simple matlab function of angular spectrum method of fast calculation of diffraction with band limitation setting, zero padding setting and so on.
简单的角谱衍射计算matlab函数，可以设置是否加入带限、是否填0等。
function[final] = ASM(cut,direction,bandlimit,quan,mu,z,pitch,lambda)
cut:output is set as the same size of input 要不要剪裁为原图大小
direction:direction of the propagation 传播方向
bandlimit:band-limited ASM 是否用带限角谱
quan:input of the complex wave field 输入复振幅
mu:zero padding size(How many times the size of the original complex wave field) 填0为原图大小的几倍
z:distance 距离
pitch: pixel size 像素大小
lambda: wavelength 波长
example，示例:
finallimit=ASM('cut','forward','limit',p,1,z,pitch,lambda);
finallimit=ASM('nocut','backward','nolimit',p,1,z,pitch,lambda);
finallimit=ASMshift('shift','nocut','backward','nolimit',p,1,z,pitch,lambda);
the band limitation method is from "Band-Limited Angular Spectrum Method for Numerical Simulation of Free-Space Propagation in Far and Near Fields"
带限角谱计算方法来自 "Band-Limited Angular Spectrum Method for Numerical Simulation of Free-Space Propagation in Far and Near Fields"