convexSweepTest never hits when tracing very small distances

[angrybaker]

Is this a bug, or expected behaviour, or am I doing something wrong.

If the movement between the From and To is 0.04 units or smaller, it seems to never return a collision.

Is that correct? Should I be elongating the distance for the check?

I basically have a bunch of cubes forming a floor, then am calling convexSweepTest on an object (tried a sphere and a box), with downwards - which doesn't add the movement to its position if it detects a hit from convexSwepTest..

It works fine if travelling 0.1 units, but passes through at 0.04 or below.

[Erwin Coumans]

If the distance is below a given threshold, the CCD will not return a hit. The default threshold value is 0.04.
You can disable this threshold by setting this threshold to zero, using

Code: Select all

collisionWorld->getDispatchInfo().m_allowedCcdPenetration = 0;

To avoid confusion, things are different in latest trunk. The allowedCcdPenetration is now passed in as optional argument to the btCollisionWorld::convexSweepTest, with default value 0 (and not 0.04).

For better performance on short swept queries, it might be better to insert a btGhostObject in the world, and leave it in the world as long as you need to perform short queries in a certain area. A btGhostObject incrementally gathers overlapping objects.
Then perform the btGhostObject::convexSweepTest, just like the btCollisionWorld::convexSweepTest. This will only test the ghost overlapping objects (instead of traversing world/broadphase acceleration structures).

So please upgrade to latest trunk or Bullet 2.73 RC5/final.
Hope this helps, and thanks for the feedback,
Erwin

[angrybaker]

Awesome, just updated - it now works exactly as I'd have expected - thanks!

btGhostObject sounds really useful. Would it be wise to surround all moving entities in a btGhostObject, which extends to their maximum velocity - or would all usefulness be eradicated by that?

[Erwin Coumans]

btGhostObject sounds really useful. Would it be wise to surround all moving entities in a btGhostObject, which extends to their maximum velocity - or would all usefulness be eradicated by that?

It depends, a ghost object has some memory/performance overhead. Typical use case for btGhostObject is a character controller or small local collision triggers for AI. The btKinematicCharacterController peforms several convexSweepTests to slide along walls and peform stair climbing.

• If the velocity/swept query is very large, the benefit of a ghost object is reduced, and using the btCollisionWorld::convexSweepTest is likely better
• If you plan to perform several short convex sweep tests for each entity, every frame, you can consider creating btGhostObjects.
• If you only use a convexSweepTest once in a while, for only a few entities, using the btCollisionWorld::convexSweepTest could be better.

So your mileage might vary, and benchmarking is recommended.
Thanks,
Erwin