Houdini 20.5 Nodes LOP nodes

Value Clip

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Since 18.0

Overview

Note

The Geometry Clip Sequence LOP should be used in preference to this node in most situations.

In USD, a value clip is a way of storing data sets in separate files.

  • This allows you to split large data across multiple files. For example, simulations or other animated data where the size of each time sample is so large that authoring all time samples to a single file is unwieldy (such as in terms of required network bandwidth).

  • Value clips are also useful in situations where you may have a library of animation clips which need to be run in a sequence or a loop. Often different primitives can share animation clips, but assemble the clips in different ways to create the appearance of variation.

  • Value clips allow flexibility in composing layer overrides. USD value resolution stipulates that the first (strongest) layer that contains any timeSample for an attribute is the source of all timeSamples for the attribute. This is often fine, but for huge numbers of timeSamples you may need more flexibility.

You should read the USD documentation for value clips to understand how they work and how you can use them:

Note

This node provides a way of authoring a value clip composition arc in USD. Future versions of Solaris may have more tools for working with and using value clips at a higher level.

Note

An alternative to using the Value Clip LOP is to use the USD Stitch Clips ROP to author a USD layer file containing a value clip definition that you can reference into a scene. This approach has the benefit that it also creates a topology layer and a manifest layer, which can improve the performance of the value clip composition.

Note

A sequence of '.bgeo' or similar non-native USD files can be loaded directly as value clips, however you must ensure that each geometry file generate its USD attribute data with the correct time sample. This can be achieved by writing the time value into the geometry file as a detail attribute created by the USD Configure SOP. Or, the time value can be specified by appending arguments to the geometry file path with the suffix '/tmp/file.$F4.bgeo.sc:SDF_FORMAT_ARGS:sampleframe=$F'.

Parameters

Primitive Path

The scene graph path of the prim on which to author a reference to the value clip. If the specified primitive does not exist, the node will create it with no primitive type (the primitive type will come from the value clip file).

Primitive Kind

If the prim at Primitive path didn’t already exist so this node has to create it, the node sets this as the new prim’s kind.

Parent Primitive Type

If any parents of the path in Primitive path do not exist, this node will automatically create them. In this case, it will create parent nodes of this type.

Clip Set

The name of the “clip set” the value clip belongs to (multiple “clip sets” can exist on a single prim).

Clip Primitive Path

The path of a prim within the value clip file that should be mapped onto Primitive path when accessing values.

Manifest File

The file path to an optional manifest file that specifies exactly which primitives and values should be brought in from the value clip files. Providing this file helps the USD framework optimize value clip resolution. See the USD documentation for more information about the manifest file.

Start Frame

The frame on the current stage at which the value clip resolution should start.

First Clip Start Frame

When this is turned on, it sets a different start time within the value

clip files. When this is turned off, the nodes uses the Start frame parameter as the clip start time.

Clip Time Scale

A scale factor applied to the time samples read in from the clip files.

Loop Until Frame

To create a repeating loop of the specified clip files, turn on this parameter and set it to the end frame of the loop. (USD does not natively support looping of animation or value clips, so this node explicitly authors all provided clips repeatedly until the requested loop end time is reached.)

Loop First and Last Frames Match

When looping an animation and using deformation or transform motion blur, it is important that the first and last frames of the looping animation are identical. In this case, enable this parameter so that the Value Clip LOP will author a “discontinuity” in the clip frames when looping from the end of the animation back to the start. This discontinuity prevents doubling up the display of the shared first and last frame, while also avoiding unexpected motion blur effects.

With this option turned off, the “clip times” metadata for a looping 100 frame clip would be (1, 1), (100, 100), (101, 1), (200, 100). In this data, the first number in each pair is the global stage frame number, and the second number is the frame within the clip that should be displayed for that global frame. So stage frame 1 is clip frame 1. Stage frame 100 is clip frame 100. Then stage frame 101 is back to clip frame 1. There are two problems here. First, if the clip’s first and last frames are equal, frame 100 and frame 101 will be identical. But even worse, when rendering with motion blur (using a shutter open/close of +/- 0.5), rendering frame 101 will show data interpolated at frames 100.5 and 101.5. But USD does this by interpolating the clip metadata, not the data stored within the clips. So the frame 101.5 data will be as expected (showing data from clip frame 1.5). But the frame 100.5 data will be generated by grabbing the clip data at a frame half way between 100 and 1, which is frame 50.5. This is obviously very unexpected.

With this option turned on, the same setup would author clip times of (1, 1), (100, 100), (100, 1), (199, 100). Note that there are two different clip times authored for the single stage time of 100. This is the “discontinuity”. But it eliminates both problems mentioned above. Stage frame 100 is clip frame 100, but stage frame 101 is now clip frame 2, eliminating the duplicated data. And for motion blur rendering frame 101, frame 100.5 will use the clip data at frame 0.5 and 1.5, exactly as expected.

Note that if the first and last frames of the clip are not equal, there is no way to get perfect looping, unless you use velocity motion blur which does not require grabbing interpolated data between frames. The first problem mentioned above (the duplicated frame at the end of the loop) will be avoided, but the clip metadata interpolation approach used by USD means sampling frame 100.5 will return data from the middle of the clip, resulting in incorrect motion blurring.

Number of Clips

You can add references to multiple clip files in one node. Set this to the number of clips to reference, or click the plus and minus buttons to add or remove clips.

Clip File or Sequence

The file path to this value clip file. This path can include number-sign (#) characters to perform numeric substitutions in a loop. For example, on disk there may be multiple files named sim_0001.usd, sim_0002.usd and so on, each containing one frame of animation. In this case you could use sim_####.usd. Numeric substitution only happens if File sequence count is turned on. See File sequence count, File index start, and File index step below.

For example, if you turn on File sequence count and set Clip file or sequence to sim_####.usd, File index start to 10, File index step to 2, and File sequence count to 3, the node will try to load sim_0010.usd, sim_0012.usd, and sim_0014.usd.

If you use numeric substitution, each file must have the same number of

frames of animation (File duration).

File Duration

The number of frames for which this clip file is active. If the Clip file or sequence path matched multiple files through numeric substitution, each clip file will get this same duration.

File Sequence Count

If you turn this on, the node evaluates the Clip file or sequence filename with numeric substitution this many times, starting the number at File index start and increasing the number each time by File index step.

File Index Start

When File sequence count is turned on, the node evaluates the Clip file or sequence multiple times with numeric substitution, starting at this number.

File Index Step

When File sequence count is turned on, the node evaluates the Clip file or sequence multiple times with numeric substitution, incrementing the number by this many each time.

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.

  • Draw Mode

    Sets draw mode properties on USD model assets.

  • 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 Color

    Adds display color and display opacity primvars to geometry.

  • 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: Crowd

    Houdini Crowd Procedural for Solaris.

  • Houdini Procedural: Hair

    Houdini Hair Procedural for Solaris.

  • Houdini Procedural: Ocean

    Houdini Ocean Procedural for Solaris.

  • Houdini Procedural: RBD

    Houdini RBD 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.

  • 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 Products

    Create multiple render products sharing common settings.

  • Karma Render Properties

    Configure Render Properties for Karma.

  • Karma Sky Atmosphere

    Creates or edits a Karma Sky Atmosphere.

  • 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 RenderMan denoiser.

  • 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 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.