Houdini 20.0 Nodes LOP nodes

Light Mixer

Lets you interactively edit USD properties for multiple lights.

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Since 18.0

How to

Light Mixer has a custom Qt-based user interface that takes over the parameter editor when this node is selected. You can switch between the custom interface and the regular parameters by clicking the Show/hide parameters button in the Parameter editor’s toolbar.

  • A tree on the left shows all lights in the scene.

  • Drag one or more lights or collections from the tree on the left into the space on the right.

  • You can switch the right side between two views of the added lights:


    Build a custom mixer interface, showing the lights and collections as sliders, similar to a light board.


    Displays the attributes on the lights in a spreadsheet format.

  • If you drag in an individual light into the mixer, it appears as an individual set of sliders for controlling different attributes (intensity and exposure, by default, plus color).

    In the spreadsheet view, individual lights appear as rows.

  • If you drag a light collection into the mixer, it first appears collapsed. Click the Collapse button in the top right corner to expand the lights inside.

    Each collection has its own “master sliders” for intensity, exposure, and a tint color, which are all multiplied with the attributes of the lights inside.

    In the spreadsheet, collections appear as folders with light rows under them.

To...Do this

Arrange items in the mixer

Drag items in the mixer to rearrange them.

Create a light collection

Select lights in the tree on the left side, then click the Create a new Collection button.

Edit a value in the spreadsheet view

Click the value.

Tips and notes

  • This node represents a first try at using custom Python-driven UI for a node. Future versions should be faster, more robust, and have many more features.

  • Currently collections are always created on /collection.

  • Collections created here will not have “/” included by default and their expansion rule is explicitOnly.

  • You can add the same light/collection multiple times.

Configuration file

You can customize the Light Mixer user interface by creating a HOUDINIPATH/LightMixerConfig.json file (for example, $HOUDINI_USER_PREF_DIR/LightMixerConfig.json. If the node finds multiple LightMixerConfig.json files on the path, it merges them such that files earlier in the path override later ones (the HOUDINIPATH is ordered from most specific to least specific, unlike many path variables in UNIX where later items override earlier ones).

The following example contains the defaults for the available settings:

    "main.default_tab": 1,
    "mixer.show_attributes": ["inputs:intensity", "intputs:exposure"],
    "sheet.show_attributes": ["inputs:intensity", "inputs:exposure", "inputs:color",
                              "inputs:colorTemperature", "$solo", "$mute", "$active"],
    "collections.search": ["/lights", "/collections"],
    "lights.search": ["/"],
    "attributes.config": { }

In your configuration file you can omit default values (unless you need to override another configuration file higher up the path). For example, if you only want to make new Light Mixers open with the Attribute Spreadsheet tab active, you can use just the following:

    "main.default_tab": 0

You can use the following configuration keys in the JSON file:


(Integer) Which tab is active in a new Light Mixer. 0 sets the Sliders tab. 1 sets the Attributes tab. 2 sets the Transforms tab. Default is 1.


(List of strings) The names of USD attributes to show sliders for in the Sliders tab. The default is ["inputs:intensity", "intputs:exposure"].


(List of strings) The names of USD attributes to show columns for in the Attributes tab. For example, "inputs:color". The strings can use the following special values:

  • You can use the special strings $solo, $mute, and $active to add columns for those controls.

  • You can “extract” a component of a vector attribute using attribute_name$index, for example inputs:color$1 to get the second component (green) of the color. You can use r, g, and b as aliases for 0, 1, and 2, for example inputs:color$r to get the red component of the color.

  • You can also use the names of custom attributes defined in the attributes.config key (see below).


(Object) This maps attribute name strings to configuration objects describing how to present the attribute in the Light Mixer user interface. The object can have the following keys:


(String) The USD name for the attribute’s data type, for example float, color3f, bool.


The default value for the attribute. The value should be a JSON equivalent of the attributes type. For example, if the type is color3f, this should be a list of three numbers.

min, max

The minimum and maximum realistic values for the attribute (for compound types, these apply to the components). The light mixer uses these to set the low and high ends of the slider.


(String) The human readable label for the attribute to display in the user interface.


(List of strings) For compound types, individual labels for each component.


(String) How to apply this attribute to a collection. If this key is not present, this attribute is not shown in the interface as applicable to a whole collection at once. The possible values are multiply (multiply the attribute on each member by the collection value), add (add the collection value to the attribute on each member), or set (set the attribute on each member to the collection value).


The value which, when applied to a collection, has no effect. This is like the default value when applied to a collection. For example, 1.0 for a scale, or 0.0 for an offset. The value should be a JSON equivalent of the attributes type. For example, if the type is color3f, this should be a list of three numbers.


(Number) Indicates the number of decimal places the user interface should show for this value (if the attribute type is numeric). For example, 0.01 to show two decimal places.

For example:

    "sheet.show_attributes": ["inputs:intensity", "mycustomvector"],
    "attributes.config": {
        "mycustomvector": {
            "type": "float2",
            "default": [1.0, 1.5], 
            "identity": [1.0, 1.0],
            "min": 0.0, 
            "max": 3.0, 
            "label": "Size", 
            "component_labels": ["Width", "Height"],
            "modify": "multiply", 
            "precision": 0.1, 


Number of Collections

The number of USD collections that this node will create. For each collection, a separate set of parameters are used to define the collection’s name, USD prim and members.

Collection # Prim

The USD prim where the collection will be authored.

Collection # Name

The name of the new collecton. Replaces any existing group of the same name on the same prim.

Collection # Includes

Space separated list of prim patterns that will be included in the collection.

Collection # Excludes

Space separated list of prim patterns that will be excluded from the include list.

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

    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 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.