main.lua

function love.load()
	canvas = love.graphics.newCanvas(800, 600)
	x = 400
	y = 300
	r = 10
	vx = math.random() * 100
	vy = math.random() * 100
end

function love.update(dt)
	x = x + vx * dt
	y = y + vy * dt
	if x < 0 or x > 800 - r then
		vx = - vx
	end
	if y < 0 or y > 600 - r then
		vy = - vy
	end
end

function love.draw()
	love.graphics.setCanvas(canvas)
	love.graphics.setBlendMode("alpha")
	love.graphics.setColor(0, 0, 0, 5)
	love.graphics.rectangle("fill", 0, 0, 800, 600)
	love.graphics.setColor(255, 255, 255, 255)
	love.graphics.rectangle("fill", x, y, r, r)
	-- draw
	love.graphics.setCanvas()
	love.graphics.clear()
	love.graphics.setBlendMode("alpha", "premultiplied")
	love.graphics.draw(canvas)
end