# Spherical Patch in 3D

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Chris Dan on 29 Mar 2022
Commented: Chris Dan on 29 Mar 2022
Hello,
I am trying to make a 3d spherical segment. I am having problems with the atan2 function. in 2D my code is
clear all; close all;
p = linspace(-1/2,1/2,200);
[X,Y] = meshgrid(p,p); % box mesh
R = p(size(p,2))/2;
r = R/1.5;
theta = atan2(Y,X);
f = figure('visible','on');
active = (X.^2 + Y.^2 <= R^2) & (X.^2 + Y.^2 >= r^2);
plot(X(active),Y(active),'o','MarkerFaceColor','blue');
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','red');
here every thing is as I want(the red patch) ,but when I try to make the same patch in 3D, I am getting getting the wrong shape.
here is my code:
clear all; close all;
p = linspace(-1/2,1/2,100);
[X,Y,Z] = meshgrid(p,p,p); % box mesh
R = p(size(p,2))/2;
r = R/1.5;
f = figure('visible','on');
theta = atan2(Z,X);
active = (X.^2 + Y.^2 + Z.^2 <= R^2) & (X.^2 + Y.^2 +Z.^2 >= r^2);
plot3(X(active),Y(active),Z(active),'o','MarkerFaceColor','blue');
active = (X.^2 + Y.^2 + Z.^2 <= R^2) & (X.^2 + Y.^2 +Z.^2 >= r^2) & (abs(theta) <= alpha);
hold on
plot3(X(active),Y(active),Z(active),'o','MarkerFaceColor','red');
saveas(f,'3d_patch','fig')
Can anyone tell me what is wrong? I figured it out to be theta...
Does any one knows how to take correct angle

Chris Dan on 29 Mar 2022
clear all; close all;
p = linspace(-1/2,1/2,100);
[X,Y,Z] = meshgrid(p,p,p); % box mesh
R = p(size(p,2))/2;
r = R/1.5;
f = figure('visible','on');
theta = atan2(sqrt(X.^2+Y.^2),Z);
active = (X.^2 + Y.^2 + Z.^2 <= R^2) & (X.^2 + Y.^2 +Z.^2 >= r^2);
plot3(X(active),Y(active),Z(active),'o','MarkerFaceColor','blue');
active = (X.^2 + Y.^2 + Z.^2 <= R^2) & (X.^2 + Y.^2 +Z.^2 >= r^2) & (abs(theta) <= alpha);
hold on
plot3(X(active),Y(active),Z(active),'o','MarkerFaceColor','red');
saveas(f,'3d_patch','fig')

KSSV on 29 Mar 2022
The shape is spherical and looks fine. You need to set the axis. Try
axis equal
at the end.
Chris Dan on 29 Mar 2022
theta = atan2(sqrt(X.^2+Y.^2),Z);

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