How to filter out annular data from square matrix

Have a square matrix...501 by 501...Z=501×501

Need to filter out an annular data 20 to 150 centered at (251,251)

Answer 1

[Image Analyst]

See the FAQ for creating a circle:

https://matlab.wikia.com/wiki/FAQ#How_do_I_create_a_circle.3F

Then to get row and column indexes for your matrix, simply round, making sure that x is the second dimension, not the first because arrays are indexes as array(y, x) NOT array(x,y)

rows = round(y);
columns = round(x);
for k = 1 : length(rows)
  Z(rows(k), columns(k)) = 0; % Filter to have whatever value you want them to have.
end

[Bhowmik.U]

Thanks a lot Sir, it works like cream! Problem solved

A last help I shall request in this thread

I converted the image to pixel format as in figure. I have a 20x20 matrix of UU_avg (attached Excel file) I am also sharing the code which I use to plot..and the resultant image (png).

*However, the resultant plot (in the same enclosed word file) shows only 2 axes..the other two seems to be non-existent

Is there a way out?*

code:

im=surf(UU_avg,'EdgeColor','none','LineStyle','none');
view(-90,90);
xtick = [0, 2.5, 7.5, 10, 12.5, 17.5, 20];
ytick = [0, 2.5, 7.5, 10, 12.5, 17.5, 20];
xticklabels = {'-150','-100', '-50', '0','50', '100', '150'};
yticklabels ={'-150','-100', '-50', '0','50', '100', '150'};

set(gca, 'XTick', xtick);
set(gca, 'YTick', ytick);
set(gca, 'YTickLabel', yticklabels);
set(gca, 'XTickLabel', xticklabels);
hh=colorbar;
title(hh,'%','FontSize',14,'fontweight','bold');
set(gca,'FontSize', 14,'fontweight','bold');
set(gca,'Linewidth',3);
axis tight;
box on;

[Image Analyst]

Looks fine to me

Probably a rendering issue. Look at the opengl() function and try some different things.

[Bhowmik.U]

Thanks again Sir!

Answer 2

[Bhowmik.U]

Thanks for the suggestion..however I tried on similar lines but that yielded concentric boxes of data..rather than circles...