Getting screenshot of a child window running OpenGL in it (Windows)

Question

I have a main window with children. I need to take screenshots programmatically to crop and draw them back on my main window. The way I do this is:

HDC hDC =         


        

Answer 1

I assume you are on win platform.

I thing you can render to bitmap even if you do not have the source code for child window.

1. First obtain correct Handle to the child window
   • correct handle search is very tricky because almost everything in win is a window (so there are hundreds handles...)
2. create bitmap
3. change handle of rendering/device context of screenshoted window to your bitmap.
   • not all windows can do that!!!
4. force repaint
   • minimize/restore win or something
5. store your new bitmap
6. restore render/device contexts of your screenshoted window.

Beware this is not very stable process (OpenGL can make conflicts during context handles change) and in some cases you even cannot change handles to contexts

More safe is to screenshot desktop it self.

TCanvas *scr=new TCanvas();
scr->Handle=GetDC(GetDesktopWindow());
// copy scr to your bitmap (do not forget to resize bitmap)

Warning do not obtain/create/release contexts or win handles too often because its a little buggy (sometimes winapi functions stops returning correct handler values) better way is obtain handle once and hope that no one closes window ... for your child wins you know exactly when to get new handler) That is true also for IsWindow() function.

For more info take a look at:

• How can I access a graphics card's output directly?