love2d postprocessing and using one shader multiple times

--[[
Reference:
local Effect = requre'effect'
local shader = love.graphics.newShader[[			-- yet another shader (wawe-effect)
	extern float time = 0;
	extern vec2 point = vec2(0, 0);
	extern float size = 100;
	extern float speed = 1;
	extern float radius = 2;
	float t = time;
	float distance(vec2 x){
		return pow(x.x*x.x + x.y*x.y, 0.5);
		};
	vec4 effect( vec4 color, Image texture, vec2 tc, vec2 sc ){
		vec2 d = point - tc;
		float dist = distance(d*vec2(1.0, 0.65));
		if (dist < radius){
			float delta = (cos(dist/time*speed*10)/2+0.5)/(dist*size);
			vec2 s = vec2(0.8, 0.6)*delta;
			return Texel(texture, tc + d*s)*color;
		}
		else return Texel(texture, tc)*color;
	}
]]


effect = Effect(shader) 							-- initialisation new shader-effect
image = love.graphics.newImage('image.png')			-- add image
framebuffer = love.graphics.newCanvas()				-- framebuffer
time = 0

function love.update(dt)
	love.window.setTitle(love.timer.getFPS())
	eff:update(dt)									-- update all shader-instances
	time = time + dt
end

function love.mousepressed(x, y, key)
	local x, y = love.mouse.getPosition(0)
	local ix, iy = love.graphics:getDimensions()
	eff:add(x/ix, y/iy, 5)							-- add new instance on 0-1 screen position by x/y and 5 seconds lifetime
end

function love.draw()
	love.graphics.setCanvas(canvas)		-- draw image on canvas
	love.graphics.draw(image)
	eff:draw(canvas)					-- apply all shader-instances on canvas
	love.graphics.setCanvas()
	love.graphics.draw(canvas)			-- draw it
	love.graphics.circle('line', love.mouse.getX(), love.mouse.getY(), 100)
end
]]



local function postprocessing()
	local graphics = love.graphics
	local buffer = graphics.newCanvas()
	local resX, resY = graphics.getDimensions()
	return function(canvas, shader, clear)
		assert(canvas:type() == 'Canvas', 'Argument #1 error: love2d-canvas expected. Use post processing only with canvases.')
		assert(shader:type() == 'Shader', 'Argument #2 error: love2d-shader expected.')
		local x, y = graphics.getDimensions()
		if x ~= resX or y ~= resY then buffer = graphics.newCanvas(); resX, resY = x, y end
		graphics.push('all')
			graphics.setCanvas(buffer)
			if clear then	graphics.clear(0.0, 0.0, 0.0, 0.0) end
			graphics.setShader(shader) graphics.draw(canvas) graphics.setShader()
			graphics.setCanvas(canvas) graphics.draw(buffer)
		graphics.pop()
	end
end

local effect = {}
setmetatable(effect, {__call =
	function(self, shader)
		local o = {}
		o.post = postprocessing()
		o.list = {}
		o.shader = shader
		self.__index = self
		return setmetatable(o, self)
	end
})

function effect:add(x, y, t)
	table.insert(self.list, {x = x, y = y, limit = t, t = 0})
end

function effect:update(dt)
	for i, v in ipairs(self.list) do
		v.t = v.t + dt
		if v.t > v.limit then table.remove(self.list, i) end
	end
end

function effect:draw(canvas)
	for i, v in ipairs(self.list) do
		self.shader:send('point', {v.x, v.y})
		self.shader:send('time', v.t)
		self.shader:send('size', (v.t+0.5)*200)
		self.post(canvas, self.shader)
	end
end

return effect