function p=profile(x,y,a,b,fmax,fmin)
% compute an angle position of the point
% al=atan2(y/b,x/a);
bet=(pi./2+atan2(y./b,abs(x)./a))./pi-0.5;
 p = (fmax+fmin)./2+4.*(fmax-fmin).*bet.^3;