I want to develop a tool which can render content over current window. The window maybe a full screen game and the game may be developed with dx or opengl etc. I know that I can create a dll and inject the destination window with different APIs accroding to different game engines. But that's a little complex and the compatibility is not good for all engines. Is there any way to realize it? I noticed this link:https://www.gamedev.net/forums/topic.asp?topic_id=359319​ but the download address is invalid now. Any help will be appreciated.

Depends on your platform. taking Windows as an example platform, there are WINAPI calls to get running processes and for every process to get a list of the windows contained by those processes. You can get the primary window of every game you like and attaching your overlay window to it as a child should work without any big trouble