Fatness

The Fatness augments the initial geometry with volume properties.

Outer Fatness

The Outer Fatness describes the distance between the shape geometry and the effective colliding surface.

_images/user_guide_fatness_2.png

Outer Fatness for a double sided collision shape.

Note

The Outer Fatness can be set to 0 for a single sided collider, but it is best to keep a small value to smooth the external surface.

Inner Fatness

The Inner Fatness for a single sided geometry describes the maximum penetration depth allowed.

The existence of an Inner Fatness (Inner Fatness > 0) means that the collider is single sided. If the Inner Fatness is zero, then the collider is double sided. Using Inner Fatness and treating the surface of a volume as single sided enhances the robustness of the collision handling but can be tricky to set up correctly.

  • If set too small, you will have to set a higher number of subdivisions for the simulation in order to prevent missing collisions; but that will slow down the simulation.
  • If set too high, the simulation will literally vacuum cloths from the other side and corrupt or even crash the simulation.

In case of a simple volume such as a cube you can easily set it up to a larger value, but still no more than half of its side’s length.

For complex geometries with thin sections, you have the following options:

  • Set Inner Fatness to a small value. This has to be at least half of the smallest surface separation distance, e.g. half of the width of a finger.
  • Remove these parts from the geometry (usually feet, face, hands/fingers).
  • Make the collision geometry double sided (by setting Inner Fatness=0) in extreme cases where the options above cannot be applied.
_images/user_guide_fatness_1.png

Inner and Outer Fatness for a single sided collision shape.

Cloth Fatness

Cloths are double sided collision shapes by nature and the fatness corresponds to the outer fatness of the collider.

There is an internal limitation as to how much of a given fatness can be used in self-colliding cloths. Internally the fatness cannot exceed half the height of the smallest triangle or edge of the cloth reference geometry. Any fatness values set above that threshold will be ignored in all self-collision situations, and the cloth won’t look as thick as indicated (but will still interact at the given fatness with other dynamic objects).

Note

  • Setting the fatness to the largest reasonable value, upfront, helps to create a good result to iterate on.
  • A larger fatness costs more collision execution time, but is also more robust when multiple layers of cloth are colliding.
  • Use the Fatness Check guide geometry to check on triangles that will be too small for the given fatness.