# Essential Math WeblogThoughts on math for games

## 3/22/2005

### Post GDC Notes: Part 3

Filed under: Mathematical,Tutorial — Jim @ 5:52 pm

Having promised to post about center-of-mass calculations, and subsequently having put it off for a week, I’m going to punt.

First, you can read the seminal Brian Mirtich paper on the subject. Then Dave Eberly responds with a more efficient method. Both rely computing solid integrals across a polytope of constant density by treating them as surface integrals across the polytope’s faces. In the end they end up with total mass (assuming a density of 1), center of mass, and the inertial tensor matrix for the polytope.

If those haven’t scared you off, Jonathan Blow has a more intuitive approach, noting that the solid integral is basically a volume calculation. By breaking the object into tetrahedrons, we can compute the total solid integral as a weighted sum of tetrahedral solid integrals. The tetrahedrons can be computed by selecting a single point — the origin or the centroid will work — and using that point together with the points on each triangular face. As he notes, it’s likely that the Eberly and Mirtich approaches are more efficient, but not as easy to understand from a geometric standpoint. Blow doesn’t provide an implementation, but you can grab a similar one from Stan Melax here.

Finally, Erin Catto pointed out to me after the tutorial that you can compute the center of mass by computing the centroids of the tetrahedrons and then performing a weighted sum of all the centroids, where the weight is the volume of a particular centroid’s tetrahedron divided by the total volume of the polytope. Blow also covers this in his paper as well. To get an intuitive sense of this, I recommend messing around with some origin-centered triangles (e.g. (1,0),(-.5,-.5),(0,-.5),(.5,-.5)) and using areas instead of volumes.

1. Hi,

Its “stan” not “sam” and I dont know if physicstools.org will remain up or not.
The following URL will stick around:
http://www.melax.com/volint/

Comment by Stan Melax — 6/16/2005 @ 7:15 pm

2. Erk, sorry. All fixed.

Comment by Jim Van Verth — 6/16/2005 @ 9:47 pm