Houdini 20.0


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

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How to configure the compositor

There are two kinds of compositing options in Houdini: global preferences and defaults which affect all compositing you do in Houdini, and project settings which affect a particular .hip file.

Global preferences

Choose Edit ▸ Preferences ▸ Compositing to open the compositing preferences.

The preferences let you control memory usage, efficiency, interface, and color behavior in the compositor.

Project settings

Choose Edit ▸ Compositing Settings to open the Composite Project Settings window.

The settings in this window control the defaults for various COP options in this scene file. Changing a project setting can affect existing nodes that use that setting.

Types of compositing operators

Most COPs belong to one of six groups. Each group has common parameters and a consistent way of behaving.


Create new images and generally create new planes.

Generators create new planes. They all have the same parameters for specifying the image and sequence information. Generators define the following information:

  • Image information

  • Sequence information

If a generator has an input, it can either add a plane to the sequence (“inline generation”) or affect an existing plane using a simple operation (“quick composite”).

Planes are generated “inline” when a generator adds a plane to an existing sequence. A generated plane can have its own data type and composition, it is not restricted to the input types.

A generator performing a “quick composite” generates the plane and uses a simple operation to combine the input plane (I) with the generated plane (G). The simple operations are Add, Subtract, Multiply, Minimum (Min), Maximum (Max), and Average. This is useful for creating masks or slates, or adding noise to images.

These techniques have the following advantages:

  • The frame range, resolution, interlace options and extend conditions are inherited from the input. This makes matching the resolution and frame range of another sequence much easier; if these are changed in the input, they automatically update.

  • The number of nodes needed is reduced; no merge or composite node is needed. This also boosts performance and memory efficiency.

Inline Generation or Quick Composite mode overrides most of the operator’s Image and Sequence parameters. Use the “Add Plane” parameter on the Image tab to specify the plane to create or modify, and the operations to perform. If the plane specified does not exist, or the operation is set to Rename or Replace, the generator creates a new plane.

Scoped filters

Filter operations which may be applied to only specified planes, channels or frames. Certain operations cannot be scoped, such as a Scale or Crop COP (which changes the resolution).

This family of operators allows an operation to be applied to selected planes and frames. By default, all planes and frames are selected.

Planes and channels that are not scoped are passed through as if the node was bypassed.

You specify scoping with a space-separated list of planes. You can specify specific channels with a dot followed by the channel name. YOu can also use wildcards. For example:


Color and Alpha are scoped.


All Planes are scoped.


The red channel of color is scoped.

Scoped filters also have options for scoping frames:


All frames are scoped.

Inside Range

All the frames within the frame range specified are scoped, with optional drop-off regions at each end of the range.

Outside Range

All the frames outside the frame range specified are scoped, with optional drop-off regions at each end of the range.

Even/Odd Frames

Only the even or odd frames are scoped.


All of the frames in the custom list are scoped.

Masked filters

Affect portions of an image through the use of a mask. This mask is another image which is similar to an alpha matte. Masked filters are a type of scoped filter.

Many operators let you apply them per-pixel using a mask image. The mask can be any plane or channel within the input sequence, or any plane or channel in the sequence of the mask input.

The mask acts as a stencil - wherever the mask is 1 (white), the operation is performed. Wherever the mask is 0, the input pixel is not affected. In between, the input pixel is linearly blended with the output pixel. The mask can be inverted so that all pixels outside the mask are affected instead.

Use the Alpha plane as a mask as a very efficient way of masking color correction operations to the actual image without affecting outside black levels.

Pixel filters

Highly optimized operations that can be combined into a single operation if they are grouped together in a chain. Pixel filters are a type of masked filter.

Houdini “collapses” multiple consecutive pixel filters in a network chain into a single cooking operation.

Pixel filter node tiles have a light blue background to distinguish them from other types of nodes.

  • Reduces quantization and cutoff errors by doing quantization once, at the end of the pixel filter chain, rather than at every node.

  • Uses less memory. Only 1 image is cooked, rather than making a copy for each pixel filter.

  • Calculates in floating point, preventing clipping and quantization error of fixed formats (8- and 16-bit integer). Can handle pixel values well above white and below black during the pixel chain without being clamped, even if the image’s native data format would clamp it. The values are clamped at the end of the chain.

  • Reduces network and quantization overhead, resulting in faster cooking.

If you use a mask, it must be the same for all pixel filters in that chain. Pixel filters with different masks will not collapse together. Houdini will log a warning to notify you that quantization is occurring at that node (to get rid of this warning, set the Quantize parameter to “At This Node”).

If a pixel filter chain branches into two pixel filter outputs, each output pixel filter will cook the chain independently of the other.

At any point in the chain, you can tell a pixel filter node to quantize using the Quantize parameter. This will terminate the chain at that point and start collapsing COPs again at the next Pixel Filter COP. Setting the display flag on a pixel filter COP has the same effect.

Timing modifiers

Move frames to different frame numbers, modify the frame range, or shift the sequence in time. Timing modifiers don’t generally cook image information, and so are very efficient in both memory and cooking performance.

Timing Modifiers perform a variety of editing operations and allow you to cut, splice, shift and sequence frames easily.

The following are timing modifier COPs:

Most Timing Modifiers do not need to cook images, so they are very quick and memory efficient. Some Timing COPs allow you to blend frames together; in this case, new images are produced and this efficiency advantage is lost.

Timing modifier node tiles have a beige background when they are passing through data without cooking.

Timing Modifiers are best viewed in the Timeline view mode . Most timing COPs have handles in the timeline view.

Compositing/blending operators

Take two image inputs and composite or blend them together in different ways. All compositing operators can be masked.

Compositing operators are the heart of the compositor. They take two or more image inputs and combine them in different ways in the output.

Compositing operators have parameters that let you choose the behavior of the operator in cases where the inputs have different frame ranges, resolution, and/or planes. You can transform the foreground elements as part of the operation, and apply motion blur to animated transforms.

See how to blend and layer images .

All compositing operations can be masked using the mask input.

Plane operators

Create, swap, or merge planes.

You will sometimes need to swap or create new planes to store information during compositing.

See working with planes.

VEX operations

Some COPs are implemented in VEX .

You can create custom compositing operators using the VEX language or VOPs. Some of the compositing operators shipped with Houdini may also be defined this way.

See how to create a custom COP with VOPs.


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.