LÖVE

The LÖVE wiki shows that love.graphics.setBackgroundColor can take four arguments, the last being alpha, but this has no effect. Is this a bug or an error with the wiki ?

Neither – it does take alpha and it does technically affect the framebuffer inside the window, it just doesn't make the window transparent (it's not currently possible to make the window transparent, and doing so would be separate from the opacity of the framebuffer inside the window).

There are currently no [wiki]BlendMode[/wiki]s which use the screen's current alpha to affect its color, but if there were then the alpha component of setBackgroundColor would affect them.

If you call [wiki]love.graphics.newScreenshot[/wiki](true) to make it include the alpha values of the screen in the screenshot's data, you can find out what the alpha values of each pixel on the screen are.