Use kill fields to delete unwanted liquid, foam, or aero particles in a Bifröst simulation. The available options allow you to affect particles based on certain criteria, when the particles are within a specific volume.

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

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

2. Select Bifrost (Add) Kill Field.

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

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

   • To delete particles outside 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.
   • To delete particles inside a volume, do the same as above and also make sure to uncheck Invert.
   • To affect all particles everywhere, uncheck Enable Boundary in the Kill Field Properties attribute group.

4. Define the criteria for deleting particles

   • To delete particles based on the accumulated time spent inside the field's volume, make sure that Mode is set to LifeSpan and then adjust the Lifespan attributes.
   • To delete particles based on their droplet value, set Mode to Droplets and adjust the Droplets attributes.
   For more information about these settings, see Bifröst Kill Field attributes.

You can use more than one kill field, but there are some issues to be aware of:

• Each particle can have only one life span. If multiple fields are set to delete based on age, then only one of the Lifespan settings are used.
• If two fields are set to delete based on life span and they overlap, then particles age twice as fast in their intersection. In addition, note that two or more fields with Invert turned on will always overlap. Even if only one of them has Invert turned on, they will still almost always overlap.

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

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