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

Began life as a copy of #324