Collision detection using Hierarchical Oriented Bounding Boxes (OBB) to detect which part of a character the bullet has hit
Nov 2006
Collision Detection
Collision Detection Using Hierarchical OBB
@ programming > game technologies

In this OGRE demo, the player is able to shoot out small bullets (considered points) to hit the character. The collision detection check is hierachical, meaning that the check is done on the larger, less accurate bounding volumes before checking their smaller, more accurate children recursively. This reduces the time complexity from O(n) to O(lg n), assuming a more or less constant branching factor, which makes the collision detection a lot faster.

OBBs are constructed for each joint of the character and a point+OBB test was to be done to detect collision. However, I transformed the colliding point into the space of the OBB so that this converts the OBB to an Axis-Aligned Bounding Box (AABB). This means the algorithm will run faster since less checks are needed to detect collisions between the colliding point and the AABBs.

In the video of the demo below, bullets are shot towards the character. Notice that the bounding boxes which the bullets have collided into have turned red, indicating that the collision detection algorithm is working.

Video showing the collision detection algorithm at work. OBBs get highlighted in red when the bullet hits them.

Note: Scene setup done by Goh Cheng Teng. Model and animation provided by Dr Golam Ashraf.

Skeel Lee Skeel Lee
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Senior FX TD / R&D
Digital Domain 3.0 (Previously at Sony Pictures Imageworks, MPC, Industrial Light & Magic, Double Negative)
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I am a Senior Technical Director with strong interests in both tech and art. My life evolves round VFX, photography, software engineering, tools programming and generally anything that looks / sounds cool.
I have done a variety of CG programming, including fluid sims, muscles, soft/rigid bodies, raytracing etc. These knowledge complement the visual works that I do as a TD in VFX.
I was interviewed by The Straits Times in May 2014 for my VFX work in X-Men: Days of Future Past.