creating an image using a matrix

Question

Raldi 2011-12-8

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https://ww2.mathworks.cn/matlabcentral/answers/23440-creating-an-image-using-a-matrix

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?

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Answer 1

Sven 2011-12-8

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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)

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Raldi 2011-12-8

ok did it thanks Sven

Sven 2011-12-8

Cool, hit "accept" if it's answered then :)

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Answer 2

Sean de Wolski 2011-12-8

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FAQ6.9

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)

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Raldi 2011-12-8

Thanks Sean, with a few modifications i did what i wanted.

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Answer 3

Andrei Bobrov 2011-12-8

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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)

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Raldi 2011-12-8

thanks Andrei.

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Answer 4

Alex Taylor 2011-12-8

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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.