Wednesday, December 9, 2009

Big or beautiful

Here is a capture from a couple of simple demos. I've made a number of improvements to the solver lately and also now gathering multiple contact points in the initial manifold. Incremental manifold is such a neat idea, but it does affect stacking stability, and objects tend to penetrate more, which in turn affects both performance and behavior. I'm now using relative rotations and reiterating GJK to find multiple points, but I'm experimenting with other methods that should be both cheaper and more stable.

All demos use a 60 Hz update and four solver iterations. Various shock propagation methods in the solver remove a lot of the sponginess you normally see at four iterations. I strongly believe in elegant algorithms, rather than hand-optimized brute force code. Hence, I think it's better to have a rather sophisticated solver that do more work per iteration rather than just increasing the number of iterations. As you can see in the demos, even a quite large stack is pretty stiff.

Speaking about demos, I am SO tired of physics demos running in slow-motion.

1) Most programmers know that taking smaller time steps increases general stability. Objects move less per time step, hence making things easier to compute.
2) Decreasing gravity improves stability, since stacks feel less weight of the objects above them.

Most people would agree that altering any of the above is cheating. People are generally very picky about keeping the time step realistic, and some are proud to use 9.82 as gravity instead of just 10. But hey, why do most physics demos run in slow-motion then? Because there's a third way to cheat that nobody cares about:

3) Making objects larger is just as bad as decreasing gravity.

Remember what Galileo taught us four hundred years ago: Large objects receive the same gravitational acceleration as small objects. So, keeping the acceleration constant and increasing the size is equivalent to keeping the same size of the object and decreasing the gravity! Increasing the size of an object might seem quite innocent "Ho ho ho, my simulation is so stable. This box just happen to by six feet tall, who can blame me for that?"

How about simulating a pile of books? Or a couple of plates and wine glasses? At 60 Hz that's quite a challenge, and which objects are going to be more common in your game? So, if you evaluate a physics engine always make sure to do it in a realistic scale. The boxes in these demos are just above one foot along the side, which I think even that is questionable. A set of physics demos that really impressed me for their natural looking motion is the Tokamak Physics Engine.


  1. Hi Dennis! I'm glad to see you have a new blog.

    I'm curious how you are able to avoid artifacts while using shock propagation. Are you able to compute realistic contact forces (which would be useful for destruction)?

  2. Hi Dennis!

    While the gravitational acceleration is independent from the size of the object the inertia is not. Larger inertias improve the convergence of the iterative solver a lot since they reduce angular non-linearities.


  3. Hi Erin, it's fun to be back again :) Yes the shock propagation I'm using does affect contact forces... I haven't thought much about it, but there's probably a way to compute (or at least estimate) the actual forces by analyzing the contact graph. There are some artifacts on large stacks tipping over, but it's quite hard to spot.

    Dirk, I'm not sure I follow. The inertia of course gets bigger with size, but the distance from contact point to center of mass (contact lever) at contact or impact also gets bigger, so the net result is exactly the same motion. My point is that scaling up objects is equivalent to scaling down gravity, even though it seems more harmless.

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