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Keys
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<F8> save screenshot to 'Snap' sub directory
<ALT> + <ENTER> switch between window/fullscreen mode (can crash if
the desktop color depth is different from the chosen fullscreen color depth)
<INSERT> show/hide the gpu version (if no FPS is displayed) or an help text
(if the FPS menu is displayed)
<DEL> show/hide FPS and option menu

How it works: Hit <DEL> and the Framerate und the menu will appear. It
looks like: 'FPS XXXX.X FL< FS OD FI SF SL FU FE GF [] % ( * 2x Ax'

What does it all mean? Here's the legend:

FPS: frames per second, higher means better... up to 50/60 fps at least
FL : Frame rate limiter (none, manual, auto-detect)
FS : Frame skipping