Delete particles with kill fields

Use kill fields to delete unwanted liquid, foam, or aero particles in a Bifröst simulation, based on various criteria. You can limit the effect to a specific volume using closed polygon meshes in the scene, or using an implicit shape.

Note that kill fields do not work well with guided liquid simulations, because guided simulations need to reseed particles to prevent gaps between the fluid and the guides.

1. Select the main Bifröst container, or a Bifröst shape node.

   For liquid simulations with foam:

   • Selecting the liquid container applies the field to both liquid and foam particles.
   • Selecting the liquid shape applies the field to liquid particles only. This will still have some effect on foam, because foam is emitted from liquid.
   • Selecting the foam shape applies the field to foam particles only. However, note that new foam particles can still be emitted by turbulent liquid even within the kill field's effect.

2. If you want to use one or more polygon meshes to define the volume of effect, then add them to the selection as well. The order of selection does not matter.

3. Select Bifrost Fluids > (Add) Kill Field.

   A bifrostKillField object is created at the world origin and automatically selected.

4. Define the volume affected by the field by doing one of the following:

   • To affect particles inside a volume, select a Boundary Shape in the field container's Boundary Controls attributes, and then scale, rotate, and translate the field. The field's wireframe representation in the viewport indicates the volume that will be affected. Note that InMesh applies only if you selected one or more meshes in step 2.
   • To affect particles outside a volume, do the same as above and also make sure to uncheck Invert.
   • To affect all particles everywhere, uncheck Boundary in the Kill Field Properties attribute group.

5. Define one or more criteria for deleting particles:

   • To delete all particles in the specified volume, use Boundary by itself and leave Lifespan and Droplets unchecked.
   • To delete particles based on their age, make sure that LifeSpan is enabled and then adjust the Lifespan attributes.
   • To delete particles based on their droplet value, make sure that Droplets is enabled and then adjust the Droplets attributes.

   For more information about these and other settings, see Bifröst Kill Field attributes.

To remove a kill field, you can either disconnect it in the Node Editor or simply delete it.