# Cutting a Circular Ring at particular points using angle-theta

1 view (last 30 days)
hamzah khan on 25 Mar 2022
Edited: Les Beckham on 25 Mar 2022
Hello,
I am drawing a circular ring using the following code:
clear all; close all;
p = linspace(-1/2,1/2,100);
[X,Y] = meshgrid(p,p); % box mesh
R = p(size(p,2))/2;
r = R/1.5;
alpha = linspace(-alpha,alpha,50);
[alpha_X,alpha_Y] = meshgrid(alpha,alpha);
theta = atan2(Y,X);
active = (X.^2 + Y.^2 <= R^2 & X.^2 + Y.^2 >= r^2);
figure()
plot(X(active),Y(active),'o','MarkerFaceColor','red');
hold on
This code is incomplete because I have to use alpha and theta to cut the ring at particular points.
The inequalities that I have are:
x^2+y^2<=R^2
x^2+y^2>=r^2
-alpha <=taninv(y/x)<=+alpha.
i have plotted the first two, I am confused as how to plot the third one, alpha one.
Does any on know how to plot the third inequality or how to use it to cut the ring at a particular angle.
the result which i get from above code is:

Les Beckham on 25 Mar 2022
Edited: Les Beckham on 25 Mar 2022
You were pretty close. See if you can adapt this to get what you want.
p = linspace(-1/2,1/2,100);
[X,Y] = meshgrid(p,p); % box mesh
R = p(size(p,2))/2;
r = R/1.5;
theta = atan2(Y,X); % check size of theta; note it is already a mesh grid because X and Y are
% the whole circle (radius constraints only)
active = (X.^2 + Y.^2 <= R^2) & (X.^2 + Y.^2 >= r^2);
plot(X(active),Y(active),'o','MarkerFaceColor','red');
% the portion that satisfies the alpha constraint as well
active = (X.^2 + Y.^2 <= R^2) & (X.^2 + Y.^2 >= r^2) & (abs(theta) < alpha);
hold on
plot(X(active),Y(active),'o','MarkerFaceColor','blue');

Simon Chan on 25 Mar 2022
Try to add the follwoing lines:
alphaA = 15*pi/180; % Set the margin, 15 degree here
[thetaA,~] = cart2pol(X,Y);
angleA = abs(thetaA)<=alphaA;
active = (X.^2 + Y.^2 <= R^2 & X.^2 + Y.^2 >= r^2 & angleA);

R2020a

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by