Average value of an annulus in a square matrix

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matnewbie 2016-5-6

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编辑： matnewbie 2016-6-18

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.

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Answer 1

Image Analyst 2016-5-6

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编辑：Image Analyst 2016-5-6

在 MATLAB Online 中打开

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.

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matnewbie 2016-6-18

编辑：matnewbie 2016-6-18

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

Zero values should not be taken into consideration when evaluating the profile.
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.

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Answer 2

Jos (10584) 2016-5-6

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https://ww2.mathworks.cn/matlabcentral/answers/282938-average-value-of-an-annulus-in-a-square-matrix#answer_221124

在 MATLAB Online 中打开

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