generation of 2D array of circular ring

Question

JK 2023-7-8

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https://ww2.mathworks.cn/matlabcentral/answers/1993453-generation-of-2d-array-of-circular-ring

编辑： DGM 2023-7-8

采纳的回答： DGM

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

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

thanks,

John

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Answer 1

DGM 2023-7-8

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https://ww2.mathworks.cn/matlabcentral/answers/1993453-generation-of-2d-array-of-circular-ring#answer_1269663

编辑：DGM 2023-7-8

在 MATLAB Online 中打开

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

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Answer 2

Jayant 2023-7-8

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https://ww2.mathworks.cn/matlabcentral/answers/1993453-generation-of-2d-array-of-circular-ring#answer_1269583

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.

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

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DGM 2023-7-8

编辑：DGM 2023-7-8

在 MATLAB Online 中打开

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.

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