Houdini 20.5 Nodes LOP nodes

Geometry Clip Sequence

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Since 20.0

Overview

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 in USD either from existing files on disk, or from data generated in LOPs fed into this node’s second input.

This node should supercede most use cases for either the Value Clip LOP or the USD Stitch Clips ROP, each of which have significant limitations. The Value Clip LOP will not author manifest or topology files (so they must already exist or USD will have to regenerate them every time the stage is loaded). The USD Stitch Clips ROP requires writing USD files to disk, running the ROP as a second step, and then referencing in the output of the ROP. And unlike either of these methods, this LOP has the ability to strip non-animated data from the clip files before they get written to disk (leaving non-animated data only in the topology file). This can result in huge disk space savings.

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

Sampling Behavior

Cooking this node can generate many USD time samples, rather than just a single time sample at the current time. This can be equivalent to having a Cache LOP following this node, but it will evaluate much faster, and does not cache data from any other nodes. This allows animated data to be authored to USD without introducing a node time dependency which would then cause all following nodes to also be time dependent. This can vastly improve playback performance of some LOP Networks.

In all sampling modes, if a parameter on this node does not vary with time, and does not rely on other time sampled data from the stage, only a single default value will be generated in USD for the corresponding attribute. USD time samples are only generated for parameters that may vary over time.

Sample Current Frame

A single time sample will be generated for the current time.

Sample Frame Range If Input Is Not Time Dependent

If the input to this node is time dependent, this node behaves as if it is in Sample current frame mode. Otherwise it behaves as if it is in Sample frame range mode.

Sample Frame Range

The Start/End/Inc parameter is used to generate multiple times at which this node’s parameters are evaluated, and a USD time sample is created for each attribute at each one of these times.

Start/End/Inc

When the Sampling behavior is Sample frame range, this parameter controls the number and spacing of base time samples to be generated by this node. The default values of this parameter are @fstart, @fend, and @finc. These values correspond to the start, end, and step size of the global Houdini animation settings when interacting with Houdini. When using a ROP node to generate a range of frames, these values correspond to the start, end, and increment values specified on the ROP node being executed. This default ensures that a USD file written to disk will contain time samples for exactly the frame range requested by the ROP (regardless of the Houdini animation settings).

Subframe Sampling

For each primary sample generated by this node, these parameters can cause additional samples to be generated around that primary sample time. This is most often used to ensure that accurate data exists at exactly the camera shutter open and close times, as well as at the primary sample time.

Shutter

Controls the method used to specify the shutter open and close times relative to the primary sample times.

Specify Manually

The Shutter Open/Close parameter values provide exact offset values relative to the primary sample time.

Use Camera Prim

The Camera Prim parameter provides the scene graph path of a camera primitive from which the shutter open and close times are extracted to provide the offset values relative to the primary time sample.

Shutter Open/Close

When Shutter is set to Specify Manually, these two offset values are added to the primary sample time to indicate the shutter open and close times. The open time should be less than or equal to zero, and the close time should be greater than or equal to zero.

Camera Prim

When Shutter is set to Use Camera Prim, this is the scene graph path of a camera prim on the input node’s stage. The shutter open and close attribute values are read from this primitive.

Samples

The number of subframe samples to create for each primary sample. These samples are evenly distributed between the shutter open and close times. Note that such an even distribution may or may not create a sample at exactly the primary sample time.

Always Include Frame Sample

When turned on, forces a sample to be created at exactly the primary sample time. If the Samples value, together with the shutter open and close times, already place a sample at the primary sample time, turning on this option has no effect. Otherwise, this option causes an addition sample to be added. This means that the actual number of samples per primary sample may in fact be one more than the number specified in the Samples parameter.

Value Clip Description

Primitive Path

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

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.

Get Geometry From Second Input

Enable this option to generate clip files from the LOP node attached to this node’s second input. The save path of the incoming stage will not be used to control where the clip files will be saved. The Save clip file path parameter controls this.

Turn off this option to load clip files from disk, in which case the connection to the second input is ignored.

Load Clip File Path

When Get geometry from second input is off, this parameter specifies the path to existing clip files on disk.

All Clip Files Have Matching Scene Graph Structure

Turn on this option if all clip files are known to have the same scene graph hierarchy, and the same set of time sampled attributes on those primitives. This allows the node to optimize the generation of the manifest and topology files by only looking at a single clip file instead of processing all of them.

To profit from the new option, a couple of settings are mandatory

  • Turn off Get Geometry From Second Input.

  • Turn on All Clip Files Have Matching Scene Graph Structure.

  • In case you have an USD ROP, set Valid Frame Range to Render Current Frame.

Track Primitive Existence to Set Visibility

LOP networks may generate vastly different scene graphs from one frame to the next. When making a value clip from these scene graphs over a frame range, the set of primitives must be fixed, because primitives not in the topology file will not appear when they show up in a clip file. Enable this option to cause this node to track a list of all primitives at each frame, and automatically author animated visibility attributes for any primitives that are added to or removed from the scene graph over time. This gives the appearance of primitives being added and removed over time. These visibility opinions are added to the toplogy layer.

Only subclasses of UsdGeomImageable primitives will be tracked this way, since these are the only USD primitive types which respect the visibility attribute.

If All Clip Files Have Matching Scene Graph Structure is enabled, we know that no animated visibility opinions are required to handle changes in the scene graph structure, so this parameter is disabled.

Flatten Clip Files

Whether the clip files are from disk or from this node’s second input, it can be useful to flatten the layers of the incoming clip file. This is because clip files read in by a Value Clip in USD are not composed. Reference, sublayer, and payload arcs are not followed. Only the data stored directly in the clip file is read in by the value clip. If this option is enabled, whether the input is from the second input or files on disk, the flattened USD file will be written to disk using the path set in the Save clip file path parameter.

Remove Unused Data From Flattened Clip Files

If the clip files are being flattened by this node, turning on this option causes all non-animated data (which will be stored in the topology file) to be stripped out of the clip file before it is saved to disk. This is an automated process which can save considerable disk space.

Save Clip File Path

If the clip files are being written to disk by this LOP (either because the clip data is coming from the second input, or because the option to flatten the clip files is turned on), this parameter controls the path on disk where the clip files will be written. As with all USD data authored by LOPs, these cilp files will only be written to disk when the whole scene is saved out with a USD ROP or USD Render ROP.

This parameter can be time varying, in which case a separate clip file will be written for each frame, or it can be constant, in which case the data from all frames will be written to a single clip file. Either approach is valid depending on the size of your clip data.

Manifest File Save Path

The file path where the automatically generated manifest file will be saved. This file specifies exactly which primitives and values should be brought in from the value clip files. The contents of this file are generated automatically from the set of time sampled attributes ni the clip files.

Topology File Save Path

The file path where the automatically generated toplogy file will be saved. This file contains all the data from the clip files that is not animated. This topology file is referenced onto the primitive holding the clip metadata.

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.

Start Frame

The first frame at which the value clip resolution should start. Prior to this frame no clip metadata is provided, which will cause USD to hold the values stored in the first clip file until the start frame is reached.

End Frame

The last frame at which the value resolution will occur. This parameter has no effect if it is not enabled, in which case clip metadata is generated for every frame for which this node is cooked. Otherwise the clip metadata will simply stop at the specified frame (and the values from the last clip file will hold forever), or the earlier clip files will be reused if Loop frames is enabled.

Loop Frames

To create a repeating loop of clips, set an End frame and enable this option. For frames after the end frame, the clip files will be reused to author the clip metadata, returning to the start frame each time the end frame is reached. Clip metadata will be authored at every frame on which this node is cooked.

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.