# Feature-based geometrical algorithms

Performing collision queries over a large number of objects is computationally expensive and therefore it’s important to use clever algorithms to avoid bottlenecks. After all potentially colliding object pairs have been sorted out during the broad phase, the remaining task is to run some algorithms to determine if and where the objects intersect. Of course there are tons of ways to do this.

motor2 mainly uses a SAT approach for this task, which is fast, easy to implement and quite robust. The only disadvantage is that you end up with an unpleasant quadratic-time complexity when shapes come into contact. In hope to improve this situation I spent some time learning different collision detection algorithms and stumbled across some papers talking about feature-based algorithms which like the name implies operate on the features (vertex, edge, face) of two polygons/polyhedra to determine if they are disjoint.
To say in advance: It’s really hard to find a better replacement for SAT in 2D because things are just so simple in two dimensions (this behaves differently in 3D, where SAT can be really slow for complex polyhedral shapes, since you need to check many more separation axes…).

The Lin-Canny algorithm [1] seems to be the most commonly used of these types of algorithms. The V-Clip, or Voronoi-Clip algorithm [2] is an enhancement of Lin-Canny and according to the author offers superior numerical robustness and can handle penetrations and degenerate situations as well.
I’m not quite sure if the well-known GJK algorithm (Gilbert-Johnson-Keerthi) falls under this category as well – from the geometrical point of view it’s simplex-based and looks at the minkowski difference.

Anyway, while I found many Lin-Canny implementations, V-Clip is a bit rare and after adopting the algorithm to 2D (fun!) I was wondering why so few people are using it until Erin Catto, the brain behind Box2D, pointed out that the algorithm is patented – I simply missed that (always read the small print!).

Here’s the result:

The V-CLIP algorithm in 2D, shaded areas represent voronoi regions of the nearest features, the solid line the minimum positive distance

Let me shortly explain how the algorithm works: The basic theorem states that if X and Y are a pair of features on two separated shapes, then if X is contained in the Voronoi Region of Y and Y is contained in the Voronoi Region of X, X and Y are the closest features and thus must contain the closest points between those features. Knowing the closest points, we can then compute the distance between them and declare a collision when the distance falls below some small value.

In a physics simulation we are usually integrating over a small time step so we can assume that objects only move by a small amount (called spatial and temporal coherence). This also means the new pair of closest features is probably near the last one from the previous time step, so an almost constant query time is achieved when re-starting the algorithm using the last result.

Talking about performance, V-CLIP is roughly 2-3 times faster than SAT and even better – in most situations it performs independently of the number of polygon vertices – which makes sense since you are only looking at the “difference” from the last state rather than the whole picture. Unluckily, the last problem I’m facing preventing it from being useful is that you have to do some extra work in creating a good contact manifold, so at the end it’s actually slower than SAT. But I have not yet given up and have some ideas to get the job done faster.

If you are interested in the code, it’s included in motor2 since version 0.9.
I have now looked more closely at the open Lin-Canny algorithm which is very similar to the Voronoi Clip algorithm, especially in 2D where I don’t need some extra cases introduced by the paper. For now I have dumped the V-Clip classes from my engine (they weren’t used anyway) and will revise the algorithm in the future.

There also other interesting applications, for example it allows you to compute the time of impact (TOI) for conservative advancement.