[ODE] ODE with non-penetration constraint

Nguyen Binh ngbinh at gmail.com
Wed Jul 11 17:19:07 MST 2007


"hard" here means the constraint will exist in the whole time step. In ODE
case, we:
1. Set up the Jacobi correspond to the constraint
2. Solve for the constraint force fc

Then because the constraint is "hard" so we will apply fc to the involved
bodies through the whole timestep. That's actually not "exact" because the
contact might be broken in-between the time step.
This problem is inherent to any force-acceleration level simulation and
along with friction, there is singularity problem also.

If you change the constraint system into velocity-impulse level then you
don't have "hard" or "soft" contact anymore because you treat all of them as

On 7/11/07, Huang Rubby <rubbybaby at gmail.com> wrote:
> Hi All,
> on ODE manual, it says, "ODE has *hard* contacts. This means that a
> special non-penetration constraint is used whenever two bodies collide."
> But in my simulation example, which basicly was a ball rolling on ground,
> somehow, I read out the contact point position and found the Z - direction
> (vertical)
> coordinates were -0.005. Which means the contact point was underground.
> How did it happened? Since ODE has Hard contacts for each collision.
> Any way to avoid such penetration? Thanks.
> Ruby
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Binh Nguyen
Computer Science Department
Rensselaer Polytechnic Institute
Troy, NY, 12180
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