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Showing posts from August, 2010

Milkmans mistake

After two posts full of boring text and graphs I feel like I have to give away a crowd pleaser. Again, not a realtime simulation. About 120.000 particles, running at almost one FPS.

The life of an impulse

For a long time I've wondered how impulses develop inside a rigid body solver. Mainly for trying to come up with a prediction of where the final magnitude would be as early as possible. I dumped the data for all impulses out to a CSV file and graphed it for a couple of simple test scenarios. Let's start with a small test case, a box pyramid of two layers (three boxes). All dirty tricks are switched off, so this is a plain vanilla solver. The Y axis is impulse magnitude for the normal impulses and the X axis is iterations.


At a first glance, there seems to be no suprises here. Impulses form asymptotic curves towards their solution and there is not a lot of action after about twelve iterations. But look again, especially at the bright yellow curve, that seems to steadily decrease and form a non-horizontal asymptote, and what about the deviation around iteration eight and nine? Let's try a more complex exmple, with a box pyramid of five rows (15 boxes). There is around 90 cont…

Explaining the rigid body solver

Following my last post about scientific papers not being written for engineers I will do an attempt at explaining a rigid body solver without equations:
Even though a rigid body scene may consists of hundreds of objects and thousands of contact points, a popular way to solve the problem is to solve each contact point in sequential order, one at a time. It sounds kind of lame, and compared to other methods it is, but if iterated a couple of times it gives really good results, and this is what most games are actually using, so let's focus on solving one contact without friction first:
So you have two objects and one contact point with a contact normal. Start by computing the velocity at the contact point for both objects and compute the difference between those vectors. Project that difference onto the contact normal (dot product). This is the contact's relative velocity along the contact normal and it indicates how much the objects are moving towards each other or away from each …