Beveled Cube from a Sphere

This post is about how one can map a sphere to a “rounded”, “smoothed” or as I prefer to call it a “beveled” cube. We will as a bonus also see how to make a inverted cube, in a very simple way using MATLAB.

The reason for doing this is I wanted to create an ordinary cube for an assignment in a visualization course. By doing this simple mapping I’m about to describe, we can actually get a very nice looking smoothed cube if we use a sphere.

In MATLAB we have the function Sphere(n) which creates coordinates for a sphere, given a resolution n, so I figured if I could map a sphere into a cube, hmm…. The reason was, I didn’t want to manually enter the coordinates for the cube, that’s too much work. I have done that in OpenGL before, I have had enough of that (hehe). Additionally if I could get a beveled cube, that would be even better.

The sphere is created using:

[x, y, z] = sphere(n);
h         = surf(x, y, z)

We can then later modify the data using set in the following way:

ex = 2^(-5*i/50);
set(h, 'xdata', abs(x).^ex.*sign(x)/2,...
    'ydata', abs(y).^ex.*sign(y)/2,...
    'zdata', abs(z).^ex.*sign(z)/2)

In the animation below, I used . The sign is telling the mapping which direction of which axis the coordinates are to be stretched. In the x axis direction, we get two directions 1, in y we get two and in z we get two. abs is balancing the stretching, if we don’t have the abs, we would not get a nice mapping.

Consider x=linspace(0,1,500); plot(x, x.^20)

Since the mapping will stay in, the same goes for my mapping and this was my first idea. By simply changing the exponent, we would get different shapes of the sphere. I had to take care of the directions using sign and abs otherwise I would get a folded sphere.

The variable that controls the shape is i:
Inverted  i = -15
Octahedron  i = -10
Sphere  i = 0
Beveled Cube i = 50

The animation shown below is stepping through these values of i with increment 1. The animation is wide, so the sphere unfortunately looks like an ellipsoid.

If we continue to increase i, choose n to be low enough, we get an ordinary cube. Anyway, comment if you like it 😉