function result=fastintm(I,factor)
%interpolates (bilinearly) matrix I given at equally spaced grid points
%to points displaced by multiplicative group centered at mid point
[l1,l2]=size(I);mid=[l1/2,l2/2];
[Y,X]=meshgrid(1:l2,(1:l1)');
XI=mid(1)+factor.*(X-mid(1));M=XI>l1.*ones(l1,l2);XI=(1-M).*XI+M.*l1.*ones(l1,l2);
M=XI<ones(l1,l2);XI=(1-M).*XI+M.*ones(l1,l2);
YI=mid(2)+factor.*(Y-mid(2));M=YI>l2.*ones(l1,l2);YI=(1-M).*YI+M.*l2.*ones(l1,l2);
M=YI<ones(l1,l2);YI=(1-M).*YI+M.*ones(l1,l2);


result=interp2(Y,X,I,YI,XI,'nearest');