Thread Subject: Circular aperture

Hi,

I am trying to create a circular matrix of ones within a
matrix of zeros. The reason being, I'm trying to simulate
fraunhofer diffraction by using the fft2 algorithm within
matlab (2 dimensional fourier transform) of a circular
aperture.

I have tried using the fspecial('disk', radius) matlab
utility but have been unable to get it to be exactly in the
centre of a matrix of zeros.

So my question is, does anyone know how to construct a
matrix of ones in the middle of a matrix of zeros, or, do
they know how to use the fspecial utility such that i can
shift it too the middle? (i've tried fftshift but it does
not quite shift it to the middle).

Thanks for your time,

T

"T Matheson Matheson" <duvorap@hotmail.com> wrote in
message <g4l645$l2k$1@fred.mathworks.com>...
> Hi,
>
> I am trying to create a circular matrix of ones within a
> matrix of zeros. The reason being, I'm trying to simulate
> fraunhofer diffraction by using the fft2 algorithm within
> matlab (2 dimensional fourier transform) of a circular
> aperture.
>
> I have tried using the fspecial('disk', radius) matlab
> utility but have been unable to get it to be exactly in
the
> centre of a matrix of zeros.
>
> So my question is, does anyone know how to construct a
> matrix of ones in the middle of a matrix of zeros, or, do
> they know how to use the fspecial utility such that i can
> shift it too the middle? (i've tried fftshift but it does
> not quite shift it to the middle).
>
> Thanks for your time,
>
> T


If I understand your problem correctly, this may get you
started:

n = 51 ; % size of matrix, odd
R = 12 ; % radius
n2 = floor(n/2) ;
[x,y] = meshgrid(-n2:n2) ;
M = sqrt(x.^2 + y.^2) < R ;

M = double(M) ; % convert from logical to double
imshow(M)

hth
Jos

Thank you Jos, this is exactly what I wanted. I see I
still have alot to learn about Matlab!

Thanks again,

T