Getting Set Up for Rigid Body Simulations

Size Does Matter

• Always keep the scale of Softimage units and your scene objects in mind when designing a rigid body simulation. As with other simulation calculations, scale is crucial when dealing with rigid bodies. Before you start to do a rigid body simulation, you need to define what unit of measurement a Softimage unit will mean in your world: 1 meter, 10 cm, 1 cm, 1 foot, etc. And you need to model your objects in reference to this and to make sure that their mass or density also reflects this.

  For example, if you create a cube to be 1 unit wide and you set its mass to 1 kg, this is going to be an extremely dense cube if you define 1 Softimage unit as 1 cm. On the other hand, if you define 1 Softimage unit to be 100 meters, the cube will be as big as a football field, so 1 kg would be lighter than air.

• Both the PhysX and ODE dynamics engines have been tuned so that 1 Softimage unit is equal to 10 cm.

• If possible, try to make the rigid bodies in your simulated world have sizes of 1 to 10 Softimage units, masses of 1 to 10 kilograms, and have moderate velocities (such as between 10 and 50 Softimage units per second). Having extremely large, extremely small or thin, or very fast moving objects can make the simulation less stable, or may require you to use many substeps for an accurate simulation.

• Because dynamics simulations imitate physical laws by performing intensive calculations, there are certain limitations. Infinite and very small values, such as an object with hardly any mass or density, should be avoided because the calculations required may introduce numerical errors into the simulation.

Gravity and Simulations

Scale also applies to gravity. Gravity is a constant that is the same for all objects regardless of their mass: every object falls at exactly the same rate. However, everything changes at the moment the object collides with another because that's where its mass, energy, and momentum play a major role.

To have the correct gravitational behavior from the objects, the size of the objects in the scene must be taken into consideration. For example, the default gravity setting is 98.1, which is earth's gravity if you define 1 Softimage unit as 10 cm. However, if you define 1 Softimage unit as 1 meter, you would need to set the gravity to 9.81.

In Softimage, the default gravity is set to 98.1 because most characters are modeled with a scale of 1 unit equalling 10 cm, which is basically the scale used for the default Softimage rigs and models.