[ODE] Contacts and materials

[Nate W]

On Thu, 28 Nov 2002, Anselm Hook wrote:

> What about finding a way to generalize material properties such that
> contacts could be generated automatically?  I'd sure enjoy finding a way
> to remove the collision callback.  

As would I - probably because I've been having fun with C# and I'm eager
to put your wrappers to work. :-)

Here's how I was thinking the problem might be approached:

The wrapper (your C# wrapper, or probably any wrapper) would define a
material data structure, something like:

typedef struct
{
  void *pUserData;
  dReal Mu;
  dReal Bounce;
  dReal SoftERP;
  dReal SoftCFM;
  // etc
} Material;

All geom objects would have Material pointers in their dGeomGetData /
dGeomSetData members, and the wrapper would implement those methods as
follows:

	// GetData implementation
	return ((Material*) dGeomGetData())->m_Data;

	// SetData implementation
	((Material*) dGeomGetData())->m_Data = p;

Then, a wrapper-defined collision callback would work something like this:

void Callback (void *, dGeomID o1, dGeomID o2)
{
  int iCount = dCollide (o1, o2, iContacts, &contact[0].geom, sizeof (dContact));

  if (iCount)
  {
    int iMax = min (iCount, iContacts);
    for (int iContact = 0; iContact < iMax; iContact++)
    {
      Material *pMaterial1 = null;
      Material *pMaterial2 = null;

      if (o1)
        pMaterial1 = dGeomGetData (o1);

      if (o2)
        pMaterial2 = dGeomGetData (o2);

      SetContactParameters (&contact[iContact], pMaterial1, pMaterial2);

      dJointID ContactJoint = dJointCreateContact (WorldID,
               CollisionJointGroupID, &contact[iContact]);

      dJointAttach (ContactJoint, Body1, Body2);
    }
  }
}

The implementation of SetContactParameters is left as an exercise for the
reader. :-)  But, as Anselm suggested, I think it would make a nice
contrib.  It would make the C# wrapper considerably more useful, and would
probably be helpful for most non-C/C++ applications that want to use ODE,
since there would no longer be a need for a callback into the application
language.  Instead, applications would just set material properties for
all of the Geoms, and the SetContactParameters method would take care of
material interactions.

Comments?

-- 

Nate Waddoups
Redmond WA USA
http://www.natew.com