Simple circular fading shader, uses screen_coords instead of texture_coords.

main.lua

local w,h
local quad
local time, value
local mask_shader

function love.load()
	w,h = love.graphics.getDimensions()
	time = 0.0
	value = math.sin(time)
	quad = love.graphics.newQuad(0,0,w,h,w,h)

	mask_shader = love.graphics.newShader[[
		extern number time;
		extern number width;
		extern number height;
		vec4 effect(vec4 color, Image texture, vec2 texture_coords, vec2 screen_coords) {
			number x = (width  / 2) - screen_coords.x;
			number y = (height / 2) - screen_coords.y;
			number xx = x * x;
			number yy = y * y;
			number sum = xx + yy;
			number dist = sqrt(sum);
			if(dist > time * min(width, height)){
				return color;
			}
			else
			{
				discard;
			}
		}
	]]

	mask_shader:send("time", 0.0)
	mask_shader:send("width",  w)
	mask_shader:send("height", h)

end

function love.update(dt)
	dt = dt / 2
	time = time + dt
	value = (math.sin(time)+1)/2
end

function love.draw()
	love.graphics.setColor(255,0,0)
	love.graphics.rectangle('fill',0,0,w/2,h/2)
	love.graphics.setColor(0,255,0)
	love.graphics.rectangle('fill',w/2,0,w,h/2)
	love.graphics.setColor(0,0,255)
	love.graphics.rectangle('fill',0,h/2,w/2,h)
	love.graphics.setColor(255,255,255)
	love.graphics.rectangle('fill',w/2,h/2,w,h)
	love.graphics.setShader(mask_shader)
	mask_shader:send("time", value)
	love.graphics.setColor(0,0,0,255)
	love.graphics.rectangle('fill',0,0,w,h)
	love.graphics.setShader()
	love.graphics.print(value,0,0)
end