说明：  低通滤波及其相关函数，可用于平滑图像。低通滤波去掉了高频信息，即细节信息，留下的低频信息代表了概貌。常用的例子，比如美图秀秀的磨皮，去掉了脸部细节信息（痘坑，痘印，暗斑等）
(%LPFILTER Computes frequency domain lowpass filters. % H = LPFILTER(TYPE, M, N, D0, n) creates the transfer function of % a lowpass filter, H, of the specified TYPE and size (M-by-N). To % view the filter as an image or mesh plot, it should be centered % using H = fftshift(H). % % Valid values for TYPE, D0, and n are: % % 'ideal' Ideal lowpass filter with cutoff frequency D0. n need % not be supplied. D0 must be positive. % % 'btw' Butterworth lowpass filter of order n, and cutoff % D0. The default value for n is 1.0. D0 must be % positive. % % 'gaussian' Gaussian lowpass filter with cutoff (standard % deviation) D0. n need not be supplied. D0 must be % positive. % Copyright 2002-2004 R. C. Gonzalez, R. E. Woods, & S. L. Eddins % Digital Image Processing Using MATLAB, Prentice-Hall, 2004 % $Revision: 1.8 $ $Date: 2004/11/04 22:33:16 $)