Real-Time Physics Simulation Forum 2023-01-10T22:40:01+00:00 https://pybullet.org/Bullet/phpBB3/app.php/feed/topic/13515 2023-01-10T22:40:01+00:00 2023-01-10T22:40:01+00:00 https://pybullet.org/Bullet/phpBB3/viewtopic.php?p=44198#p44198 <![CDATA[Re: Is it safe to retrieve objects transforms from one thread while stepSimulation is in other thread?]]> 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: 

    // 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.

Statistics: Posted by drleviathan — Tue Jan 10, 2023 10:40 pm

]]> 2023-01-09T06:53:21+00:00 2023-01-09T06:53:21+00:00 https://pybullet.org/Bullet/phpBB3/viewtopic.php?p=44193#p44193 <![CDATA[Is it safe to retrieve objects transforms from one thread while stepSimulation is in other thread?]]> 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

Statistics: Posted by graphene9932 — Mon Jan 09, 2023 6:53 am

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