Li : Dev Actionscript Development Blog

OBB Collision Detection

Click on image to view the demo (view source enabled).

Its just a basic implementation of the method of separating axes for 3D OBB's (oriented bounding boxes). It detects collisions between cubes in a way that is slightly more advanced than spherical collisions, which are merely based on distances and radii. Collisions are not resolved in the demo though, only detected.

The point of the experiment is to show that it isn't hard at all to detect collisions between 3D objects if considering only their bounding boxes for their 'physical volume' is enough; if we just need to detect if objects are colliding or not, we don't actually need to bring in a heavy, cpu thirsty, 3D physics engine.

The code is not optimized (in fact quite the contrary so that its easy to read), but it runs quite smoothly with 15 cubes, where each cube is checked with all others for collision. If, for example, a character needed to be checked for collisions against static objects in a 3D scene, we would just need to compare his bounding box with the scene's static bounding boxes, so a large number of physically active objects could be used to describe a pretty complex scene. Furthermore, if any of these boxes were axis aligned, they are no longer OBB's, but rather AABB's (axis aligned bounding boxes) and collision checks could be made much faster. Something like this I imagine:

return (obj1MinX < obj2MaxX) && (obj1MaxX > obj2MinX) &&
(obj1MinY < obj2MaxY) && (obj1MaxY > obj2MinY) &&
(obj1MinZ < obj2MaxZ) && (obj1MaxZ > obj2MinZ);

The experiment is clarifying in understanding how the method of separating axis works, and I guess it gave me a quick glimpse at how a advanced physics engines like JigLibFlash might work. The code is heavily commented, so have a look if you're interested in understanding OBB collisions.