Bounding Boxes, and the Effect On Performance

    While making editing an avatar for VRChat recently I found out that the avatar was marked as 'Very Poor' performance wise, due to its bounding box being too large on a particular axis. The avatar was pretty simple, albeit very large, but it doesn't hit any other performance conditions besides that, so surely it wouldn't be that big of a problem? 

    Regardless, this isn't a discussion on VRChat's avatar grading system, this is on bounding boxes. The avatar got me thinking, how exactly does that box affect performance? In case you don't know, a bounding box represents the limits of where something exists. Wikipedia gives a more specific definition here. These boxes can be used to speed up calculations. 

    When a game renders a scene, it doesn't need to render every single object within that scene, only those that the camera can see. In Unity, this is called frustum culling. You can also use occlusion culling to hide objects that are hidden behind other objects. More info on that here. When it comes time for the game to decide if the object should be rendered or not, it needs to decide if it's inside the camera view. One way this could be done is to go through every vertex on the model and check if it's within the boundaries of the camera. For smaller models, this could be fine, but gets increasingly intensive for models with many vertices. Instead, we can use a precomputed bounding box, and check if any part of that is in view. This brings that visibility calculation down to a constant time, regardless the size of the model. That's a rough explanation, so definitely don't cite me on that, but that's roughly how it's done. 

    How does the size of the bounding box affect performance? Well, evidently, if the size of the bounding box is increasingly large, the more likely it is to be drawn. If it's rendered, there's a bunch of calculations that need to be done, including deformations, shading, and all those dynamic bones people put on their stuff. We'd like to avoid doing that as much as possible, so smaller boxes are typically better.