[ODE] Some objects pass through other objects

[Keith Wiley]

I have taken the "crash" example provided with ODE and tried to add a  
simple trebuchet, a counterweight powered ballistic projectile  
machine.  This is not the classic trebuchet with a cable and pouch.   
It is simpler, basically the way you launch peas off the end of a  
spoon.  My intention is to add a cable-pouch mechanism once I get the  
spoon approach working.

If I construct the trebuchet from separate bodies joined by fixed  
joints, it mostly works.  The trebuchet loads and launches just  
fine.  However, the fixed joints don't entirely work.  Some bodies  
that are connected by fixed joints aren't very "fixed".  They  
oscillate and eventually the simulation dies due to extreme errors.

If I construct the trebuchet as just two bodies (frame and pendulum)  
connected by a hinge joint, where each body is a composite of  
multiple geoms, then the structure is quite stable and seems to  
perform exactly as expected.

However, when I drop a canonball into the cup of the pendulum, it  
falls right through the pendulum to the ground as if there is no  
collision detection going on.  Even weirder, the ball does  
successfully collide (and rest against) the pendulum sometimes, but  
not always.  In particular, it falls through the pendulum under two  
specified circumstances.  One, when the cup of the trebuchet is  
resting on the ground (so the canonball drops into a cup that is  
already at ground level, ready for launch).  It passes right through  
and rolls away on the ground.  Two, if I "trick" it into accepting  
the canonball by not adding the canonball until a moment *after* I  
release the trebuchet (so the cup is already moving upwards and no  
longer on the ground when the canonball is added), then the canonball  
gets pushed by the pendulum and cup for some of the rotation, but  
before the rotation completes, it falls through the pendulum (you can  
also get the pendulum to "accept" the canonball by quickly releasing  
the trebuchet before the canonball falls through the pendulum).  I  
can't make any sense of this.

I have tried playing with the surface parameters that are generated  
for the joints corresponding to the surface contacts between the  
canonball and the trebuchet.  Nothing seems to fix this problem.  I  
also tried decreasing the stepsize to dWorldQuickStep, but this  
didn't solve the problem either.

I have included a link to the file below (which is mostly the  
original crash example, with a few additions and modifications for  
the trebuchet, plus some other minor changes).  If anyone would like  
to compile it and see what I am trying to do, I would love some  
feedback on what I am doing wrong.  I am, quite frankly, tapped for  
ideas.  I have been banging against this bug/error for a day solid  
and I simply don't know what else to try modifying at this point.

Thank you.

Instructions:

When you run the simulation, the trebuchet will automatically push  
itself down to the ground.  This may take one or two swings.  Wait  
until it settles down.  Then hit the '7' key to drop a canonball into  
the cup.  Notice the bug, the canonball falls through the cup to the  
ground and rolls away (or rests under the cup).  You can release the  
trebuchet with the '8' key (Reset the simulation to try again with  
the 'R' key, as per the original crash example).  If you release the  
trebuchet before the canonball falls completely through, you will be  
able to see the pendulum push the canonball for some of its rotation  
(so some collision detection is successful), lifting the canonball  
successfully into the air a little bit, but the canonball will  
eventually fall through the pendulum before the trebuchet completes  
the launch.

Any help is greatly appreciated.

The file can be downloaded at:
http://www.cs.unm.edu/~kwiley/test_crash_with_trebuchet.cpp

________________________________________________________________________
Keith Wiley         kwiley at cs.unm.edu         http://www.unm.edu/~keithw

"Luminous beings are we, not this crude matter."
                                            --  Yoda
________________________________________________________________________