generation of 2D array of circular ring

I am looking for a way to generate 2D array of circular ring in retangular coordinate.

1. outer diameter (D)
2. inner diameter (d)
3. value of the ring = 1
4. outside of D & inside of d = 0
5. center of the ring = (x,y)

thanks,

John

Answer 1

[DGM]

In this case, I'm going to do an antialiased image instead of a binary image.

sz = [300 400]; % image size [y x]
c = [150 200]; % circle center [y x]
dmin = 175; % diameters
dmaj = 250;

maskinner = drawcircle(sz,c,dmin/2);
maskouter = drawcircle(sz,c,dmaj/2);
annmask = maskouter.*(1-maskinner);

imshow(annmask,'border','tight')

% draw smooth circle
function circ = drawcircle(sz,c,r)
    xx = 1:sz(2);
    yy = (1:sz(1)).';
    circ = sqrt((xx-c(2)).^2 + (yy-c(1)).^2); % no quick tricks this time
    circ = min(max((1+r-circ)/2,0),1);
end

Answer 2

[Jayant]

Here are the steps how you can write a function which takes D, d, x, y, width and height as its input and gives a 2D array as output.

1. Create a ringArray of (width, height) dimesion.
2. Traverse the ringArray. For each element (i, j), calculate the euclidean distance from the centre(x,y).
3. If the distance>=d or <=D, the assign 1, else assign 0.
4. The function returns the ringArray which is the required 2D array.

[DGM]

No loops necessary.

sz = [300 400]; % image size [y x]
c = [150 200]; % circle center [y x]
dmin = 175; % diameters
dmaj = 250;

xx = 1:sz(2);
yy = (1:sz(1)).';
rr = (xx-c(2)).^2 + (yy-c(1)).^2; % squared euclidean distance
annmask = (rr <= (dmaj/2)^2) & (rr >= (dmin/2)^2); % compare against square radii

imshow(annmask,'border','tight')

The bit with using squared distance simply avoids doing an expensive sqrt() on the entire array, so it's an easy way to make simple comparisons like this faster.