get("#348") -- rgb functions 

-- returns value between 0 and 1
-- img must be cropped already
function triangleness(img)
  local w, h = img.width, img.height
  local stepx = w < 40 and 1 or 4
  local stepy = h < 40 and 1 or 4
  
  local total = 0
  for y = 0, h-1, stepy do
    local sum = 0
    for x = 0, w-1, stepx do
      sum = sum+bright(rgb(img.getInt(x, y)))
    end
    sum = sum/math.floor(w/stepx)
    local expected = 1-(y+1)/h
    total = total+math.abs(sum-expected)
  end
  
  return 1-total/math.floor(h/stepy)
end

Began life as a copy of #496