Real-Time Physics Simulation Forum

Hi all, I'm just trying to understand a bit more about constraint solvers, in particular the Gauss-Seidel algorithm. My understanding is that Box2D uses Sequential Impulses, which is analagous to "Projected Gauss-Seidel", and that PGS is simply Gauss-Seidel but with an additional step that...

Thanks Dirk. Really appreciate all your replies.
After about 9 years of studying this stuff in my spare time I almost feel ready to code a simulator

Thanks Dirk, I think I mostly understand it now. Just to clarify, are you saying that this For n iterations Update J, M and C using p Solve J * M^-1 * J^T * lambda = -C Update dp Update p (Assuming p, dp, C and lambda are vectors and J and M are matrices, ie we're solving all constraints simultaneou...

Hi Dirk, thanks again for your reply. The Hairer paper is a bit beyond me at this stage but I remember reading the Jacobsen paper a while back and it seemed to make sense, the thrust of it being to resolve constraints on the position level directly by projecting bodies out of penetration. I understa...

Thanks Dirk. I suppose I'm still not clear on it... from Erin Catto's "Modeling and solving constraints" presentation, for the velocity constraints, we use: Newton's law -> v2 = v1 + M^-1 * p Virtual work -> p = J^T * lambda Velocity constraint -> J * v2 = 0 and we form the linear equation...

Hi, One more question from me regarding 2D physics :) Following the evolution of Box2D, I see that different approaches were tried in terms of position stabilization. I understand Baumgarte stabilization (feeding a fraction of the position error back into the velocity solver) and can see some potent...

Thanks Dirk. I was only interested in this question because I've never seen it addressed anywhere before - constraint forces were used initially, then at some point there was a shift to impulses. Reasons were given as to why impulses make things easier, but no real analysis of the compromises or dow...

Thanks guys. I had a feeling that solving on the velocity level would be less accurate but I'm still hazy on how exactly. From what I understand, we accumulate forces the same way, but instead of correcting these with constraint forces, we disregard the acceleration constraints and defer our correct...

Thanks for your reply Dirk, that makes more sense now.

Hi, I've noticed that earlier papers such as those by David Baraff discuss contact solvers that solve for contact forces, but later papers seem to deprecate this idea in favour of solving for contact impulses (such as the sequential impulse solver used by Box2D). I remember reading that the reason f...

Hi, This question is not specifically about Bullet, but it's related to the derivation of the contact constraint used in Box2D. I apologise if this is not the correct forum for this question, but it didn't seem to attract any attention on the Box2D forum, so I thought someone here might be able to a...