# Essential Math WeblogThoughts on math for games

## 3/25/2005

### Angular Dynamics: Matrices or Quaternions?

Filed under: Mathematical,Tutorial — Jim @ 9:00 pm

One of the questions asked during the tutorial was whether matrices or quaternions were more efficient for angular rigid body dynamics. My response was that a matrix would be more efficient, because a quaternion requires a conversion to a matrix to compute the transformed inertial tensor matrix. If we analyze the operation count, this certainly appears to be true:
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## 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.

## 3/17/2005

### Quick Update

Filed under: General,Tutorial — Jim @ 8:26 pm

Due to sickness and unexpected work, I haven’t had a chance to do some of the updates I’d planned. I’m working on a write-up about computing center of mass, since I botched that badly during the presentation. I still need to do a little more research to make sure I got it right, though. The Mirtich and Eberly presentations are mostly formula manipulation, and not intuitive from the geometric standpoint.

Until then, here’s an article by Joe van den Heuvel and
Miles Jackson regarding sphere collision: Pool Hall Lessons. Someone recommended it as an alternative method for dynamic sphere-sphere collision. Looking at it, it appears that it might be faster than the one I came up with; certainly so in the average case, as it culls out obvious misses early. But again, I haven’t had a chance to do an in-depth analysis of it.

## 3/14/2005

### Post-GDC notes: Part 2

Filed under: General,Tutorial — Jim @ 8:09 pm

The Win32 sample code has been updated to match what was shown at GDC. The ZIP file also includes the old Mac base code because someone (didn’t catch your name, sorry) expressed an interest in porting it. If you’ve downloaded the code previously, it includes three new examples: an integration methods demo which shows the effect of using three different integration techniques with a spring force; a resting contact demo using impulses to resolve the collision; a resting contact demo which uses contact forces to resolve the collision. For the last two, I have commented out the code which slows it down to 10 fps, so it should look fairly smooth in both cases. If you want to uncomment it, it’s in Game::Update().

For those who picked up the book, there are two code demos in the Chapter 11 folder that are also apropos. The first shows the bounding hierarchy for a simple submarine model. The second shows a one-dimensional example of the use of sweep and prune.

## 3/13/2005

### Post-GDC notes: Part 1

Filed under: General — Jim @ 10:01 pm

I’ve uploaded the latest tutorial slides to the site. I made a few changes, mostly removing demo links (which require specific paths and wouldn’t necessarily work) and replacing some of them with direct web links. I also fixed the positive and negative elements in the collision response slides. In case I missed one or two, basically if the relative velocity along the normal or the relative force along the normal is positive, then the objects will be pushed together. So the correction (either impulse or constraint force) is in the opposite direction of the normal, and needs to be negative.

I’ll be doing a few more updates throughout the week. Tomorrow I’ll upload the latest demo code, at least for Windows. There was also a lot of good conversation during the breaks and after the talk, so I’ll post some comments, corrections and clarifications based on that. So stay tuned.