Angular Impulse

Why have you not consulted the erratum on O'Reilly's website ?

Inertia tensors are defined by the geometry of your objects. See page 19.

Icg is a typo, should be I₁ and I₂
l (L) is a typo too, should be 1 (ONE).

Both the book and your website quote say the same thing. See figure page 97.

Determining the collision parameters (time and geometry) is the hard part. There is no easy answer. I doubt your solution will work.

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[edited by - Fruny on December 26, 2002 4:10:24 AM]

Ok, I almost have this working, but I am sure these equations are incorrect:

v1+ = v1- + (Jn) / m1
v2+ = v2- + (-Jn) / m2


Unfortunatley the Erratum doesnt cover these. I definitely need to have r1 and r2 in there somewhere. Can anyone supply me with the correct equations ?

Nitzan

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Actually let me rephrase my question, since its possible the equation is correct but my code for it is not....

This is my interpretation of the following equations:

v1+ = v1- + (Jn) / m1
v2+ = v2- + (-Jn) / m2
w1+ = w1- + (r1 x Jn) / I1
w2+ = w2- + (r2 x -Jn) / I2

v1f is v1+
model1->returnVelocity() is v1-
model1->returnMass() is m1
w1f is w1+
model1->returnAngularVelocity() is w1-
model1->returnInertia() is I1

Here is my code:

v1f.x = model1->returnVelocity().x + ((J*n.x) / model1->returnMass());
v1f.y = model1->returnVelocity().y + ((J*n.y) / model1->returnMass());
v1f.z = model1->returnVelocity().z + ((J*n.z) / model1->returnMass());

v2f.x = model2->returnVelocity().x + ((-J*n.x) / model2->returnMass());
v2f.y = model2->returnVelocity().y + ((-J*n.y) / model2->returnMass());
v2f.z = model2->returnVelocity().z + ((-J*n.z) / model2->returnMass());

w1f = crossProduct(r1, n);
w2f = crossProduct(r2, n);

w1f.x = w1f.x * J / model1->returnInertia().x + model1->returnAngularVelocity().x;
w1f.y = w1f.y * J / model1->returnInertia().y + model1->returnAngularVelocity().y;
w1f.z = w1f.z * J / model1->returnInertia().z + model1->returnAngularVelocity().z;

w2f.x = w2f.x * J / model2->returnInertia().x + model2->returnAngularVelocity().x;
w2f.y = w2f.y * J / model2->returnInertia().y + model2->returnAngularVelocity().y;
w2f.z = w2f.z * J / model2->returnInertia().z + model2->returnAngularVelocity().z;

Anyone know what I am doing wrong ? The rotation seems to be ok, but the velocities appear to be wrong. The values I am using is two cubes from 1, 1, 1 to -1, -1, -1.

Model1 is translated to 4.0, 0.9, 0.0 with velocity -0.1, 0.0, 0.0

and

Model2 is translated to -4.0, -0.9, 0.0 with velocity 0.1, 0.0, 0.0

The collision point I get is 0.0, 0.0, 0.0, and the normal is 1.0, 0.0, 0.0

My normal might be where I am going wrong. I understood its the normal of the colliding surface of Model1. Is this incorrect ?

Thanks for the help.

Nitzan

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www.geocities.com/nitzanw
www.scorchedearth3d.net
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Hi,
I''m trying to improve my collision system. Currently my collisions work only if object colliding with static object. I''v been reading and searching for info how to do collisions between two rigid bodies and this seems to be what I''m lookig for.

quote:

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J = -vr ( e+1 ) / {1/m1 + 1/m2 + n . [(r1 x n / I1] x r1 + n . [(r2 x n / I2] x r2}

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I''d just like to know what is the ''vr'' in front there? (I don''t have the book) Otherwise it seems ok, so I''d like to try how it works

Wow!! It seems to be working
Should the 'vr' be dotProduct(relative_velocity, contact_normal)?

About your code, I think you should apply negative angular values to model 2.

quote:

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Original post by nitzan
v1f.x = model1->returnVelocity().x + ((J*n.x) / model1->returnMass());
v2f.x = model2->returnVelocity().x + ((-J*n.x) / model2->returnMass());

w1f.x = w1f.x * J / model1->returnInertia().x + model1->returnAngularVelocity().x;
w2f.x = w2f.x * (-J) / model2->returnInertia().x + model2->returnAngularVelocity().x; // <========== Should be (-J)????

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[edited by - stefu on December 29, 2002 10:44:13 AM]

I''ve always assumed that inertia was constant regardless of the axis, and it''s not such a bad approximation most of the time.

There are two types of collisions between polyhedras:

- Vertex/Face: In that case, N is the normal of the face

- Edge/edge: N is the cross-product of the vectors of the two edges

Baraff (use Google) uses the same equations. His papers are online.

HTH,

Cédric

I finally realized that the equation I was using was correct and that I coded it correctly (except for that -J). The collision and response didnt look realistic because I am not accounting for friction or elasticity.

I thought that if an Edge collides with a Face then you take the average of the two normals. The cross product gives you a vector perpendicular to the two normals which seems to be, well... useless. Dont you want a vector between the two ?

Nitzan


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