数字图像的分段线性灰度变换的方法，其中假定a=30,b=180,c=120,d=230,e=255 且假定图像的最大灰度值为Mf-Digital image of the piecewise linear gray level transformation method, which assumes that the a = 30, B = 180, C = 120, d = 230, e = 255　and it is assumed that the maximum gray value of Mf image