该函数可将输入值低于m的灰度级压缩为输出图像中较暗灰度级的较窄范围内，可将输入值高于m的灰度级压缩为输出图像中较亮灰度级的较窄范围内，输出的是一副具有较高对比度的图像。-This function can be lower than the input value for compressed grayscale m output image is dark gray level of a narrow range, but will the input value is higher than the gray level compression for m output image lighter gray level of a narrow range, the output is a pair of high contrast images.