code

HWND hwnd;
	hwnd = FindWindow(L"some app", NULL);
	RECT r;
	::SetWindowPos(hwnd,HWND_TOP ,0,0,0,0,SWP_NOMOVE|SWP_NOSIZE);
	HDC dstDC = ::GetDC(NULL);
	HDC srcDC;
	::GetClientRect(hwnd, &r);
	SIZE sz = { r.right - r.left, r.bottom - };
	if ( <= 0 || <= 0)
		return false;
	srcDC = ::GetDC(hwnd);
	HDC memDC = ::CreateCompatibleDC(srcDC);
	HBITMAP bm = ::CreateCompatibleBitmap(dstDC,,;
	HANDLE oldBM = ::SelectObject(memDC, bm);
	::BitBlt(memDC, 0, 0,,, srcDC, 0, 0, SRCCOPY);
       ::SelectObject(memDC, oldBM);

what I need to add for grab openGl window?

Aero - off

Uh, I overlooked the fact that you’re trying to do this to a different application’s window. So ignore what I said.