Work with Bifröst colliders

Colliders are obstacles to the flow of a Bifröst simulation. You can use them to form a barrier around which smoke must flow, or create a basin that prevents liquid from falling away under gravity, or animate them to create waves and splashes.

Add colliders

1. Select one or more polygon meshes to act as colliders (instances are not supported), as well as either one of the following:
   • The main container (e.g. bifrostLiquid or bifrostAero) to add all selected meshes to a new shared collider property. Alternatively, you can select the shape (e.g. liquid or aero)
   • An existing collider property to share its attribute values with the selected meshes.
2. Select Bifrost Fluids > (Add) Collider.

Remove colliders

1. Select the following:
   • One or more existing colliders.
   • Either the container (e.g. bifrostLiquid or bifrostAero) or the shape object (e.g. liquid or aero).
2. Select Bifrost Fluids > Remove > Collider.

   When you remove the last mesh that uses a particular property, the property node is removed as well.

Control fluid interaction with colliders

You can use the Enable option in the Properties attribute group of a collider object to toggle whether the object acts as a collider or not. In addition, you can use the Conversion options to control how the collider is voxelized.

The Refine Nearby Fluids attribute on colliders prevents the fluid resolution from being coarsened in regions that are close to the collider when Spatial Adaptivity is enabled on the liquid properties node. Turn this option off on colliders where less detail is needed, for example, on the bottom and sides of pools, and turn it on for colliders where you want full detail. This does not affect the free surface (air boundary), which always uses full resolution.

In addition, the Stickiness section of a liquid emitter property controls how fluid particles from the associated emitters interact with any collider. High Stickiness Strength values cause the particles to adhere and slow down as they move across the surface of colliders, and Stickiness Bandwidth controls how close the particles need to be to the collider surface to be affected by the stickiness.

If you are using a collider as a basin to hold a pool of liquid from a static emitter in place, it's a good idea to have the emitter and collider overlap a little bit. This prevents small gaps of air that can create sudden velocities as the liquid fills the gap.

You might notice that fluid is somewhat offset from colliders' geometry when Thickness is 1.0 or higher in the collider mesh property's Conversion attributes, or that fluid penetrates the collider geometry by a small amount when Thickness is lower. This should not cause problems and becomes less perceptible at higher resolutions (lower Master Voxel Size).