使用迭代式阈值算法来提高图像对比度。可以指定某个色阶作为阈值，所有比阈值亮的像素转换为白色；而所有比阈值暗的像素转换为黑色。

-The use of iterative threshold algorithm to enhance image contrast. You can specify a color scale as the threshold, all pixels lighter than the threshold into white　and all pixels darker than the threshold value is converted to black.