[ODE] ODE world stepping + TOI based collision detection problem

Roland Plüss roland at rptd.dnsalias.net
Sun Aug 20 11:52:18 MST 2006

i'm trying to get a system working here in which i'm using TOI based
collision detection functions from my own collision library together
with ODE's stepping system. somehow though i can't get things working
like it should and i suspect that i don't get the system correctly.

for example i use a step size of 0.01s for the ODE system. now i detect
for example a collision at TOI 0.4 ( hence 0.004s ). now i can have
either a physics response ( using a contact joint ) or a custom response
( from my game scripts ). in this example i use a very simply sliding
plane response giving me something similar to what a contact joint would
do with no bouncyness.

now the problem is how to get it working. if i step by 0.004s i am
exactly at the position of contact from where on the repelling force has
to start applying. now if i have a custom response i know the force
applied ( from the sliding plane ) and i can do another collision run
but if this is a contact joint what will happen? if i place the contact
joint in the step of 0.004s will it apply force? because if i apply it
in the next step after the 0.004s one i have the problem that i don't
know the new velocity of the object ( as the joint has not applied force
yet ) and therefore i can not do another collision detection run.

i'm somehow a bit stuck here on how the ODE stepping system exactly
works. in fact i need to know the velocity of the objects after each
step but without applying the displacement to the objects as this way i
can do a collision test and then apply the change if no collision
happens ( or up to the next collision point ). am i misusing the system
in this case? or do i simply not grasp the way it works?

help appreciated.


Leader and Head Programmer
- Game: Epsylon ( http://epsylon.rptd.dnsalias.net/ )
- Game Engine: Drag(en)gine (
http://epsylon.rptd.dnsalias.net/dragengine.php )
- Normal Map Generator: DENormGen (
http://epsylon.rptd.dnsalias.net/denormgen.php )

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