Houdini 19.0 Nodes LOP nodes

Reference

References the contents of an external USD file into a branch of the existing scene graph tree, or removes/replaces existing references.

On this page
Since 17.0

Overview

Referencing takes the tree in the referenced file and “grafts” it onto a branch in the current tree.

For example, if you start with this tree:

Scene tree before reference


/Lights/
    light1
    light2
/Models/
    tableside_lamp
        

If /Models/tableside_lamp references lamp.usd and that file contains:

Contents of lamp.usd


/lamp/
    base
    bulb
    shade
    socket
    switch

Then after referencing the file, the tree will look like this

Scene tree after reference


/Lights/
    light1
    light2
/Models/
    tableside_lamp/
        base
        bulb
        shade
        socket
        switch
  • The prim containing the reference (/Models/tableside_lamp) and the referenced prim (/lamp from lamp.usd) are composed together (with overlapping property values decided by opinion strength), but the prim keeps the name of the original referencing prim.

  • When you reference in content, you must specify a prim in the referenced file (/lamp in the example above), and that prim cannot be the root (/). So when you're creating a file that is meant to be referenced in by other layers, you should organize the contents under a single root primitive.

    (It’s possible to specify the default primitive of a file. If you reference a file and don’t specify a specific prim in the file, you get the file’s “default primitive”.)

Referencing is useful to graft smaller individual assets into a larger scene. In particular, referencing is the only way to load the same layer file more than once at different locations. (Because sublayering works on the entire tree, sublayering the same file more than once would have no effect.)

Simple trash can asset

#usda 1.0
(
   defaultPrim = "TrashCan"
)

def Xform "TrashCan" (
   kind = "component"
)
{
   def Cylinder "Can"
   {
       token axis = "Y"
       bool doubleSided = 0
       double height = 2
       double radius = 1
   }
}

File which references the trash can asset 3 times, for 3 different rooms in a set

#usda 1.0
()

def Xform "Scene"
{
   def Xform "Set"
   {
       def "BathroomTrashCan" (
           append references = @./trashcan.usda@
       )
       {
           double3 xformOp:translate = (2, 0, 1.4)
           uniform token[] xformOpOrder = ["xformOp:translate"]
       }

       def "KitchenTrashCan" (
           append references = @./trashcan.usda@
       )
       {
           double3 xformOp:translate = (16.01, 5, -43.072)
           uniform token[] xformOpOrder = ["xformOp:translate"]
       }

       def "OfficeTrashCan" (
           append references = @./trashcan.usda@
       )
       {
           double3 xformOp:translate = (-7.12, 0, 11.9)
           uniform token[] xformOpOrder = ["xformOp:translate"]
       }
   }
}

The Reference LOP and Stage Manager LOP create references. The Graft Stages LOP is similar, but instead of referencing from a file, it inserts a branch into the scene graph tree from elsewhere in the LOP network.

Tips

  • If the prim you want to reference onto already has an existing reference, the new reference will be composed with any existing references. See the Pre-operation parameter for how to clear existing references before adding a new one.

How to

The “target” that gets referenced onto a primitive in the scene graph tree is often in an external file, however this node can also reference a prim in the output of another LOP node.

To...Do this

Reference a prim in an external file

  1. Set the Primitive path to the scene graph path at which the referenced prim should be attached.

  2. Set the Reference type to “Reference files” or “Payload files” (see payloads for more information).

  3. Set the Reference file to the file path of the USD file to reference in.

  4. Use the Reference primitive pop-up menu if you want to choose the prim in the external file to load in.

  5. Usually you can leave Reference primitive set to “Reference automatically chosen primitive”. If you want to import a specific prim, change Reference primitive to “Reference specific primitive” and enter the prim path in Reference primitive path.

Reference a prim from the output of a LOP node

  1. Connect the LOP node containing the stage you want to reference to this node’s multi-input.

  2. Set the Reference type to “Reference from multi-input”.

  3. Set the Reference primitive to “Reference specific primitive”.

  4. For a single addition, you can set the Reference primitive path to the prim to import.

    If you have multiple inputs connected to this node’s multi-input:

    • You can set Reference primitive path to a scene graph path to import from the same prim in all inputs, or write an expression which is evaluated from each input connected to this node’s multi-input. In the expression, you can use the @input local variable ($IIDX is deprecated) for the current input index (starting at 1). (Remember that in string parameters, you must enclose expressions in backticks.)

    • Next to each input in the table below the parameters, click in the Reference Primitive Path and enter the scene graph path of the prim you want to reference in from that input. If an input has a Reference Path in the table, it ignores the value of the Reference primitive path parameter.

Reference another prim in the scene graph tree in the first input

  1. Set the Primitive path to the scene graph path at which the referenced prim should be attached.

  2. Set the Reference type to “Reference from first input”, “Inherits from first input”, or “Specialize from first input”, depending on the type of composition you want. See inherits and specializes for more information about how these choices differ from a simple reference.

  3. Set Reference primitive to “Reference specific primitive”.

  4. Set Reference primitive path to the scene graph path of the prim you want to reference in.

Block all existing references

  1. Set the Primitive path to the scene graph path of the prim that currently has a reference you want to block.

  2. Set the Pre-operaiton menu to “Clear all references”.

Block one existing reference

  1. Set the Primitive path to the scene graph path of the prim with the reference you want to remove.

  2. Set the parameters to specify the reference you want to remove. For example, if you want to remove a reference to the file lamp.usd, set Reference type to “Reference files” and Reference file to lamp.usd.

  3. Set Reference operation to “Remove references”.

Parameters

Destination Primitive

Primitives

In edit mode, the node has this parameter to specify any number of existing primitives. This lets you specify the prim(s) the node should operate on. You can click the select button beside the text box to select the primitives from the scene graph tree. You can also use primitive patterns for advanced matching, including matching all prims in a collection.

Primitive Path

In create mode, the node has this parameter to specify a single primitive path. This is the scene graph path where the referenced branch will be attached. The referenced prim will be overlayed onto this prim, and the referenced prim’s descendants will become this prim’s descendants. If this prim doesn’t exist, the node will create it.

Action

Whether this node should create a new prim, or edit existing prims. Create mode will also edit a prim if it already exists, but only a single prim can be specified in that mode. Edit mode allows for adding references to any number of prims with one node.

Primitive Kind

If the prim at Primitive path doesn’t exist, this node will create it. If it has to create the prim, it will set it to have this kind.

Parent Primitive Type

If the prim at Primitive path doesn’t exist, this node will create it. If it has to create the prim, it will give any intermediate prims it has to create this type.

Make Instanceable

Mark the new or modified prims as instanceable (as in “native instancing”). This saves memory if you reference the same branch into multiple places, but makes the descendants of the referenced prim not editable. If this option is off, the prims are not explicitly marked as not instanceable. Instead, the current instanceable value on each prim is left unchanged.

Composition

Reference Type

The type of composition arc to create to the referenced prim.

Reference Files

Reference in a prim from an external USD file.

Payload Files

Reference in a prim from an external USD file as a payload.

Reference From Multi-input

Reference in a prim from one of the nodes connected to this node’s multi-input.

Payload From Multi-input

Reference in a prim from one of the nodes connected to this node’s multi-input as a payload.

Reference From First Input

Reference in another prim from the scene graph tree in the first input.

Inherit From First Input

Create an “inherits” composition arc to another prim from the scene graph tree (in the first input).

Specialize From First Input

Creates a “specializes” composition arc to another prim from the scene graph tree (in the first input).

Handle Missing Files

If the reference is to an external file, this controls what the node does if the referenced file does not exist.

Error for Missing Files

The node errors, preventing cooking the rest of the network.

Warn for Missing Files

The node shows a warning, then passes through the first input unchanged.

Ignore Missing Files

The node silently ignores the missing file, and passes through the first input unchanged.

Allow Missing Files on the Stage

Author the reference to the sublayer even if it currently doesn’t exist. This node and the following nodes will probably print warnings from the USD libraries about missing files.

Pre-Operation

If you set this to a value other than “No pre-opertation”, the node clears existing references on the node.

No Pre-Operation

Do not clear existing references on the prim at Primitive path.

Clear Reference Edits in Active Layer

Clear any additions or removals to the list of references on this prim made in the currently active layer.

This is not a non-destructive USD edit. It’s more like undo-ing any changes from previous nodes that are currently “pending” in the active layer.

Clear All References

Authors an override of the specified reference type which masks all existing references on this primitive, regardless of the layer in which they were authored. This provides a blank slate so that only the references specified by this node will remain.

Reference Operation

How new references added by this node interact with existing references on the prim at Primitive path.

When editing references on a primitive, you are really making edits to a list. These edits are expressed as adding to the beginning or the end of the list created by composing all weaker layers, or by explicitly removing certain items from the list.

Add as Strongest References in Prepend List

Make new references stronger than any references from lower layers, and stronger than any previous “prepend” edits in the currently active layer.

Add as Weakest References in Prepend List

Make new references stronger than any references from lower layers, but weaker than any previous “prepend” edits in the currently active layer.

Add as Strongest References in Append List

Make new references weaker than any references from lower layers, but stronger than any previous “append” edits in the currently active layer.

Add as Weakest References in Append List

Make new references weaker than any references from lower layers, and weaker than any previous edits in the currently active layer.

Remove References

The layers or primitives referenced by this node are removed from the list of references on the destination primitive rather that being added.

Because this removal is expressed as a list edit, you can selectively remove specific layers from the list of references, no matter what layer authored the original reference.

Number of Files (in File-based modes and primary input modes)

This node can add multiple references to the specified prim, however the reference type, and operation will be the same for all additions.

Reference File

Enable

Perform the reference in this instance of the multi-parm.

Reference File

When Reference type is “Reference files” or “Payload files”, the file path to the file to reference in.

Reference Primitive

When Reference type is “Reference files” or “Payload files”, this controls which prim to import from the external file. When Reference type is “from first input” or “from multi-input”, you should set this to “Reference specific primitive” (the other options are usually pointless in that case).

Reference Automatically Chosen Primitive

If the referenced file has a default primitive set, use that. If there is no default prim, use the first Xform prim at the root. If there is no Xform prim at the root, use the first root prim of any type.

Reference Default Primitive

If the referenced file has a default primitive set, use that. If not, display a warning on this node and ignore this instance of the multi-parm.

Reference Specific Primitive

Reference in the scene graph path in the Reference primitive path parameter.

Reference Primitive Path

When Reference primitive is “Reference specific primitive”, this is the scene graph path to the prim to reference in.

(If you set this field automaticPrim or defaultPrim, it mimics the behavior of setting Reference primitive to “Reference automatically chosen primitive” or “Reference default primitive”.)

Time Offset (in Frames)

Offsets all time samples from the file. You can use this to re-time animations in the references file.

Time Scale

Scales all time samples from the file. You can use this to re-time animations in the references file.

Reload Files

Click to reload the contents of all files imported by this node.

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.

  • 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.

  • 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 Prototype

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

  • 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.

  • Follow Path Constraint

    Constrains a prim to follow a path curve.

  • For Each

    The end node of a For-Each loop block.

  • 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.

  • 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

    Creates a constant volume within a box.

  • Instancer

    Create multiple render products sharing common settings.

  • Karma

    Renders the USD scene using Houdini’s Karma renderer.

  • Karma Procedural

    Sets up render-time instantiation for Karma.

  • Karma Render Properties

    Configure Render Properties for Karma.

  • Karma Standard Render Vars

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

  • LOP nodes

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

  • 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 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.

  • 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.

  • 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.

  • 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 of how to a fracturing simulation in USD, also useful as a canned effect.

  • Reference

    References the contents of an external USD file into a branch of the existing scene graph tree, or removes/replaces 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 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.

  • 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 ROP

  • 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.