[ODE] Should trimeshes react to physics?

[Megan Fox]

Well, it would of course depend on your setup, but "usually," the big
terrain trimesh has no body.  If it has no body, it can not experience
a force... math-wise, it is an infinitely massive object that is
unaffected by gravity.

If, instead, you were to assign a body to your huge trimesh?  It would
experience gravity, and it would recieve forces, and you would have to
set a mass to that body.  This scenario isn't terribly useful, though.


The concept of mass is unrelated to the geometry you assign to a
shape, incidentally.  The mass tensor matrix you specify can be
calculated via a particular shape (the dMassSetBox and similar
functions), but this doesn't have to match the shape of the body in
question...

For instance: you could create a body, attach a single sphere geom,
but set a mass calculated via dMassSetBox.  This would create a shape
which had the inertial properties of a box, but the collision and
friction properties of a sphere.  (which isn't sensical or useful, and
you probably wouldn't notice the difference, but you can do it)

In my scenes, everything just uses mass properties calculated via
dMassSetBox.  I never notice the difference, and it's simple and
general (easy to determine from the shape's visual AABB, and anything
physical is visible in my sim).  This beats trying to calculate a
to-the-frame accurate intertial representation of a multi-limbed human
entity that is constantly deforming (animating).

On 11/21/05, Keith Wiley <kwiley at cs.unm.edu> wrote:
> I'm trying to understand trimesh roles in ODE.  I realize that,
> according to the docs, they are new and not fully developed.  I am
> using one massive trimesh for a nonoverlapping extruded terrain right
> now.  One thing I have noticed is that the trimesh is unaffect by
> gravity.  If I create the entire terrain at some height up in the
> air, it doesn't fall down, it just floats permanently in space, even
> though other objects are rolling and crashing around on the top
> surface of the terrain, so it must clearly be enabled due to all the
> collisions it is experiencing.
>
> Likewise, since a trimesh is infinitely thin, I'm curious how mass
> works with respect to trimeshes.
>
> There is an option to precalculate the face normals for a trimesh and
> give them to the trimesh building routine.  When should this be used
> and when shouldn't it be used?
>
> And lastly, there is a brief mention of temporal coherence, which as
> I understand it, reduces overall collision checks by assuming that
> objects don't move very far between adjacent timesteps.  So, it
> sounds like a good idea.  When would it *not* be a good idea to use?
>
> Thanks.
>
> ________________________________________________________________________
> Keith Wiley         kwiley at cs.unm.edu         http://www.unm.edu/~keithw
>
> "Luminous beings are we, not this crude matter."
>                                             --  Yoda
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--
-Megan Fox
Lead Developer, Elium Project
http://www.elium.tk