## Drawing Pentatope Cross-Sections in three.js

A triangle is the simplest regular figure in 2 dimensions. Its 3 dimensional analogue is a tetrahedron. Its 4 dimensional analogue is the pentatope. Another term for these 3 geometries is: simplex. Many articles online explain these further.

I wanted to view what it would look like if a pentatope passed through our 3-dimensional space. There are many examples of what the projection of a pentatope onto 3-d space would look like (ie its shadow), but I didn’t want to do that because while projections can be pretty, they seem less intuitive than cross-sections.

To summarize, the plan was: take 3-dimensional cross-sections of the pentatope (a 4 dimensional shape). The actual drawing will be done using the 3-dimensional drawing toolkit called three.js.

Our equation for the 3-dimensional cross-section is:

ax+by+cz+K = w

This is a linear equation; we’re not dealing with curved cross-sections for now, though that would be fun. a,b, and c are constants. x,y,z, and w are the 4 axes. K is a constant that we change to get different cross-sections.

But why do we need 4 dimensions to describe a 3-dimensional space? Because we are locating that 3-d space in 4 dimensions. Just as the equation for a plane in 3-d space requires 3 dimensions (e.g. x+y = z describes a plane in 3-d space).

What about our equation for pentatope? Well we don’t have an equation, but we have a collection of points. Our pentatope has 5 corners. Lines connect each point to every other point, resulting in 10 lines. These 10 lines in turn form 10 faces (2-d planes). These 10 faces in turn from 5 3-d tetrahedrons.

Note that:

– The intersection of a 4-d line and a 3-d space is a point.

– The intersection of a 4-d plane and a 3-d space is a line.

– The intersection of a 4-d volume and a 3-d space is a plane.

Our cross-section is going to look like a wire-frame, in other words a collection of lines. So we will capture the intersection of the pentatope’s faces with our cross-sectional space.

Our pseudo-code would look like this:

foreach (face in faces) { // 10 of these // Each face is a triangle. Lets name the vertices p0, p1, p2. These points have 4-d coords. // point1 and 2 will be the points we use to draw a line in 3-d space using three.js. // they have 3-d coords of course. point1 = calculateIntersectionOfLineWithSpace(p0,p1); point2 = calculateIntersectionOfLineWithSpace(p0,p2); drawLineUsingThreeJS(point1, point2); }

So the interesting bit is in ‘calculateIntersectionOfLineWithSpace()’:

We must use parametric equations to work with lines in dimensions higher than 2. The parametric form for a line defined by the points p0 and p1 is:

p = p0 + t*v (1) where v = p1 - p0.

We need to calculate what the value of t is at the intersection with our cross-sectional space. (p0 and p1 are the vertices of one of the faces of our pentatope.) So we need to solve for t.

t = (Pt - P0)/V

Pt is the point of intersection of the line with the cross-sectional space. Given our formula above for our 3-d cross-sectional space, the equation for t is:

t = (W0-aX0-bY0-cZ0-K)/(aXv+bYv+cZv-Wv) Where P0 = (X0, Y0, Z0, W0) and V is (Xv, Yv, Zv, Wv).

Plugging t into equation (1) above we calculate the point of intersection of our 4-d line with our 3-d space. We return (x,y,z) from ‘calculateIntersectionOfLineWithSpace()’ above, discarding w.

And we simply change K to see different cross-sections. Here is the animated result:

https://rwoodley.org/MyContent/WIP/30-Simplex/index.html

A still: