Remove a "plane" from my 3d plot

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]

If x^2 > y^2, won't the real part of your square root be 0?

[davidcu]

I have tried with that restriction, but I got this error "mesh: X, Y, Z arguments must be real". Or in what way do I have to write the restriction?

[Ravi Narasimhan]

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.

Answer 1

[DGM]

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")