直方图均衡法　利用原始图像的灰度直方图把比较集中的某个灰度区间变成在 全部灰度范围内的均匀分布，把给定图像的直方图分布改变成“均匀”分布直 方图分布. -Histogram equalization method using the original image histogram to a relatively concentrated into a gray zone in all gray uniform distribution within the scope