# Remove a "plane" from my 3d plot

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davidcu on 14 Oct 2021
Edited: Ravi Narasimhan on 14 Oct 2021
When I plot the surface it results into the surface itself but there is something weird in the middle of the surface, I mean it's kind like it is in the xy plane, I do not know how to remove it, here is my code:
clear
figure, hold on
[x,y]=meshgrid(-50:50)
xlabel ("x");
ylabel ("y");
zlabel ("z")
z=sqrt(9*(y.^2)-9*(x.^2))
w=-sqrt(9*(y.^2)-9*(x.^2))
surf(x,y,real(z))
surf(x,y,real(w))
zlim([-150,150])
Ravi Narasimhan on 14 Oct 2021
You could perhaps set any value of real(z) or real(w) = 0 to NaN (Not a Number) using the find function. Those items wouldn't display on the surface plot, leaving a space between the top and bottom halves. It would look like
z(find(real(z) == 0)) = NaN;
The find function returns the indices where real(z) == 0 and the z(find(...)) = NaN sets those positions in the z array to NaN
You'd tweak it accordingly for w.
Addendum: Posted while @DGM's answer came in.

DGM on 14 Oct 2021
You could try to clip off the excess points with NaN and be left with a gap in the surface at the joint, or you could just avoid the problem entirely.
Consider this:
N = 50;
th = linspace(0,2*pi,N);
r = linspace(-50,50,N).';
[x z y] = pol2cart(th,r,r);
z = z*3; % your example stretches on z
h = surf(x,y,z);
set(h,'facealpha',0.8,'edgealpha',0.5)
axis equal
xlabel ("x");
ylabel ("y");
zlabel ("z")