Real-Time Physics Simulation Forum

Re: center of mass not at mesh origin

2011-11-21T18:15:00+00:00

#BigPet, there is no btTransformShape right now, but if you need it for simple convex shapes (btBoxShape etc) you can create it indeed
(you will need to create a new collision algorithm too, not sure if it will be much more efficient than a btCompoundShape)

We'll have a more efficient method for Bullet 3.x (see the Physics Effects code in https://github.com/erwincoumans/experiments )
Thanks,
Erwin

Statistics: Posted by Erwin Coumans — Mon Nov 21, 2011 6:15 pm

Re: center of mass not at mesh origin

2011-11-20T22:05:51+00:00

I do this kind of things a lot because of the meshes we get from maya
are basically never centered around their pivot.
Instead of adding a transform I transform the vertices of the shape
so that the vertices are centered around the pivot, and no need for a top transform,
once that is done I apply an invert offset on the transform to move the object back
into the correct position.

Statistics: Posted by majestik666 — Sun Nov 20, 2011 10:05 pm

Re: center of mass not at mesh origin

2011-11-18T07:06:14+00:00

Sorry for necroing this thread but I thought that this would fit nicely in here.
So, I know that for the translation of objects a btCompoundShape has been recommended in multiple threads. btCompoundShape may have a negligible overhead if you wrap a single complex shape into it but using it to offset the center of gravity of a lot of simple shapes (like btBoxShapes in my example) does produce a significant overhead.

basically I created a huge stack of cubes and let the tower fall. When using btCompoundShapes around the btBoxShapes I had spikes of over 100ms while the spikes of using simple btBoxShapes were in the range of 25-35ms. For this use-case (offsetting the center of mass of a lot of simple shapes) btCompoundShape seems unfit. I am currently looking into writing a "btTransformShape" that just hosts one child shape and applies a transformation to it (an equivalent to the ode GeomOffsets). If something like this already exists, please let me know.

here's the demo Video:
http://www.youtube.com/watch?v=KLDDExjgosk

Statistics: Posted by Bigpet — Fri Nov 18, 2011 7:06 am

Re: center of mass not at mesh origin

2009-05-02T09:11:19+00:00

Hello,

In my project Im using Ogre together with btOgre. I have exported a car chassis and 4 wheels from 3dsmax
to Ogre mesh. All this meshs are loading as seperate entitys and are not alligned to orgin.
Without gluing the car to bullet, the position is ok in render.

When I glue it to bullet with btOgre, there is an offset in calculation. The debug-drawer shows that the collision
spere are alligned to the mesh, but the calculation is not.

I try different solutions found here in the forum and look in some demos without success.

Here is my code of creating a wheel body:

Code:

Wheel::Wheel(Model *Model, BTPhysics *Physics, WHEEL_POSITION WheelPosition, std::string path, std::string name)   {   m_Model   = Model;   m_Physics = Physics;   m_Part    = (MODEL_PARTS)WheelPosition;      m_Node = m_Model->get_Node((MODEL_PARTS)WheelPosition);                        // get scene node   Ogre::Entity *ent=(Ogre::Entity *)m_Node->getAttachedObject(0);                  // get attached entity   Ogre::AxisAlignedBox BBox = ent->getBoundingBox();                                      // get bounding box   m_CylinderShape = new btCylinderShape(Convert::toBullet(BBox.getHalfSize()));     m_ColShape       = new btCompoundShape;      btTransform localTransform;   localTransform.setIdentity();   // car is facing in -Z direction, rotating cylinder to fit wheel orientation   localTransform.setRotation(Convert::toBullet(Ogre::Quaternion(Ogre::Degree(-90),Ogre::Vector3::UNIT_Z)));   // move shape to graphical center   localTransform.setOrigin(Convert::toBullet(BBox.getCenter()));   m_ColShape->addChildShape(localTransform,m_CylinderShape);   btScalar mass(30.0);                                                //    btVector3 localInertia(0,0,0);                                     // Is this ok when it is not at orgin ?   m_ColShape->calculateLocalInertia(mass,localInertia);    //   btTransform MassTransform;   MassTransform.setIdentity();   MassTransform.setOrigin(Convert::toBullet(BBox.getCenter()));   MassTransform.setRotation(Convert::toBullet(Ogre::Quaternion(Ogre::Degree(-90),Ogre::Vector3::UNIT_Z)));   localTransform.inverse();      // inverse shift   MassTransform.inverse();     // inverse massposition   m_BodyState = new RigidBodyState(localTransform, MassTransform, m_Node);    btRigidBody::btRigidBodyConstructionInfo rbInfo(mass,m_BodyState,m_ColShape,localInertia);   m_WheelBody = new btRigidBody(rbInfo);   Physics->getWorld()->addRigidBody(m_WheelBody);   }

The code I use to set the wheel to a specific position

Code:

void Wheel::set_Position(Ogre::Vector3 Position)   {   btTransform transform;    transform.setIdentity();                                                   transform.setOrigin(Convert::toBullet(Position));                   m_WheelBody->setWorldTransform(transform);                     }

What is the right way to setup a rigid body which is not orgin alligned ?
I have not found a working solution withing the last 7 days and I realy tryed everyting
found in forum and demos.

I need your help guys.

( sorry for my bad english )

Statistics: Posted by Heizer — Sat May 02, 2009 9:11 am

Re: center of mass not at mesh origin

2008-09-09T16:05:15+00:00

The rigid body world transform is the center of mass.

If your graphics mesh needs an offset, you can use a btCompoundShape to shift the graphics mesh, relative to the center of mass, and use the btDefaultMotionState to apply the inverse shift.

Hope this helps,
Erwin

Statistics: Posted by Erwin Coumans — Tue Sep 09, 2008 4:05 pm

Re: center of mass not at mesh origin

2008-09-09T15:23:36+00:00

Hello Paul,
can you post your working solution. I have the same problem and can't figure out, how i can fix it

Thanks

Statistics: Posted by peacemoon — Tue Sep 09, 2008 3:23 pm

Re: center of mass not at mesh origin

2008-06-05T23:02:16+00:00

Thanks for the input, all. Here's what I have finally done as a solution...

Yes, the Bullet mesh must have center of mass at the origin, and I must do something to make that work, such as applying an offset to each vertex or using a CompiundShape and transform as mentioned earlier.

In addition to that, I have a class derived from btDefaultMotionState, and I override setWorldTransform to drive the position of the geometry in my visual scene. It turns out btDefaultMotionState has a member m_centerOfMassOffset. In my override of setWorldTransform, I first call the base class's setWorldTransform, and this folds the m_centerOfMassOffsset into the m_graphicsEWorldTrans -- and then I just use that to transform my visual scene geometry.

The problem I had up until now was that I had tried both of those (transforming the mesh, and setting the m_centerOfMassOffset) in isolation. Separately, they don't provide a complete solution, But together they resolve the issue.

The nice thing about this is that I can hide the center of mass offset entirely in the code module that glues Bullet together with my rendering code, and it's all managed transparently there.

Thanks again for everyone's input.

Statistics: Posted by PaulMartz — Thu Jun 05, 2008 11:02 pm

Re: center of mass not at mesh origin

2008-06-05T09:23:50+00:00

That said, I agree it would be nice to be able to set center of mass on an object - but I can't think of any pratical use for doing so when the intent is to render images based off the physics data.

There's lots of uses - e.g.

1. Artists may want to set the origin of models at ground level - e.g. level with the bottom of the feet for a table, and it would be nice if the game code didn't have to store the offsets.

2. You may have a car object that has different configurations based on its CoM - i.e. you can have different versions of the car differing only in handling characteristics (including different CoM positions). Again, awkward to absorb the offsets into the game code.

3. A model aircraft flight simulator may want to move the CoM as the plane flies - e.g. as a result of geometry moving (swept wing etc) or the fuel tank emptying, or simply to allow the flight params to be modified whilst flying.

I don't think generally artists will care exactly where the CoM is, or be able to construct the model geometry so that the origin is at the CoM, so there will have to be an offset somewhere in the code. It makes sense to put this offset in the physics engine code rather than game code for any physics engine that wants to support more than just boxes and spheres.

Statistics: Posted by DannyChapman — Thu Jun 05, 2008 9:23 am

Re: center of mass not at mesh origin

2008-06-04T19:51:02+00:00

Ultimately if you are rendering an object that is represented in the physics engine, you will (most likely) want your model origin and the physics center of mass to coincide with each other. If they do not you will have to do a center of mass offset translation before doing any rotation/scaling translations as part of your render pipeline. Failing to do so will always result in the the visual representation of your model not being in sync with the physics representation of it's location and orientation.

Consider your box as a scenario. If center of mass is 1, 1, 1 and you rotate it around the z-axis by 90 degrees, then in the physics system your box is taking up the exact same space and volume that it was prior to the rotation - or rather the min/max corners as represented in the physics system in both cases will be (0, 0, 0) - (2, 2, 2). Graphically if you then render based on a rotation of 90 degrees from the model origin (one of the corners) it will go from occupying the min/max corners of (0, 0, 0) - (2, 2, 2) to (-2, 0, 0) - (0, 2, 2) - occupying a completely different volume of space.

That said, I agree it would be nice to be able to set center of mass on an object - but I can't think of any pratical use for doing so when the intent is to render images based off the physics data.

Statistics: Posted by bbangerter — Wed Jun 04, 2008 7:51 pm

Re: center of mass not at mesh origin

2008-06-04T17:16:30+00:00

Hi,

i also had to get used to this. However, there is really no other way to set the center of mass. You can go through Bullets OpenGl debug drawer code to see how to get the correct model matrix.

Statistics: Posted by pico — Wed Jun 04, 2008 5:16 pm