## Search found 57 matches

- Mon Oct 27, 2014 5:33 pm
- Forum: General Bullet Physics Support and Feedback
- Topic: btTransform * operator giving the inverse ??
- Replies:
**7** - Views:
**6562**

### Re: btTransform * operator giving the inverse ??

Bullet's transforms are stored row-major in memory. They are still column vectors with post multiply, though. i.e. T*v_body = v_world, where T is the rigid body transform. They use the same math conventions as OpenGL and... well, _math_ ( not Directx ). But the storage is different from OpenGL, wher...

- Fri Jul 13, 2012 12:23 am
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: Lagrange multipliers
- Replies:
**8** - Views:
**8678**

### Re: Lagrange multipliers

Thanks Erwin, and pass on my Thanks to Kenny!

- Fri Jul 13, 2012 12:22 am
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: Lagrange multipliers
- Replies:
**8** - Views:
**8678**

### Re: Lagrange multipliers

Awesome. Very helpful. That's makes sense to me now. It matches up well with the Witkin/Kass Space-Time Constraints paper. Still a lot of work to do though! Basically I'd like to see if some of the tricks we use to reduce the computational complexity of the Rigid Body Dynamic Solvers we all know and...

- Thu Jul 12, 2012 5:59 pm
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: Lagrange multipliers
- Replies:
**8** - Views:
**8678**

### Re: Lagrange multipliers

Thanks Erwin, good to be back. :) Both those papers use the same trick as Baraff's "Linear Time Dynamics Using Lagrange Multipliers". Which skips an important step. The objective function isn't stated anywhere. I don't know what is being minimized. This is the Lagrange Multiplier Method: 1) problem ...

- Mon Jul 09, 2012 9:09 pm
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: Lagrange multipliers
- Replies:
**8** - Views:
**8678**

### Re: Lagrange multipliers

I also have written physics engines without knowing how Lagrange multipliers work. If you view the constraints geometrically and directly apply the principle of virtual work, you don't even need to know calculus. However, now I'm attempting to do some research into extending a "solver" to work with ...

- Mon Feb 15, 2010 5:00 pm
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: on integrating rigidbody orientations
- Replies:
**7** - Views:
**3791**

### Re: on integrating rigidbody orientations

Ah, I see how the exponential map collapses into the rotation formula I posted. I just don't rotate the body if the angle < epsilon. I'm quite interested in the fact that most physics engines drop the non-linear part of Euler's equation. (w x I w). Not for gyroscopic effects so much as for torque-fr...

- Fri Feb 12, 2010 4:12 pm
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: on integrating rigidbody orientations
- Replies:
**7** - Views:
**3791**

### on integrating rigidbody orientations

The classic approach to integrating rigidbody orientation is: R' = R + dR/dt*dt with dR/dt = w~R or dR/dt = .5*wq R = rigidbodies orientation matrix w = angular velocity w~ = cross skew matrix from w q = the quaternion from R Seems to me calculus has blinded folks to the simple geometric ( physical ...

- Sun Jul 20, 2008 5:20 am
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: b-spline evaluation near t=1
- Replies:
**1** - Views:
**2022**

### b-spline evaluation near t=1

When I evaluate a non-periodic b-spline near the end t ~= 1 the basis functions return 0. for a b-spline of degree 2. Knot vector k = [0,0,0,0.33, 0.66, 1, 1, 1] with 4 non-zero control points P0,...P3. Using the recursive formulation: f(t) = P0*N0,2(t) + P1*N1,2(t) +... P3*N3,2(t) Ni,0(t) = 1 if k ...

- Thu Dec 20, 2007 12:42 am
- Forum: Links, Papers, Libraries, Demos, Movies, Comparisons
- Topic: Position Correction
- Replies:
**12** - Views:
**19311**

### Re: Position Correction

I found that using a stability factor caused some bad artifacts when simulating non-trivial cloth. So I left it out. For console performance we were getting killed by load-hit-stores. This is caused by reading from a vertex that has just been changed ( still on it's way to being stored in the cache ...

- Fri Oct 19, 2007 8:48 pm
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: Newton's cradle
- Replies:
**8** - Views:
**11031**

### Re: Newton's cradle

IMHO, the spheres have a gap, even if they are touching. That is because reality uses the penalty method (e.g. Hertz contact for spheres). So there has to be some movement before force is built up. Agreed. This happens at the speed of sound through the material, though, which is a bit finer resolut...

- Thu Oct 18, 2007 12:54 am
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: Newton's cradle
- Replies:
**8** - Views:
**11031**

### Newton's cradle

Hello, anyone managed to simulate a newton's cradle properly, where all the balls are in contact simultaneously? If there is a slight gap between the objects I can get it simulating properly, but if all the balls are in the LCP solver at the same time, I get the wrong result. This is what I would li...

- Tue Oct 16, 2007 9:50 pm
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: Resting Contact in rigid body simulation. . .
- Replies:
**38** - Views:
**18421**

### Re: Resting Contact in rigid body simulation. . .

Yup, I understand that, that's why I said it was accurrate in the limiting case where t -> 0. ( 1/infinity )

- Tue Oct 16, 2007 5:53 pm
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: Resting Contact in rigid body simulation. . .
- Replies:
**38** - Views:
**18421**

### Re: Resting Contact in rigid body simulation. . .

The velocity constraints are linear, right? Only if your timestep is 1/infinity :) For a sphere penetrating an immovable flat plane, or a point-to-point constraint on one particle achored to an immovable body; if we have an initial velocity of zero; our linear system is an exact model of the physic...

- Mon Oct 15, 2007 10:11 pm
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: Resting Contact in rigid body simulation. . .
- Replies:
**38** - Views:
**18421**

### Re: Resting Contact in rigid body simulation. . .

Yes, my mistake. J1 != J2. I have my Jacobians split into a linear and angular contribution. For my linear contribution J1x = -J2x, but I have different J1w and J2w angular. I went over that quickly to get to my main point. Anyways, the point of my post remains the same. You can extend the system to...

- Mon Oct 15, 2007 6:46 pm
- Forum: Research and development discussion about Collision Detection and Physics Simulation
- Topic: Resting Contact in rigid body simulation. . .
- Replies:
**38** - Views:
**18421**

### Re: Resting Contact in rigid body simulation. . .

GS can handle multi-body constraints quite easily with a special constraint. I look at GS/SI like so: 1) Project the bodies velocities onto the constraint I think of the Jacobian 'J' as a projection from normal world-space onto the constraint manifold ( constraint-space ) To do this, sum all the vel...