-- negative (from a gluas sample)
-- origine http://www.mark0.net/plugins-doglua-e.html   



 local x, y, r, g, b



 for y = 0, height - 1 do

   for x = 0, width - 1 do

     r, g, b = get_rgb(x, y)

     r = 1 - r

     g = 1 - g

     b = 1 - b

     set_rgb(x, y, r, g, b)

   end

   progress(y / height)

 end