Houdini 20.5 Nodes LOP nodes

Simulation Proxy

Generates low-poly collison geometry suitable for physical simulation and creates a proxy relationship to the original model.

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Since 18.0

Overview

Often, doing physics simulation (such as the Edit tool’s “Use Physics” mode, or the Drop tool) with highly-detailed models is slow. To make simulation faster, you can simulate a lower-resolution proxy of the original model, which has the same basic shape but eliminates unnecessary detail.

There are two basic ways of using this node to create proxy geometry:

  • This node can automatically generate proxy geometry from an original model. The node has multiple methods for simplifying geometry, and you can use multiple methods at the same time.

  • You can dive inside this node and use the SOP subnet inside to simply build the proxy geometry manually.

    The input to the subnet is the result of any automated simplifications you applied on the node above, so you can combine automatic reduction and hand-editing.

When you create the proxy geometry, the node automatically sets up a USD proxy relationship between the proxy and the original model. When USD-aware tools such as Edit and Drop see this relationship, they know to simulate the proxy geometry.

Tips

  • While you can make the proxy geometry visible, it overlaps with the original model. Unfortunately, you can’t hide the original without also hiding the proxy because of how USD visibility is inherited.

    It’s inconvenient, but you can see the collison proxy alone by first hiding the original in the USD scene graph tree, and then diving into the node. While you're in the SOP network, the SOP viewer shows the output of the SOP network, no matter what’s visible in USD.

  • In general, you want to reduce to a proxy that reasonably matches the shape of the original, with the fewest polygons possible. For simulation purposes, the “quality” of the polygons doesn’t really matter (for example, it’s fine if the proxy has long skinny polygons).

  • If you enable multiple simplification methods, they are applied in the order they appear in the parameters.

  • VDB Resurface is excellent at simplifying models that are bumpy or full of holes (it’s a way of “blurring” out fine detail in the original model).

  • If you are using the Convex Hull method with Decomposition, it can produce extraneous polygons. You can use the “Final Polygon Reduction” step to clean them up, however don’t set the “percent to keep” too low. Poly reduction can change the shape enough that it destroys the convexity, or introduces other problems such as zero-volume enclosures.

  • If you use the SOP network inside this node to generate polygonal proxy geometry, all faces must have a SOP primitive string attribute named usdprimpath where the value is the prim path of the original model’s polygon mesh (for example, /Geometry/basket/mesh_0).

Parameters

Primitives

The primitive(s) the node should operate on. You can drag primitives from the scene graph tree pane into this textbox to add their paths, or click the Reselect button beside the text box to select the primitives in the viewer, or ⌃ Ctrl-click the Reselect button to choose prims from a pop-up tree window. You can also use primitive patterns for advanced matching, including matching all prims in a collection (using /path/to/prim.collection:‹name›).

Note

This parameter needs to be exposed in order for the transform handles to be context-aware. Without it, the handles may not be positioned properly.

Exclude Primitives with Existing Proxy

If a prim selected in Primitives already has proxy geometry, ignore it.

Display Proxy

Display the generated proxy geometry in the viewport. See Tips above.

File Cache

These parameters set up a proxy geometry cache on disk. This makes cooking the node faster, since it doesn’t have to recompute the proxy unless the input geometry changes. (Note that once you write the scene to USD and use it in a Houdini scene, the USD file itself is a file cache containing the proxy geometry.)

Load from Disk

Load the proxy geometry from the file specified below.

Geometry File

File path to save the generated geometry to. Use .bgeo.sc as the file extension.

Save to Disk

Click to regenerate the proxy geometry and save it to the file path above.

Polygon Reduction

Enable

Uses PolyReduce to produce a proxy with fewer polygons while keeping the same general shape.

Percent to Keep

The percentage of polygons to keep. For example, if the original model has 1000 polygons, setting this to 10 will generate a proxy with 100 polygons.

VDB Resurface

Enable

Converts geometry to a VDB surface volume and then back to polygons. This is a good method for getting clean/simple surfaces out of very complex/bumpy geometries like woven fabrics or baskets (it essentially “blurs” out small details).

Voxel Size

The voxel size of the VDB surface, in world units. The larger the voxel size, the chunkier the proxy will be.

Dilation

The distance in voxels to dilate & erode the VDB surface by. You can use this to close any gaps and small holes in the geometry for a cleaner, smoother surface.

Post Percent to Keep

This node automatically uses PolyReduce on the surface generated from the VDB volume, since it tends to have many polygons, even though the overall shape is simpler. This is the percentage of polygons to keep. For example, if the surface has 10000 polygons, setting this to 5 will generate a proxy with 500 polygons.

Convex Hull

Enable

Turn meshes into convex hulls, which are ideally suited for physics simulation.

Granularity

The Bullet RBB solver is much more efficient with purely convex collision geometry than with complex/concave geometry. If the model can be well represented by a “skin” stretched around its outside, choose “Convex Hull Per Primitive”. If the original model has space between disconnected pieces, choose “Convex Hull per Connected Piece”. If your model is concave, you can try using “Decompose Each Connected Piece” to see if the computer can approximate the original shape using multiple convex shapes (this can benefit from using VDB Resurface to blur out details first).

Convex Hull Per Primitive

Create a single convex hull for each mesh primitive. Use this for primitives that only need to be represensted roughly by their outer shape. This is great for creating simulation friendly geometry from even very compex primitives. For example turning a detailed flower pot into a single convex hull will remove all the flower & leaf details while still keeping an accurate representation of the bottom of the pot, which might be all you're interested in when putting it on a table or shelf.

Convex Hull Per Connected Piece

Generates individual convex hulls for each connected piece of geometry within each prim, potentially resulting in a much more detailed representation.

Decompose Each Connected Piece

Decompose each connected piece into approximate convex segments. Lowering the value of the Max Concavity parameter will create more segments and have a better fit to the original geometry.

Max Concavity

When Granularity is “Decompose Each Connected Piece”, the maximum distance from a point on the geometry to its closest convex hull edge (in world units). Lowering this may give you a better approximation of a concave original, but increases complexity of the proxy.

Final Polygon Reduction

Enable

Uses PolyReduce on the results any enabled steps above. This can be useful for a final cleanup for “Convex Hull”, which can sometimes leave behind useless co-planar polygons.

Post Percent to Keep

The percentage of polygons to keep.

See also

LOP nodes

  • Add Variant

    Adds one or more variants to a variant set on a primitive. This node creates the primitive if it doesn’t exist.

  • Additional Render Vars

    Create multiple render vars.

  • Asset Reference

    Reference, Transform, and select variants of a USD Asset.

  • Assign Material

    Assigns a material to one or more USD primitives. You can use also programmatically assign materials using VEX, programmatically override material settings for each assignment, and programmatically assign materials to geometry subsets.

  • Assign Prototypes

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

  • Attribute VOP

    Create/edit USD attribute values using a VOP network.

  • Attribute Wrangle

    Create/edit USD primitive attributes using a VEX snippet.

  • Auto Select LOD

    Automatically selects a level-of-detail variant based on the primitive’s distance from the camera.

  • Background Plate

    Sets up hold-out or matte objects that leave holes in the scene through which the background is visible. These prims still take shadows and contribute to reflections as if they were the background.

  • Bake Skinning

    Bakes animation driven by a UsdSkel into transforms and point positions.

  • Basis Curves

    Creates or edits a basis curves shape primitive.

  • Begin Context Options Block

    This node begins a block of LOP nodes, within which certain context options have certain values.

  • Blend

    Partially applies edits to a layer’s attributes based on a fractional weight.

  • Blend Constraint

    Blends transforms according to a list of weights specified as parameters.

  • Cache

    Caches the results of cooking the network at different times, increasing playback speed.

  • Camera

    Adds a USD camera to the scene.

  • Capsule

    Creates or edits a capsule (tube with hemispherical ends) shape primitive.

  • Collection

    Creates/edits collections using primitive patterns.

  • Component Geometry

    Geometry container or import source, in a network created by the Component Builder tool.

  • Component Geometry Variants

    Sets up geometry variants, in a network created by the Component Builder tool.

  • Component Material

    Assigns materials to geometry in a network created by the Component Builder tool.

  • Component Output

    Assembles the final Component prim, in a network created by the Component Builder tool.

  • Cone

    Creates or edits a cone shape primitive.

  • Configure Layer

    Edits metadata on a layer.

  • Configure Primitives

    Edits various metadata on one or more primitives.

  • Configure Properties

    Configures metadata on properties (relationships and attributes).

  • Configure Stage

    Configures metadata for how to load layers into the stage and asset resolution.

  • Coordinate System

    Define named coordinate systems used in shaders.

  • Copy Property

    Copy properties from one primitive to another, or renames properties on a primitive.

  • Create LOD

    Uses the PolyReduce SOP to automatically generate multiple levels of detail from a high-res model, and stores them as USD variants.

  • Cube

    Creates or edits a cube shape primitive.

  • Cylinder

    Creates or edits a cylinder shape primitive.

  • Distant Light

    Creates or edits a USD Distant Light, representing a far-off light source such as the sun. Adds some useful Karma-specific attributes.

  • Dome Light

    Creates or edits a USD Dome Light prim. A dome light emits light inward, simulating light coming from the sky/environment surrounding the scene.

  • 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.