Houdini 20.0

Volume visualizer

Visualizes a cross-section of a volume.

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The volume visualizer lets you view the values in a cross-section of a volume, using color, vectors, or advection trails.

See visualizers for more information.

Visualizing pyro fields

When visualizing pyro fields such as vel (velocity) and temperature, remember the following:

  • In the pyro_import object, put the display flag on the import_pyrofields DOP I/O node.

    To avoid having to put all the settings back the way they were before rendering, you may prefer to duplicate the pyro_import node and put the display flag on the copy.

  • in the import_pyrofields node’s parameters, on the Fields tab, make sure the field you want to visualize is imported, and set the Visualization for the field to something other than “Invisible”.

  • You may want to turn off import of other fields.


Source group

You can specify up to three primitives to visualize, using group syntax. For a volume, this will usually be a field accessed using @name= syntax. (for example @name=vel or @name=temperature).

If you specify 2-3 scalar fields, you can visualize them as if they were a 2-3 component vector field. This might be useful in some cases to visualize the relationship between two scalar fields at the same time in a color map.


Choose the orientation of the cross-section.


How far into the volume to display the cross-section. Drag the slider to move the cross-section through the volume.

Ghosting level

Controls the visibility of the volume. You can make the volume completely invisible to see only the visualiztion.

The number of levels of visibility available depend on the Antialias samples setting on the Display Options window's Effects tab. Better graphics cards can use a higher setting and get more possible visibility levels.


Visualize values as

How to display the voxel data in the cross section. Different styles have different controls.


You can visualize a scalar field using a color ramp, or a vector field as RGB, or convert a vector field to a ramp using various functions such as the magnitude of the vector.


Draws a line from the point based on a vector field’s direction and length.


Visualizes trails left by imaginary particles advected through a velocity field.

Style: Colors

Color mapping

How to display scalar data as colors. Choose a preset color ramp, or choose “Custom” to get a ramp control.


To map attribute values onto the ramp, Houdini needs to know the full range of possible values so it can scale that range onto the range of the ramp.


Looks at the actual values of the attribute in the visualized geometry and picks the minimum and maximum.

Min and Max

Allows you to manually specify min and max values. This can be useful when there’s a “natural” range of values larger than what’s currently in the geometry. You can also use a narrower range than what’s currently in the geometry as a way of highlighting values outside the range (see the Out of range values paramter).

Center and Width

Like Min and Max, except you specify a center value and the width of the range around that value.

Treat As Scalar

Convert vector attribute types (with more than one component) to a single value. This enables a menu for choosing how to convert the values.


How to convert multi-component values to a single number, when Treat as scalar is on.


Pick one of the components.

Component Abs.

Pick one of the components, and use its absolute value.

Component Sum

Sum up the component values.

Component Abs. Sum

Sum up the absolute values of the components.


Use the length of the vector.

Length Squared

Use the square of the length of the vector.

Dot Product

Use the dot product of the vector with a given vector. This may be useful for measuring deviance from a certain direction.

Text labels

Show values as text

Draw the sampled values as text on the cross-section. This can quickly become unreadable for dense grids, so you can use the Visibility setting to only show values around the mouse pointer.


Draw the values in this color.

Font size

The relative size of the values in the viewport.


Whether to show all values, only the value under the mouse pointer, or all values within a certain radius of the mouse pointer.

Style: Vectors

Length Scale

Allows you to scale the lines length up or down. This lets you make long lines shorter to reduce clutter, or make short lines longer to be more visible.

Normalize vectors

Normalize the values to the 0-1 range for display.

Show Arrow Tips

Draw a small barb at the end of each line.


How to color the lines representing the vectors.

Fixed color

All quills have the same color.

Vector values

Use raw vector values as RGB.

Vector directions

Normalize vectors, then scale and offset to the center of the color cube. This makes direction vectors visible as color regardless of magnitude and sign.

Style: Trails


Whether to advect the imaginary particles forward or backward through the velocity field.


How many “time steps” to advect the particles for. Increase this value to get longer trails. (This value has an arbitrary unit, it is not related to playback time or simulation timesteps.)

Per trail samples

How many timesteps to take within the Duration. Increase this to get higher-resolution trails.


How to color the trails. (This color is computed at each step along the trail, so trails may change color along their length.)

Fixed color

All trails use the same color.

Vector values

Use raw vector values as RGB.

Vector directions

Normalize vectors, then scale and offset to the center of the color cube. This makes direction vectors visible as color regardless of magnitude and sign.


Sampling rate

A percentage of the original volume resolution to sample at. This lets you sample at a low resolution to speed up visualization of a high-res volume, or oversample to see interpolation. For example, 1 samples at the full resolution of the volume, 0.5 samples at half the resolution of the volume.

Sampling grid shift

An offset between the sampling grid and the actual volume’s grid. Values less than 0 or greater than 1 will wrap around.

Snap slicing plane to sampling grid

When this is on, You can only move the cross-section Offset to positions on the sampling grid. When this is off, you can see interpolated sample values between grid positions.

Component order

Lets you shuffle the mapping between components in the field and RGB/XYZ in the visualization. This might be useful to change the direction of quills to make them easier to see, or change colors around to a more contrasty palette.

Display sampling grid

Draws lines on the cross-section to make the sampling grid clearer.


Enable extraction

Moves the cross section away from its actual position inside the volume, to make it easier to see the visualization and the volume at the same time.


Moves the cross-section. Values are multiples of the volume size in each direction, so a value of -1 or 1 will move the cross-section next to the volume.


Rotates the cross-section.

Uniform scale

Scales the cross-section.

Transform extract relative to slicing plane

Makes the values in the Translate and Rotate fields relative to the current Orientation of the cross-section.

Houdini 20.0

Getting started

Using Houdini

  • Geometry

    How Houdini represents geometry and how to create and edit it.

  • Copying and instancing

    How to use copies (real geometry) and instances (loaded or created at render time).

  • Animation

    How to create and keyframe animation in Houdini.

  • Digital assets

    Digital assets let you create reusable nodes and tools from existing networks.

  • Import and export

    How to get scene, object, and other data in and out of Houdini.

  • Executing tasks with PDG/TOPs

    How to define dependencies and schedule tasks using TOP networks.

  • MPlay viewer

    Using Houdini’s stand-alone image viewer.

Character FX

  • Character

    How to rig and animate characters in Houdini.

  • Crowd simulations

    How to create and simulate crowds of characters in Houdini.

  • Muscles and tissue

    How to create and simulate muscles, tissue, and skin in Houdini.

  • Hair and fur

    How to create, style, and add dynamics to hair and fur.

  • Feathers

    How to create highly realistic and detailed feathers for your characters.


  • Dynamics

    How to use Houdini’s dynamics networks to create simulations.

  • Vellum

    Vellum uses a Position Based Dynamics approach to cloth, hair, grains, fluids, and softbody objects.

  • Pyro

    How to simulate smoke, fire, and explosions.

  • Fluids

    How to set up fluid and ocean simulations.

  • Oceans and water surfaces

    How to set up ocean and water surface simulations.

  • Destruction

    How to break different types of materials.

  • Grains

    How to simulate grainy materials (such as sand).

  • Particles

    How to create particle simulations.

  • Finite elements

    How to create and simulate deformable objects


  • OBJ - Object nodes

    Object nodes represent objects in the scene, such as character parts, geometry objects, lights, cameras, and so on.

  • SOP - Geometry nodes

    Geometry nodes live inside Geo objects and generate geometry.

  • DOP - Dynamics nodes

    Dynamics nodes set up the conditions and rules for dynamics simulations.

  • VOP - Shader nodes

    VOP nodes let you define a program (such as a shader) by connecting nodes together. Houdini then compiles the node network into executable VEX code.

  • LOP - USD nodes

    LOP nodes generate USD describing characters, props, lighting, and rendering.

  • ROP - Render nodes

    Render nodes either render the scene or set up render dependency networks.

  • CHOP - Channel nodes

    Channel nodes create, filter, and manipulate channel data.

  • COP2 - Compositing nodes

    Composite nodes create, filter, and manipulate image data.

  • TOP - Task nodes

    TOP nodes define a workflow where data is fed into the network, turned into work items and manipulated by different nodes. Many nodes represent external processes that can be run on the local machine or a server farm.

  • APEX - APEX nodes

Lighting, rendering, and compositing

  • Solaris

    Solaris is the umbrella name for Houdini’s scene building, layout, lighting, and rendering tools based on the Universal Scene Description (USD) framework.

  • Rendering

    How to render images and animation from the 3D scene.

  • HQueue

    HQueue is Houdini’s free distributed job scheduling system.

  • Materials

    How to assign materials and create custom materials for shading.

  • Compositing

    Houdini’s compositing networks let you create and manipulate images such as renders.


  • Menus

    Explains each of the items in the main menus.

  • Viewers

    Viewer pane types.

  • Panes

    Documents the options in various panes.

  • Windows

    Documents the options in various user interface windows.

  • Stand-alone utilities

    Houdini includes a large number of useful command-line utility programs.

  • APIs

    Lists all the reference documentation for the ways you can program Houdini.

  • Python scripting

    How to script Houdini using Python and the Houdini Object Model.

  • Expression functions

    Expression functions let you compute the value of parameters.

  • HScript commands

    HScript is Houdini’s legacy scripting language.

  • VEX

    VEX is a high-performance expression language used in many places in Houdini, such as writing shaders.

  • Properties

    Properties let you set up flexible and powerful hierarchies of rendering, shading, lighting, and camera parameters.

  • Galleries

    Pre-made materials included with Houdini.

  • Houdini packages

    How to write and combine multiple environment variable definition files for different plug-ins, tools, and add-ons.

  • Houdini Engine

    Documents the Houdini Engine C, Python APIs, and Houdini Engine plugins

  • hwebserver

    Functions and classes for running a web server inside a graphical or non-graphical Houdini session.