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?

Answer 1

[Sven]

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)

[Raldi]

Thanks guys....but i was thinking more...of choosing only the circle...and not the area outside. So i need to create only the outline. Thanks thow...ill see if i can modify the code.

[Sean de Wolski]

That shoudl be simple. Create the circle, and then create another circle with radius a pixel or two shorted

C(bigcirc&littlecirc) = false;

[Raldi]

ok did it thanks Sven

[Sven]

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

Answer 2

[Sean de Wolski]

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)

[Raldi]

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

Answer 3

[Andrei Bobrov]

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)

[Raldi]

thanks Andrei.

Answer 4

[Alex Taylor]

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.