How should I handle tiny objects in a physics engine?

Question

I am making a 3D physics engine for throwing dice. Up until now a dice has been 1x1x1m and gravity has been 9.82 m/s^2. This of course does not look realistic as the dice will respond to everything in a way that looks a little bit slow.

To fix this I tried to just change all sizes to make a dice be about 0.02m along a side. This in turn made my inverse masses and inverse inertia matrices get very large values which lead to numerical instability.

What is the best route to handle this? Can I keep the scale up so that a dice is 1x1x1 and just change mass or size of acting forces? Or should I tweak something else?

I guess the same problem could turn up if you made a game with very large physical bodies like spaceships or something so I hope that someone has run into something similar before.

Answer 1

You could

• increase the gravity constant? Games often use 2 * 9.81 m/s^2. Can't remember where I read this, but games usally use none-sensical units. Whatever constants feel right (make the game fun). Using a realistic constant makes the physics seem slow and floaty.
• run the simulation at a faster rate. If you're using a fixed timestep, this will increase the rate of animation.
• use higher precision floats?

Answer 2

In case you are having troubles with small numerical values, I suggest that you simply scale everything down. You might even not use "real" measurment units, but just some "generic" units that only make sense inside the engine.

The first option, scaling down, would require that you use millimeters or centimeters as a base measurement unit. Your cube's dimensions wouldn't suffer from loss of precision, then.

The second solution is basically the same thing, but then you wouldn't need to think about the actual units.

Also, why don't you just change the cube's mass?

Answer 3

Bullet Physics library has a wiki page about scaling the physics world. This information is probably useful with other physics engines as well.

Answer 4

I thought I should share my final solution to the problem. Increasing the gravity constant as suggested did indeed make the dice move faster thus giving the impression of correct scale. This, however, increased issues of microbouncing. I added microcollisions which mitigated the problem although did not solve it perfectly. Note that this becomes a problem only for "small" objects, if you are having the reverse issue and want to simulate very large objects (spaceships or something like that) then you won't be getting any microbouncing problems at all and can contend with that solution.

So I found the highest level of gravity I could have without getting way too much "microbouncing". Then I used a simple shake-detection on the phone which responded to large changes from the accelerometer readings and applied impulses to all the objects when such shakes are detected. It isn't perfect but as it will set the dice moving fast when phone is shaken it will give an impression of "tiny" dice. It's a bit of a hack but it will have to do :) Here's a clip of the engine in action.