Love2d Postprocessing

function postprocessing()
    local graphics = love.graphics      --optimisation cache
    local buffer = graphics.newCanvas() --buffer in upvalue
    local resX, resY = graphics.getDimensions()
    return function(canvas, shader)
        local x, y = graphics.getDimensions()
        if x ~= resX or y ~= resY then buffer = graphics.newCanvas(); resX, resY = x, y end
        graphics.setCanvas(buffer)
        graphics.setShader(shader)
        graphics.draw(canvas)
        graphics.setShader()
        graphics.setCanvas(canvas)
        graphics.draw(buffer)
        graphics.setCanvas()
    end
end

--using:
function love.load()
-- new postprocessor
  post = postprocessing()
-- framebuffer
  canvas = love.graphics.newCanvas()
-- loading shaders
  shader1 = love.graphics.newShader('shader1.glsl')
  shader2 = love.graphics.newShader('shader2.glsl')
end

function love.draw()
  love.graphics.setCanvas(canvas)
  love.graphics.rectangle('fill', 100, 200, 100, 200)
  love.graphics.setCanvas()
-- applying shaders
  post(canvas, shader1)
  post(canvas, shader2)
-- draw framebuffer
  love.graphics.draw(canvas)
end