Work with Bifröst emitters

Emitters are the sources of fluid in Bifröst simulations.

Unlike in purely particle-based fluids, there is no "emission rate" for particles. Instead, particles flow out of emitters based on the velocity voxel field. This is determined by various factors, including gravity, pressure, temperature, and accelerators and other influences. The main role of the particles is to keep track of where the fluid is, rather than to drive the flow themselves.

Particles automatically inherit velocity from their emitter. For additional control over their velocity, you can use an accelerator (see Work with Bifröst accelerators).

Add emitters to an existing Bifröst simulation

1. Select one or more polygon meshes to act as emitters (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 emitter property. Alternatively, you can select the shape (e.g. liquid or aero).
   • An existing emitter property to share its attribute values with the selected meshes.
2. Select Bifrost > (Add) Emitter.
   If no particles appear in the viewport, there are a few things to check:

   • Make sure that the fluid is not hidden by the emitter itself. For example, you can switch to Wireframe display, or make the emitter into a template (unselectable), or simply hide the emitter.
   • Make sure that Renderer is set to Viewport 2.0 in the viewport's menu bar.
   • Make sure that the current frame is the start of the simulation (by default, frame 1).
   • Reduce Master Voxel Size on the container node in the case that the emitter objects are too small to contain any voxels.

Remove emitters

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

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

Edit emitters

To modify the properties of a specific emitter as well as the fluid emitted from it, edit the attributes of the corresponding emitter property. The emitterProps tab appears in the Attribute Editor when you select the emitter mesh. Alternatively, you can select the property directly in the Outliner.

In addition, you can paint certain emission attributes. See Paint Bifröst attributes.

Continuous Emission

Continuous Emission is the most important attribute for determining the behavior of an emitter.

• If you want to fill a static pool, emit a single drop, or create a single puff of smoke, turn off Continuous Emission. Particles get emitted only on the first frame of the simulation.
• If you are using an emitter as the source for something like a flowing river or a belching smokestack, leave Continuous Emission on. The emitter's voxels automatically get refilled with particles as they empty due to gravity or the effect of an accelerator object.
• If you want a jet, like liquid from the nozzle of a hose, leave Continuous Emission on, and in addition make the emitter into an accelerator or use a separate accelerator object to push particles out in a particular direction. You can parent the emitter and accelerator to animate them together, and use expressions or DAG connections to update the acceleration direction as they move.

You can animate Enable or Continuous Emission to turn emitters on and off.

Master Voxel Size

Unlike colliders and accelerators, there is no extra control over the global voxel scale for emitters. Both the emitters and the fluid itself use the Master Voxel Size set on the main simulation properties.