get("#348") -- rgb functions

-- returns value between 0 and 1
function averageBrightness(img, x1, y1, x2, y2)
  local sum, count = 0, 0
  local w, h, g = img.width, img.height, img.getInt
  for y=y1, y2-1 do
    if y >= 0 and y < h then
      for x=x1, x2-1 do
        if x >= 0 and x < w then
          count = count+1
          sum = sum+bright(rgb(g(x, y)))
        end
      end
    end
  end
  return count == 0 and nil or sum/count
end