Finding a circle in a 2D array

This is not circle finding in an image, which has been well covered, but in an array.

I have an array P with several approximately-circular areas of a low value. I would like to find the centres of those areas to calibrate my device. Here is a contour 2D plot of P. The contour plot looks a bit odd because I only obtained a patch of data around each circle (the box) and it is doing a linear interpolation between those.

This is a one-time calibration so I'm happy to do some manual setting up work.

Thanks in advance!

Jack

This is how I display the contour:

figure(1)

pcolor(X,Y,P);

shading interp

hold on

colorbar

hold off

PS this is my first question so do say if there is anything I can improve for next time.

Answer 1

[Matt J]

load P

BW=median(P(:))-P>50;

BW=bwpropfilt(bwareaopen(BW,100),'Eccentricity',6,'small');

c=vertcat(regionprops(BW,'Centroid').Centroid);

imagesc(BW);colormap(gray); axis off; hold on
plot(c(:,1),c(:,2),'r.','MarkerSize',12); hold off

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[Jack Lavender]

Thanks very much for your answer!

That's great.

[Matt J]

You're welcome, but please Accept-click the answer that you decided to use.

Answer 2

[Walter Roberson]

There are no functions to find circles in "images" : there are only functions to find circles in arrays. When it is necessary to find circles in "images", the program must first read the image file into an array, and then find circles in the array that was read in.

There is no difference in circle finding routines between the case where the array was read from an image file, compared to the case where the array was computed.

[Jack Lavender]

Thanks very much for your explanation, makes perfect sense.

Cheers!