Hi!
Let's start a library of transition effects!
They should take one argument called progress. If you want to fade out instead of in you just use fade(1-progress).
My first contribution:

function fade(progress)
	for xx=0,15 do
		for yy=0,15 do
			circfill(xx*8+4,yy*8+4,8*(1-progress)+sin((yy)/15),0)
		end
	end
end