Game overlay is for rendering contents on top of game screen. Representative examples are Discord and Steam in-game overlay. They are complicated because it has to hook rendering part of a game.

Asdf overlay provides easy to use interfaces for rendering on top of game screen. I recognize game performance degradation due to overlay rendering, so GPU shared texture was used to avoid CPU framebuffer copy. Asdf Overlay is capable of rendering full screen overlay without noticeable performance loss.