Average value of an annulus in a square matrix

I have a square matrix of values and I need to evaluate the average value of an annulus between coordinates r and r+dr and repeat this step from the center of the circle to its radius R. Finally, I want to plot the radial profile of the N values, where N is the number of annuli.

How could I do this in MATLAB? Thank you in advance.

Answer 1

[Image Analyst]

It's in the FAQ: http://matlab.wikia.com/wiki/FAQ#How_do_I_create_a_ring.3F

So then take your ringPixels, which you get from the snippet in the FAQ, and then use mean to get the average value of your matrix within the ring:

meanValue = mean(yourImage(ringPixels))

By the way, I've attached a related thing. The demo computes the average radial profile (from a point you select) in an image and plots it.

[matnewbie]

The script is ok, but I want to change two things:

1. Zero values should not be taken into consideration when evaluating the profile.
2. I want to obtain an overall radial profile on a 3D isotropic matrix (NxNxN) with center located at XCEN,YCEN and radius RAD. In this case I need to average the values on 3D annuli from 1 to RAD.

Answer 2

[Jos (10584)]

R = 3 ;
M = magic(2*R+1) ;
dr = 1 ;

[ri,ci] = ndgrid(1:(2*R+1)) 
D = hypot(ri-R-1,ci-R-1)
for k=dr:dr:R
   TF = D > (k-dr) & D <= k 
   tmp = M(TF)
   Result(k) = mean(tmp)
end