collision impulse and normal

[linksan]

Hi, I'm relatively new to bullet and need help on how to get information from collisions

I have a room full of rigid body boxes, how could I determine the impulse force from collisions on these boxes ?
How could I determine the normal vector from a collision these boxes?

thanks in advance

[Erwin Coumans]

Easiest and best way is to iterate over all btPersistentManifold, each holds up to 4 contact points (but it can be empty).

Each contact point (btManifoldPoint) has the applied impulse (m_appliedImpulse), contact normal (m_normalWorldOnB) etc.

See CollisionInterfaceDemo how to access the contact points (Bullet\Demos\CollisionInterfaceDemo\CollisionInterfaceDemo.cpp)

Hope this helps,
Erwin

[linksan]

Thanks for your help, I got it working

[pico]

Hi,

what is "m_appliedImpulse" exactly? The impulse that is needed to seperate two bodies after collision?

Especially when one object crashes into another then "m_appliedImpulse" is often zero for me. Is this
the normal behaviour? The collision normal acts as expected, also in this case.

[Erwin Coumans]

what is "m_appliedImpulse" exactly?

It is the total accumulated impulse applied by the constraint solver during the last internal substep.

Bullet might perform multiple internal substeps, and a later substep might clear out the applied impulse. So if the m_appliedImpulse is access outside of the stepSimulation, it might be zero.

Try to access the m_appliedImpulse during a contact callback (see ContactProcessedCallback) or during an internal tick callback (see setInternalTickCallback in the dynamics world).

Hope this helps,
Erwin

[pico]

This makes sense, because we let Bullet perform 4 substeps for each frame as otherwise small collisions go through small walls:

simulationStepsNum=4;
bltWorld->stepSimulation(1.0/frameRate,simulationStepsNum,(1.0/frameRate)/(float)simulationStepsNum);

If we now catch "ContactProcessedCallback", as you describe, wouldn't we then get more then one callback for our single frame because we have 4 subframes?
How is this problem solved generally?

Currently we just iterate over all btPersistentManifold and send callbacks to when "contactManifold->getNumContacts()>0".

thanks