Houdini 20.5 Vellum

Emitting fluids and grains

Emit fluid or grain particles once, per frame or per substep.

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Overview

By default, the Vellum solver sources Vellum objects and constraints only on the start frame. Imagine, you want to emit water from a tap to fill a glass. In this case you need a continuous stream of particles. This configuration requires additional setup.

Working with emission types

Create an object to emit from. You can source fluids and grain in two different ways

  1. Select the object and click Vellum Grains tool on the Vellum tab. Dive into the DOP Network and click the Vellum Source DOP node. In the node’s parameters pane tab, look for Emission Type.

  2. Create your own node network. Dive into the network, and go to the Vellum Soruce node. There, look for Emission Type.

The standard selection is Once Only. For a continuous stream of particles, select Each Frame or Each Substep. With Each substep, emission relies on the number of Substeps in the Vellum solver node. For better results, we recommend 10 for Vellum fluids, and 5 subteps for Vellum grains. Higher substeps create a more natural emission and more particles. Note that higher subteps will also increase simulation times.

How to

To...Do this

Avoid gaps between particles

  1. Increase the number of substeps.

  2. Switch Emission Type from Each Frame to Each Substep.

Stop emission

For example, you want to stop emission at frame 20 through an expression:

  1. Select the Vellum Source node and set Emission Type to Each Frame or Each Substep.

  2. Go to Activation and enter the following expression: $FF<21. The variable $FF stands for the exact frame and will be updated by Houdini.

Another way is to animate Activation. This is, for example, helpful if you want to interrupt emission for a certain time, and reactivate it later:

  1. Select the Vellum Source node and set Emission Type to Each Frame or Each Substep.

  2. Go to Activation. With 0, emission is deactivated, with 1 it’s active.

  3. Drag the timeline slider to the desired frame and Alt + on the parameter to create a keyframe.

  4. Repeat the previous step if necessary.

Control emission direction and speed

Vellum recognizes a velocity attribute (v) on your source points. You have two methods to control emission. The first method uses an Attribute Adjust Vector:

  1. Add the node between the Vellum Configure Grain and the GEO Null node.

  2. Fill in values for Constant Value to determine speed and direction.

The second option is based on an Attribute Wrangle node:

  1. In the Attribute Wrangle node’s VEXpression field enter, for example: v@v = set(0,1,0);

  2. This expression will emit the particles along the Y axis with a speed of 1. Change the vector values to alter emission direction.

Avoid regular emission patterns

  1. Set the Vellum Configure Grain node’s Seed to $T to displace newly created particles with every timestep. The variable $T stands for the current time and will be updated by Houdini.

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