Square function for binary grating

Hi everyone,

I try use square function to generate binary grating. But I found when I increase frequency, the period of pattern changed? Does anyone know this problem? Any suggestion would be very helpful.

Thank you so much.

Xiaolei

Code:

wDeg=1;%size of image(in degrees)
nxPix=80; 
nyPix=40;
[x,y]=meshgrid(linspace(0,wDeg,nxPix+1),linspace(0,wDeg,nyPix+1));
x=x(1:end-1,1:end-1);
y=y(1:end-1,1:end-1);
sf=20;

grating =1+square(2*pi*sf*x);