Thank you for your answer!
Okey, what i meant with "a program create print screens" was that the program - as you said - copies the entire screen. Is there any way a program can know when the other program takes the capture of the screen? I don't know in what way the program will copy the screen. The program copies the screen from a game called "Counter-Strike" which is usually played with OpenGL and full-screen.
Is there anything you need to know before you can give me a hint?
