[ODE] Only setting surface params for contacts actually generated

[Marty Rabens]

In the ODE sample apps, the collision callback generally does something
like this:

  dContact contact[MAX_CONTACTS];
  for (i=0; i<MAX_CONTACTS; i++)
  {
      contact[i].surface.mode = dContactBounce | dContactSoftCFM;
      contact[i].surface.mu = dInfinity;
      contact[i].surface.mu2 = 0;
      contact[i].surface.bounce = 0.1;
      contact[i].surface.bounce_vel = 0.1;
      contact[i].surface.soft_cfm = 0.01;
  }
  int numc = dCollide (o1,o2,MAX_CONTACTS,&contact[0].geom,
sizeof(dContact));
  for (i=0; i<numc; i++)
  {
      dJointID c = dJointCreateContact (world,contactgroup,contact+i);
      dJointAttach (c,b1,b2);
  }
  
So it's setting the surface properties for MAX_CONTACTS every time, even
if only some (or none) of those contacts is actually generated by
dCollide().

Could you do this instead?

  dContact contact[MAX_CONTACTS];
  int numc = dCollide (o1,o2,MAX_CONTACTS,&contact[0].geom,
sizeof(dContact));
  for (i=0; i<numc; i++)
  {
      contact[i].surface.mode = dContactBounce | dContactSoftCFM;
      contact[i].surface.mu = dInfinity;
      contact[i].surface.mu2 = 0;
      contact[i].surface.bounce = 0.1;
      contact[i].surface.bounce_vel = 0.1;
      contact[i].surface.soft_cfm = 0.01;
      dJointID c = dJointCreateContact (world,contactgroup,contact+i);
      dJointAttach (c,b1,b2);
  }

The question is, does dCollide() use any of the data that you set?  If
not, the second method should work, and saves a little overhead (as well
as being less code).  I've tried it this way, and it seems to behave
identically.  I just don't know if it's because dCollide() doesn't use
any of the surface data, or if I'm just getting lucky (due to old memory
contents, etc.)

Marty Rabens