function tablelength(T)

  local count = 0

  for _ in pairs(T) do count = count + 1 end

  return count

end

function getcrop(img)

  local w, h, g = img.width, img.height, img.getInt

  local x1, y1, x2, y2 = 0, 0, w, h

  -- left side

  local colors = {}

  while x1 < x2 do

    for y=y1, y2-1 do

      colors[g(x1, y)] = 1

    end

    if tablelength(colors) > 1 then break end

    x1=x1+1

  end

  -- top side

  colors = {}

  while y1 < y2 do

    for x=x1, x2-1 do

      colors[g(x, y1)] = 1

    end

    if tablelength(colors) > 1 then break end

    y1=y1+1

  end

  -- right side

  colors = {}

  while x2 > x1 do

    for y=y1, y2-1 do

      colors[g(x2-1, y)] = 1

    end

    if tablelength(colors) > 1 then break end

    x2=x2-1

  end

  -- bottom side

  colors = {}

  while y2 > y1 do

    for x=x1, x2-1 do

      colors[g(x, y2-1)] = 1

    end

    if tablelength(colors) > 1 then break end

    y2=y2-1

  end

  if x2 <= x1 or y2 <= y1 then -- weird result? return full image

    x1, y1, x2, y2 = 0, 0, w, h

  end

  return x1, y1, x2, y2

end