[ODE] Collision triangle culling

[Erwin de Vries]

What we did was quite easy. Check if the center of the box is at the
positive side of the plane. Beware though. This does not give perfect
results in all cases. This was also a problem i didnt quite know how to
solve.

Erwin

----- Original Message -----
From: "Ruud van Gaal" <ruud@marketgraph.nl>
To: <ode@q12.org>
Sent: Wednesday, December 05, 2001 23:01
Subject: [ODE] Collision triangle culling


> Hi all,
>
> I have the following situation: in my carsim I have tracks with fences
that are
> made of 2 polygons; one for each direction it faces (there are no
doublesided
> faces).
> I have an (ODE) OBB for the car body and create an ODE plane when I detect
a
> hit of the OBB with a track (or fence) triangle. This is done with
AABBtrees so
> it's quite ok; mostly 0 triangles are even checked, and the tree gets down
to
> the correct potential triangles in O(logN).
>
> Now the car hits a fence; when colliding with the wrong side (polygon) of
the
> fence, the normal is flipped, and ODE thinks the OBB is way inside the
triangle
> (understandably). The result is a fierce push dragging the car *through*
the
> triangle.
> At first I thought I could solve this by checking the car's velocity
w.r.t. the
> triangle's normal (normal dot car linear velocity for a rough check). This
> works a little bit, but as ODE allows penetrations, the *next* step, the
car is
> still somewhat inside both fence triangles, but this time, it's moving OUT
the
> fence (because of the previously generated contact points).
> Now, the reverse is happening. The car is moving into the other direction
> (correctly) but this causes my 'normal dot car velocity' to indicate the
OTHER
> triangle should now be checked. Since the car isn't taken out of the
> penetration entirely, the next collision check indicates the car is very
far
> into the triangle, and we still get an explosion.
>
> What I've done now is to check both the car's velocity (in the direction
of the
> fence triangle's normal), and  a penetration depth that looks weird; for
> example, if(penetration_depth>0.5 (meter) ) then forget about this contact
> point. Still, this looks awfully hacked.
> Does anybody have a suggestion on how to tackle this? Using just 1 polygon
for
> the fence doesn't seem an option, as I must pick 1 face normal when the
car
> hits the triangle from the one or the other side (btw I create the contact
> points using ODE's dCollide() with the OBB of the car and the plane of the
> triangle).
>
> Thanks, and I hope this is clear enough,
> Ruud
>
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