Houdini 18.5 Nodes LOP nodes

Python Script

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

On this page
Since 17.0

Overview

For technical or integration tasks that would be difficult to express using LOP nodes, you can use this node to access the full power of the USD API to read and modify the scene graph tree, the power of HOM to access and control the rest of Houdini, as well as the full power of Python and its libraries to access external data and integrate with your pipeline and other systems.

This node is often useful to script custom pipeline-specific tasks.

See Pixar’s Python API tutorials for help with the APIs.

Scripting the stage

  • Call hou.pwd() to get a reference to this node as a hou.Node. You can get either the current stage through this reference (see below). You can also use it to access spare parameters you might add to this node.

  • Call hou.pwd().editableStage() to get an editable pxr.Usd.Stage object. This represents the output of this node, including any changes you make using the API.

    (The USD Edit Target is automatically set to the active layer for you. Do not change the edit target. Doing so will have unpredictable negative consequences.)

Examples

Add new prims to the stage

stage = hou.pwd().editableStage()
xform = stage.DefinePrim('/shapes', 'Xform')
cube = stage.DefinePrim('/shapes/cube1', 'Cube')

Loop over every prim in the scene graph tree

stage = hou.pwd().editableStage()
for prim in stage.Traverse():
    # Access or edit the prim
    pass

Use USD’s high-level APIs to manipulate complex attributes

from pxr import UsdGeom

stage = hou.pwd().editableStage()
cube = stage.GetPrimAtPath("/shapes/cube1")

# Change attributes directly
cube.GetAttribute("size").Set(1.5)

# APIs can have useful shortcuts
cube_api = UsdGeom.Cube(cube)
color = cube_api.GetDisplayColorAttr()
color.Set([(0, 0, 1.0)])

# Use the XformCommonAPI to manipulate transforms instead of messing
# with matrix attributes directly
UsdGeom.XformCommonAPI(cube).SetTranslate((4, 5, 6))

Loop over all lights, copying their exposure into their intensity and clearing the exposure

from pxr import UsdLux

stage = hou.pwd().editableStage()

# UsdLux has a special API for finding all lights in the scene efficiently
api = UsdLux.ListAPI(stage.GetPseudoRoot())
for lightprim in light api.ComputeLightList():
    exposure_attr = lightprim.GetAttribute("exposure")
    intensity_attr = lightprim.GetAttribute("intensity")

    intensity = intensity_attr.Get()
    exposure = exposure_attr.Get()
    if exposure:
        intensity_attr.Set(intensity * (exposure ** 2))
        exposure_attr.Set(0)

Reduce the intensity of all lights to half their current values, using a hou.LopSelectionRule.

node = hou.pwd()

# First, find all the light prims on our input.
# We must do this before getting our editable stage to
# avoid an expensive recook of the input node.
ls = hou.LopSelectionRule()
ls.setPathPattern('{ usd_istype(0, @primpath, "UsdLuxLight") }')
paths = ls.expandedPaths(node.inputs()[0])

# Now that we have our set of prim path, we can get the
# editable stage and start making changes.
stage = node.editableStage()
for path in paths:
    prim = stage.GetPrimAtPath(path)
    intensity = prim.GetAttribute('intensity')
    intensity.Set(intensity.Get() * 0.5)

Scripting the active layer instead

  • Instead of manipulating the stage using Usd APIs, you can edit the active layer using lower-level Sdf APIs. You cannot do both in the same script.

  • You probably only want to do this if you have a reason to prefer the lower-level APIs. pxr.Usd methods are higher level methods that can be much easier to use. They perform more error checking and data validation. However the pxr.Sdf APIs can be dramatically faster, especially when creating new USD primitives.

  • To get an editable pxr.Sdf.Layer reference to the active layer, call hou.pwd().editableLayer(). If you do this, do not also call hou.pwd().editableStage() in the same script.

Tips

  • When you are editing code, the node will cook if you click outside the editor, or press Alt + Enter.

  • You can add spare parameters onto the node to give the node a graphical interface. For example, you could add a text box named primpath that takes a primitive path. Then you can access the parameter value in the script:

    node = hou.pwd()
    primpath = node.evalString("primpath")
    
  • If you make a useful Python Script node you want to share, you can turn a node instance into a LOP digital asset. Right-click the node in the network editor and choose Create digital asset. The script is stored as part of the digital asset, and the node functions the same way, but as an asset that can be shared and versioned.

  • The pop-up menu button to the right of the editor pulls snippets from any PythonScripts.txt files in the Houdini path. You can create a $HOUDINI_USER_PREFS_DIR/PythonScripts.txt file and add snippets to have them appear in this menu. Read $HFS/houdini/PythonScripts.txt to see the required file format.

  • If your script needs to "find all prims that…", instead of using the USD traversal methods, consider using hou.LopSelectionRule. This object implements Houdini’s USD prim pattern matching. It generates a list of path strings. You can turn a path string into a Prim object using stage.GetPrimAtPath(pathstring). When using this approach, evaluate the selection rule on the input node, not the current node, to avoid recursion errors. Also evaluate the selection rule before getting this node’s editable stage to avoid an expensive duplication of the input stage.

Parameters

Python Code

The Python code to run when cooking this node. See the main help above for tips and examples.

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.

  • 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.

  • 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 a collection using selected prims.

  • Collection

    Creates/edits collections using primitive patterns.

  • 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 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.

  • Karma Procedural

    Sets up render-time instantiation for Karma.

  • Karma Render Properties

    Configure Render Properties for Karma (Beta).

  • Karma Standard Render Vars

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

  • LOP nodes

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

  • 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.

  • 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 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).

  • Retime Instances

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

  • 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 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 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

  • 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.