let inline bresenham fill (x0, y0) (x1, y1) =
  let steep = abs(y1 - y0) > abs(x1 - x0)
  let x0, y0, x1, y1 =
    if steep then y0, x0, y1, x1 else x0, y0, x1, y1
  let x0, y0, x1, y1 =
    if x0 > x1 then x1, y1, x0, y0 else x0, y0, x1, y1
  let dx, dy = x1 - x0, abs(y1 - y0)
  let s = if y0 < y1 then 1 else -1
  let rec loop e x y =
    if x <= x1 then
      if steep then fill y x else fill x y
      if e < dy then
        loop (e-dy+dx) (x+1) (y+s)
      else
        loop (e-dy) (x+1) y
  loop (dx/2) x0 y0