function tablelength(T)
  local count = 0
  for _ in pairs(T) do count = count + 1 end
  return count
end

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

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

-- left side

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 "Crop: "..x1..", "..y1..", "..x2..", "..y2

Better than #315 (always returns an actual rect)