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creating an image using a matrix

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Raldi
Raldi on 8 Dec 2011
lets say we have a 50x50 matrix. All of its values are zero. I wanted to create a way to add ones only to those places of the matrix that would create a sircle if i used imshow(). So i thought if yi,yj are the coordinates for each cell i maybe could use a function like (i-yi)^2+(j-yj)^2=r^2 to find each cell that i need. Does anyone has any ideas on that or maybe a better way?

Accepted Answer

Sven
Sven on 8 Dec 2011
Raldi, try this:
% Define the image size and where you want the circle
rows = 50;
cols = 50;
radius = 20;
center = [25 25]; % In [X,Y] coordinates
% Make the circle
[xMat,yMat] = meshgrid(1:cols,1:rows);
distFromCenter = sqrt((xMat-center(1)).^2 + (yMat-center(2)).^2);
circleMat = distFromCenter<=radius;
figure, imshow(circleMat)
UPDATE to match your comments:
% To get just the border
B = circleMat & ~bwmorph(circleMat,'erode',1);
figure, imshow(B)
  4 Comments
Sven
Sven on 8 Dec 2011
Cool, hit "accept" if it's answered then :)

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More Answers (3)

Sean de Wolski
Sean de Wolski on 8 Dec 2011
Or:
r = 11; %radius
cx = 25; %xcenter
cy = 25;
sz = 50; %mat size
C = bsxfun(@(x,y)hypot(x-cx,y-cy)<=r,(1:sz)',1:sz);
imshow(C)
  1 Comment
Raldi
Raldi on 8 Dec 2011
Thanks Sean, with a few modifications i did what i wanted.

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Andrei Bobrov
Andrei Bobrov on 8 Dec 2011
A = single(zeros(50));
A(ceil(m/2),ceil(n/2)) = 1
d = bwdist(A);
r = 20;
A(abs(d-r)<.5)=1;
imagesc(A)

Alex Taylor
Alex Taylor on 8 Dec 2011
The createMask method of the ROI tools was designed for the case in which you want to generate a binary image that is one where a specific ROI tool lies over an image or axes.
For example:
imshow(zeros(50));
h = imellipse(gca,[5 5 40 40]);
mask = h.createMask();
imshow(mask)
You can interactively or programmatically (as done above) any of the ROI tools, and then call the createMask method. In the case above, imellipse expects the bounding rectangle of an ellipse as the position argument. Don't know if that helps, but I thought it might be relevant to this problem.

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