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Recomputing AABBs for animated actors

Johan Jons · 2015-10-26T02:53:21

I'm using assimp and it's convention of dealing with nodes and meshes. Basically each model has a node, which can have any number of children and so on, and each node may or may not have a mesh. I go through the node and mesh hierarchy and compute AABB around the entire model and for each node aswell for fine-grained culling. These AABBs are used for view frustum culling later. This works fine when the models are static and not animated as I could just compute it once and forget it. Now when doing animations where some rotation/translation is applied to some or all nodes in the model the whole AABB is invalidated. The only solution I see is to recalculate the entire AABB structure each frame which seems very expensive - say in a model with 100 nodes, which is not uncommon, apply rotation/translation to each node just to recalculate the AABB. It sounds helluva expensive to do this every frame. Is there anything I am missing? How is this usually handled in other games and engines?

General and Gameplay Programming Programming

Started by KaiserJohan October 23, 2015 11:32 AM

12 comments, last by Buckshag 8 years, 6 months ago

Alundra

2,326

October 23, 2015 10:40 PM

Instead of looping over each vertex, just loop over each joint's animated position + some per joint radius.

OBB is better and is the common way used in all big animation system.

JoeJ

4,199

October 24, 2015 02:49 PM

OBB is better and is the common way used in all big animation system.

OBB isn't better than bounding capsule for characters - both methods will end up having similar accuracy.
I just wanted to mention the method because no one else did, and it's the fastest.

* There may be many characters and performance matters
* Or there may be ragdolls from physics engine, and physics may have computed a bounding box for each ragdoll already.

ankhd

2,304

October 24, 2015 09:17 PM

Hey there.
You could export a collision mesh just boxes with the object. I'm using a mesh, sphere and aabb that can be converted to a obb

Buckshag

899

October 26, 2015 02:53 AM

In EMotion FX I offer different ways to calculate the bounds.

1.) Based on the node's (bones) global space positions

2.) Based on the mesh vertices in case software skinning is used

3.) Based on the bone OBBs, taking all corners into account

4.) Based on the bone OBBs, taking just the transformed min and max into account (faster, less accurate)

5.) Based on a static precomputed AABB, which is calculated by calculating the mesh based AABB and then rotating the character in all directions to find the max bounds.

Number 1 to 4 are dynamic and can be calculated once per frame or at another update rate. They require the bone transforms to be known.

Number 5 is the most practical one in games. This also allows you to only process motion extraction/root motion once the character walks out of screen. You move this precomputed AABB with the characters position. This way you do not need to calculate any bone transforms when the character walks out of screen. Only the motion extraction node needs to be calculated, which can be a huge win in performance.

When using method 5, you can of course also use a sphere or capsule or any other shape.

EMotion FX - Character Animation System

Recomputing AABBs for animated actors

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