Houdini 20.0 Nodes LOP nodes

Background Plate

Sets up “hold-out” or “matte” objects that leave “holes” in the scene through which the background is visible. These prims still take shadows and contribute to reflections as if they were the background.

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Since 19.0


This node sets up various render settings needed to render holdout (also known as “matte”) objects, as well as to render with a background image.


The Backround Plate LOP assumes that the elements will be rendered separately for tweaking them in compositing. To include the background image to the final render, follow the steps from Add background plate to the final render below.

The plane, the model is sitting on, is set as a matte.
The plane is not visible in the render, though it does catch shadows.

Behind the scenes, this node sets the Holdout Mode render setting on the matte objects to “background”, sets the matte objects for phantom shadow capture, and sets a render background image.

How to

  1. Create the model(s) you want to be holdouts.

  2. Create a Background Plate node in your LOP network.

  3. In the Background Plate node’s parameters:

    • Set the Primitives to a space separated list of prim paths of the objects you want to be hold-outs.

    • Set Plate to the file path of the background image to use. (You can use the Scale and Translate parameters to position the background image relative to the camera view.)

    If you're using frame-by-frame background images in an animation, you can use a filename expression for the plate path, for example $HIP/Footage/Images/Frame_$F.exr.

  4. After the Background Plate, add a Karma Render Settings node if your network doesn’t already have one.

  5. Wire the Background Plate’s second output (which contains AOV settings) to the Karma Render Properties node’s second input (which accepts extra AOV settings).

  6. On the Karma Render Properties node’s parameters, click the Image Output tab, then click the AOVs (Render Vars) sub-tab, then turn on Import Render Vars From Second Input.


By default, the node creates AOVs for diffuse bounces on holdout objects. You can use these in compositing to better integrate the CG elements with the background.


In order to match the viewport render, the compositing should be done such as “Beauty” over (Plate * invert("Holdout Shadows" + "Holdout Reflection Occlusion") + "Holdout Diffuse" + "Holdout Emission" + "Holdout Glossy Reflection")

Add background plate to the final render

The background plate is only displayed in IPR (= Interactive Photorealistic Rendering) for preview purposes, but not included to the final render. We assume that you want to have the plate as a separate compositing layer. However, you can render image and background together.

  1. After the Karma Render Settings LOP, add a Karma Render Settings Edit LOP and connect it.

  2. Open the Karma tab’s Global section and scroll down to Image.

  3. Click the ring-shaped button next to Use Background and choose Set or Create to make the parameter accessible.

  4. From the dropdown menu, choose On.


Output stage

Contains the USD stage, modified by this node. This is the standard output of a LOP node.

Output render vars

Contains the AOV (render var) settings specified in this node. If you are using the Karma node for rendering, you should connect this output to the Karma node’s second input to have them override the Karma node’s default settings.



A space-separated list of prim paths for the objects you want to act as hold-outs.


Rendersettings Primitive Path

The prim path of a USD Render Settings prim to use to set up the background and matte settings. The default is /Render/rendersettings, which is the default for where the Karma render node looks for render settings. If this prim doesn’t already exist in the incoming stage, this node will create it.

Allow Shader Parameter Animation

When it is turned off, all USD values are assumed non-animatable and will be authored at a default time code. And, with this option turned on, shader node parameters that are time-dependent will get translated to USD values at non-default time codes, while time-independent parameters will still author a value at a default time code.

Note, checking parameter time dependency can be costly and may add up to a large portion of the shader translation time. Since the vast majority of shader nodes don’t have animated parameters, this toggle is off by default to save the work involved in checking it.


The file path of the background image to use.


Horizontal and vertical scaling factors for the background image. These are normalized relative to the rendered image size, so for example a horizontal scale of 1 is the width of the rendered image.


Horziontal and vertical offsets for the background image. These are normalized relative to the rendered images size, so for example a horizontal offset of 0.25 moves the background image to the right 1/4 of the width of the rendered image.

Use Offscreen Texture

If the background plate does not perfectly cover the camera view, the “real” background (empty space) will show through. Use this to replace empty space with a texture. If you leave this off, the node will fill empty space with the Offscreen color instead.

Offscreen Texture

When Use offscreen texture is on, the file path of the texture image to use to cover empty space.


When Use offscreen texture is on, this sets the rotation of the offscreen texture in 3D space.

Offscreen Color

When Use offscreen texture is off, the color to use for any “empty space” if the background plate does not fully cover the camera view.

Shader Parameters


Values closer to 1 give the shader a more metallic appearance.

This is done by fading out the diffuse component and driving reflectivity by Base Color. This makes it easy to create the colored reflections typical of metallic materials.


Controls the reflectivity of surfaces facing the viewer. This is also affected by the Index of Refraction.

When a surface is rotated away from the viewer, more and more light is reflected regardless of this parameter’s value. At 90 degrees away from the viewer, 100% of the light is reflected, as in the real world.


How shiny an object appears mostly depends on its Roughness rather than its Reflect value. When you intend to vary shininess using a texture, Roughness is usually the better parameter to use.


For additional artistic control, you can disable reflections completely by setting Reflect exactly to 0.0. When using a texture, this effect also occurs in completely black areas of the texture.


The effect of this parameter diminishes with increasing Metallic values, since metallic reflectivity is tied to Base Color.


Controls how dull a surface appears. A value of 0.0 results in a smooth surface with perfectly sharp reflections. A value of 1.0 produces a completely dull surface.

This simulates microscopic bumps which become more pronounced the higher the Roughness, causing reflected light to be scattered more.

The effect of Roughness on a material with Metallic set to 1.0:

The effect of Roughness on a material with Metallic set to 0.0:


Higher values reflect more light and bend refraction rays more.

The impact on reflectivity is strongest on surfaces facing directly towards the viewer. Surfaces facing less directly towards the viewer generally reflect more light, regardless of this parameter’s value.


The shininess of a surface also depends greatly on its Roughness.


To disable reflections completely, set this parameter to 1.0. This can speed up rendering where reflections are not needed. When using a texture, this effect also occurs in completely black areas of the texture.

The effect on a shiny black object in a white environment. Note how reflections at the edges are not affected, apart from the special value 1.0:


The effect of this parameter diminishes with increasing Metallic values, since metallic reflectivity is tied to Base Color.

Holdout AOVs


Whether to record all holdout shadows in a single AOV, or generate a separate AOV for each light LPE tag.


Generate an AOV for diffuse bounces on holdout objects.


Generate an AOV for light emissions on holdout objects.

Glossy Reflection

Generate an AOV for glossy reflections on holdout objects. When disabled, the background shader will become diffuse only, reflectivity and metallic shader parameters will be set to 0.

Reflection Occlusion

When Glossy Reflection is enabled, whether to record all holdout reflection occlusion in a single AOV, or generate a separate AOV for each light LPE tag.

See also

LOP nodes

  • Add Variant

    Adds one or more variants to a variant set on a primitive. This node creates the primitive if it doesn’t exist.

  • Additional Render Vars

    Create multiple render vars.

  • Asset Reference

    Reference, Transform, and select variants of a USD Asset.

  • Assign Material

    Assigns a material to one or more USD primitives. You can use also programmatically assign materials using VEX, programmatically override material settings for each assignment, and programmatically assign materials to geometry subsets.

  • Assign Prototypes

    Switch point instances or USD instanceable prims to instance a different prototype.

  • Attribute VOP

    Create/edit USD attribute values using a VOP network.

  • Attribute Wrangle

    Create/edit USD primitive attributes using a VEX snippet.

  • Auto Select LOD

    Automatically selects a level-of-detail variant based on the primitive’s distance from the camera.

  • Background Plate

    Sets up hold-out or matte objects that leave holes in the scene through which the background is visible. These prims still take shadows and contribute to reflections as if they were the background.

  • Bake Skinning

    Bakes animation driven by a UsdSkel into transforms and point positions.

  • Basis Curves

    Creates or edits a basis curves shape primitive.

  • Begin Context Options Block

    This node begins a block of LOP nodes, within which certain context options have certain values.

  • Blend

    Partially applies edits to a layer’s attributes based on a fractional weight.

  • Blend Constraint

    Blends transforms according to a list of weights specified as parameters.

  • Cache

    Caches the results of cooking the network at different times, increasing playback speed.

  • Camera

    Adds a USD camera to the scene.

  • Capsule

    Creates or edits a capsule (tube with hemispherical ends) shape primitive.

  • Collection

    Creates/edits collections using primitive patterns.

  • Component Geometry

    Geometry container or import source, in a network created by the Component Builder tool.

  • Component Geometry Variants

    Sets up geometry variants, in a network created by the Component Builder tool.

  • Component Material

    Assigns materials to geometry in a network created by the Component Builder tool.

  • Component Output

    Assembles the final Component prim, in a network created by the Component Builder tool.

  • Cone

    Creates or edits a cone shape primitive.

  • Configure Layer

    Edits metadata on a layer.

  • Configure Primitives

    Edits various metadata on one or more primitives.

  • Configure Properties

    Configures metadata on properties (relationships and attributes).

  • Configure Stage

    Configures metadata for how to load layers into the stage and asset resolution.

  • Coordinate System

    Define named coordinate systems used in shaders.

  • Copy Property

    Copy properties from one primitive to another, or renames properties on a primitive.

  • Create LOD

    Uses the PolyReduce SOP to automatically generate multiple levels of detail from a high-res model, and stores them as USD variants.

  • Cube

    Creates or edits a cube shape primitive.

  • Cylinder

    Creates or edits a cylinder shape primitive.

  • Distant Light

    Creates or edits a USD Distant Light, representing a far-off light source such as the sun. Adds some useful Karma-specific attributes.

  • Dome Light

    Creates or edits a USD Dome Light prim. A dome light emits light inward, simulating light coming from the sky/environment surrounding the scene.

  • Drop

    Runs a simulation to drop primitives under gravity.

  • Duplicate

    Creates copies of a prim (and its descendants).

  • Edit

    Interactively transforms prims in the viewer. Can use physics collisions to position props realistically.

  • Edit Context Options

  • Edit Material

    Allows you to edit an existing USD material by modifying parameters and shader connections. This can be useful if the existing material is on a non-editable layer.

  • Edit Material Properties

    Lets you build a spare parameter interface that reflects material or shader input attributes to directly edit their values.

  • Edit Properties

    Lets you build a spare parameter interface to directly edit attribute and relationship values.

  • Edit Properties From Node

    Lets you refer to the parameter on another node to directly edit attribute and relationship values.

  • Edit Prototypes

    Modify the prototypes of native or point instances in-place, without disturbing the instancing setup.

  • Edit Target Layer

    Allows you to apply edits directly in a lower layer, instead of overriding prims and attributes in the active layer.

  • Error

    Generates a message, warning, or error, which can show up on a parent asset.

  • Explore Variants

    Visualize, set, or extract variants on primitives.

  • Extract Instances

    Converts (heroes) an instance into a real editable prim.

  • Fetch

    Grabs the output of another LOP, potentially in another LOP network.

  • File Cache

    Caches (writes out once and then reads from) USD layers (possibly animated) to disk.

  • Follow Path Constraint

    Constrains a prim to follow a path curve.

  • For Each

    The end node of a For-Each loop block.

  • Geometry Clip Sequence

  • Geometry Sequence

    Imports a sequence of geometry files into LOPs as animated geometry.

  • Geometry Subset VOP

    Creates USD geometry subsets within geometry prims (similar to groups in SOPs) based on evaluating a VEXpression or VOP network.

  • Graft Branches

    Takes prims/branches from the second input and attaches them onto branches of the scene graph tree in the first input.

  • Graft Stages

    Takes scene graph trees from other inputs and attaches them onto branches of the scene graph tree in the first input.

  • HDA Dynamic Payload

    Cooks a OBJ or SOP asset on disk and imports the animated geometry output as a USD payload.

  • Hermite Curves

    Creates or edits a hermite curves shape primitive.

  • Houdini Feather Procedural

    Generates feathers for rendering.

  • Houdini Preview Procedurals

    Invokes Houdini Procedurals while working interactively.

  • Houdini Procedural: Hair

    Houdini Hair Procedural for Solaris.

  • Houdini Procedural: Ocean

    Houdini Ocean Procedural for Solaris.

  • Inline USD

    Parses usda code representing a layer and adds it to the layer stack.

  • Insertion Point

    Represents a point in the node graph where nodes can be inserted.

  • Instancer

    Instances or copies primitives onto points.

  • Instancer

    Create multiple render products sharing common settings.

  • Isolate Scene

    Work in masked areas of the stage.

  • Karma

    Renders the USD scene using Houdini’s Karma renderer.

  • Karma Cryptomatte

    Setup Cryptomatte AOVs for Karma.

  • Karma Fog Box

    Creates a constant volume within a box.

  • Karma Physical Sky

    Creates a Karma Sky Dome and Sun Light rig.

  • Karma Render Properties

    Configure Render Properties for Karma.

  • Karma Sky Dome Light

    Creates or edits a Karma Sky Dome Light.

  • Karma Standard Render Vars

    Create standard karma render vars (AOVs/Image Planes).

  • LPE Tag

    Manage Lights' LPE Tags.

  • Labs Karma AOVs for RenderMan Denoiser

    Generates AOVs for the Pixar RerderMan denoiser.

  • Labs RizomUV Optimize

  • Labs RizomUV Rectangularize

  • Labs RizomUV Unwrap

  • Layer Break

    Starts a new active sublayer that subsequent nodes will edit, and indicates all previous layers will be discarded when saving to disk.

  • Layer Replace

    Replaces all uses of a certain layer with a substitute layer from its second input.

  • Layout

    Provides tools for populating a scene with instanced USD assets. You can place individual components, paint/scatter components in different ways using customizable brushes, and edit existing instances.

  • Light

    Creates or edits a USD Light prim. This node also adds some useful Karma-specific attributes.

  • Light Filter Library

    Authors USD light filter primitives from VOP nodes.

  • Light Linker

    Creates USD light link properties based on rules.

  • Light Mixer

    Lets you interactively edit USD properties for multiple lights.

  • Load Layer for Editing

  • Loft Payload Info

    Adds basic information from inside a payload to the primitive that loads the payload.

  • Look At Constraint

    Constrains a prim to always point toward a target.

  • Mask from Bounds

    Sets a primvar based on whether/by how much selected prims are inside a bounding shape.

  • Match Size

    Resizes and recenters the input geometry to match a reference bounding box.

  • Material Library

    Authors USD material primitives from shader VOP nodes.

  • Material Linker

    Creates material assignments based on rules.

  • Material Variation

    Creates attributes/primvars to override material parameters per-prim/instance.

  • Merge LOP

    Merges the layers from incoming stages into a unified layer stack.

  • Merge Point Instancers

    Merges point instancers into a single consolidated point instancer.

  • Mesh

    Creates or edits a mesh shape primitive.

  • Modify Paths

    Modify asset path attribute values.

  • Modify Point Instances

    Modifies point transforms and property values for individual point instances.

  • Motion Blur

    Adds time samples to allow motion blur when rendering.

  • Null

    This node does nothing. It can be useful to insert a Null into a network as a fixed point in the network that you can refer to by name in expressions/scripts.

  • Output

    Represents the output of a subnetwork. Allows you to design a node asset with multiple outputs.

  • Parent Constraint

    Makes a primitive appear to inherit the transform hierarchy of another prim somewhere else in the tree.

  • Points

    Creates or edits a Points shape primitive.

  • Points Constraint

    Position and Orient primitives using point positions from a geometry.

  • Primitive

    Bulk-creates one or more attributes of a certain type.

  • Prune

    Hides or deactivates primitives and point instances.

  • Python Script

    Lets you write Python code in the node to use the USD API to directly manipulate the stage.

  • RBD Destruction

    An example for a fracturing simulation in USD, also useful as a canned effect.

  • Reference

    References the contents of a external USD files and/or layers created by other LOP nodes into a branch of the existing scene graph tree. Can also remove or replace existing references.

  • Render Geometry Settings

    Applies renderer-specific geometry settings to geometry in the scene graph.

  • Render Product

    Creates or edits a UsdRenderProduct prim, which represents an output of a renderer (such as a rendered image file or other file-like artifact produced by a renderer), with attributes configuring how to generate the product.

  • Render Settings

    Creates or edits a UsdRenderSettings prim, which holds the general settings for rendering the scene.

  • Render Var

    Specifies a custom variable computed by the renderer and/or shaders, either a shader output or a light path expression (LPE).

  • Resample Transforms

    Generates interpolated transform time samples from existing time samples on USD prims.

  • Restructure Scene Graph

    This node has various operations for editing prim paths, variant sets, and composition arcs.

  • Retime Instances

    Offsets and/or scales the timing of animation on selected instances.

  • SOP Character Import

    Imports a character or animation from a SOP network into the USD scene graph.

  • SOP Create

    Lets you create geometry in a SOP subnetwork inside this node, so you can create geometry in-place in the LOP network instead of needing a separate SOP network.

  • SOP Crowd Import

    Imports a crowd from a SOP network into the USD scene graph.

  • SOP Import

    Imports geometry from a SOP network into the USD scene graph.

  • SOP Modify

    Converts USD geometry into SOP geometry, runs the SOP subnet inside this node on the geometry, and converts the changes back to USD overrides.

  • Scene Doctor

    Validates primitives on a USD stage.

  • Scene Import

    Imports models, materials, and lights from the Object level into the LOP network.

  • Scope

    Creates a scope primitive. Scope is the simplest form of grouping, and does not have a transform. Scopes can be useful for organizing the scene tree.

  • Set Extents

    Sets the bounding box metadata of selected primitives.

  • Set Variant

    Selects (switches to) one of the variants stored in a variant set on a primitive.

  • Simulation Proxy

    Generates low-poly collison geometry suitable for physical simulation and creates a proxy relationship to the original model.

  • Sphere

    Creates or edits a sphere shape primitive.

  • Split Point Instancers

    Splits a point instancer into two or more instances, which divide up the original instances.

  • Split Primitive

    Splits USD geometry prims into child primitives based on geometry subsets or primvar values.

  • Split Scene

    This node splits a scene graph into two disjoint sets of primitives.

  • Stage Manager

    Provides a convenient interface to reference in many files at once and place them in the scene graph tree.

  • Store Parameter Values

    Lets you store temporary (unsaved) data in the stage.

  • Sublayer

    Imports from USD files or other LOP node chains into as sublayers, or removes/replaces/reorders existing sublayers.

  • Subnet

    Encapsulates a LOP subnetwork, allowing you to organize and hide parts of the network.

  • Surface Constraint

    Constrain a prim to stick to a surface.

  • Switch

    Passes through one of several inputs, based on a parameter choice or expression.

  • TimeShift

    Outputs the stage as it is at a different point in the timeline.

  • Transform

    Edits the transforms of selected USD primitives.

  • Transform UV

    Moves, rotates, and scales texture coordinates on USD primitives.


  • USD Render ROP

  • Unassign Material

    Unbinds a material from one or more USD primitives.

  • Value Clip

  • Vary Material Assignment

    Assign different materials across a number of prims to create variation.

  • Volume

    References volume data on disk into a volume prim containing field prims.

  • Xform

    Creates or edits an Xform prim. Xform (and its sub-classes) represents a transform in the scene tree.