Real-Time Physics Simulation Forum

Posted: **Mon Jan 09, 2023 6:53 am**

I do physical simulation and rendering in my program.
People recommend using fixed step simulation independently of rendering thread and framerate.

So I decided to call stepSimulation in, let's say, thread 0 and rendering in thread 1.
To render btRigidBody objects I need their transforms. As far as I know body->getWorldTransform() is the best way of doing it.

So, the question is following:
Is it safe to call getWorldTransform() while simulation is ongoing in other thread?

I heard that modifying state of rigid bodies is not thread safe, but I haven't seen anything about reading their states.

Sorry for my bad English

Posted: **Tue Jan 10, 2023 10:40 pm**

Actually body->getWorldTransform() is not necessarily the best way to do it.

When your render rate is NOT the same as your simulation step rate you can get aliased results from body->getWorldTransform() which can sometimes produce a visible jitter in the motion of otherwise smoothly moving objects. What you want is a predicted transform of the object that includes an extrapolation for any not-yet-sub-stepped time between your render time and the physics world's simulation time. The typical way to do this is to use a btMotionState for each physical body that also has a render component.

btMotionState is a pure virtual class with two methods that must be overridden:

Code: Select all

    // simulation requests outside context for new world transform
    // called once per substep for active kinematic objects, but also once for all objects when body is first added to the world
   ·virtual void getWorldTransform(btTransform& worldTrans) const = 0;

    // simulation stores the extrapolated world transform
    // called once per substep for active dynamic objects
   ·virtual void setWorldTransform(const btTransform& worldTrans) = 0;

I added the extra comments above in an attempt to summarize how the Bullet simulation uses the btMotionState API.

Bullet provides one non-virtual class derived from btMotionState for you called btDefaultMotionState. That may suit your needs, or you can write your own if you need special behavior.

Real-Time Physics Simulation Forum

Is it safe to retrieve objects transforms from one thread while stepSimulation is in other thread?

Is it safe to retrieve objects transforms from one thread while stepSimulation is in other thread?

Re: Is it safe to retrieve objects transforms from one thread while stepSimulation is in other thread?