Houdini 18.0 Nodes LOP nodes


Runs a simulation to drop primitives under gravity.

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This node creates a Bullet physics simulation where the selected prims are dynamic rigid bodies under the influence of gravity, and you set other prims act as static colliders that don’t move.

This node shows a time-based simulation that replays in the viewer when you press play. However, this node does not actually create USD animation (it does not write time samples to the prims). The workflow is to find a frame of the animation where you like the positioning of the objects, and "snapshot" those transforms. See how to use the node below.

This can be useful to drob a bunch of objects in a natural-looking pile. However, in general the Use Physics mode of the Edit node is much easier to understand and use.

How to

To...Do this

Drop objects using a physics simulation

  1. Add a Drop node.

  2. Set Active primitives to the prims you want to drop.

  3. Go to frame 1 and click in the playbar to play the simulation. Find a frame where the simulated prims have a position you want to keep.

  4. Click Stash transforms.

    This automatically turns on Use stashed transforms. This switches the node to set the stashed transforms on the selected prims, instead of running a simulation.

(You may want to go back to frame 1 or whatever frame you were working on.)

Re-run the simulation to choose a different frame

  1. Turn off Use stashed transforms.

  2. Go to frame 1 and click in the playbar to play the simulation.

  3. Find the frame where the simulated prims have a position you want to keep, the click Stash transforms.

Tips and notes

  • By default the simulation uses the full prim geometry. For very detailed geometry, this can be slow and inefficient. See the Simulation Proxy node for how to generate low-res collision geometry for prims.

  • You can change physics constants (such as friction between the prims, or increasing gravity) to change the simulation outcome.

  • Increase both gravity and drag to make objects settle faster.



Active 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).

Active Geo Representation

How bullet treats geometry when computing collisions.

Convex Hull

The default. This tends to be fast and stable in most situations. Acts like the objects have a skin wrapped around outside (ignores concave parts).


More realistic, but more expensive to compute. You should use this sparingly, such as for terrain, which cannot be well represented using the methods described below.


Let the Bullet solver try to fit a basic shape to the object.

Create Rigid Body Per

Create a rigid body per USD Group, Component, or Leaf Primitive.

No Traversal means that each primitive specified in the Active Primitives pattern will be turned into a rigid body.

Passive Primitives

Scene graph paths/patterns selecting prims to collide with. These prims don’t move, but the "active" prims will be affected by them.

Passive Geo Representation

Like Active Geo Representation but for the static colliders.

Prune Other Primitives

Deactivates primitives that don’t participate in the simulation. This can speed up interaction and playback while working on the simulation. Once you're finished simulating, turn this off to get the rest of the scene graph tree back.


Use Stashed Transforms

Apply "stashed" transforms to the selected prims, rather than simulating them. See how to use this node above.

Stash Transforms

Take the transforms of the simulated prims, and stash them inside this node. Automatically turns on Use stashed transforms. Click this when you find a simulation frame you want to use as the new positions of the selected prims. See how to use this node above.

Delete Stashed Transforms

Delete any stashed transforms inside this node. Automatically turns off Use stashed transforms so the node starts simulating again. See how to use this node above.

Collision Padding

A padding distance between shapes, which is used by the Bullet engine to improve the reliability and performance of the collision detection. You may need to scale this value depending on the scale of your scene. The actual collision geometry is shrunk by the same amount to prevent the collision shape from growing by this padding amount.


Initially lift the selected primitives by this amount. This moves objects in the opposite direction of gravity.


Initially spaces active primitives by this amount in the opposite direction of gravity.


The gravity vector, defaults roughly to earth gravity, assuming Houdini units are meters.

Linear Drag

A resistance to (drag on) changes in position.

Torque Drag

A resistance to (drag on) changes in rotation.


Higher numbers make objects more "sticky", lower numbers make objects more "slippery".

Reset Simulation

Clears the simulation cache, causing the simulation to recook up the current frame.

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.

  • 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 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • Extract Instances

    Converts 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 Subset VOP

    Creates USD geometry subsets within geometry prims (similar to groups in SOPs) based on evaluating a VEXpression or VOP network.

  • Graft

    Takes scene graph trees from other inputs and attaches them onto branches of the scene graph tree in the first input.

  • Inline USD

    Parses usda code representing a layer and adds it to the layer stack.

  • 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.

  • Layer Break

    Starts a new active sublayer that subsequent nodes will edit.

  • 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

  • Look At Constraint

    Constrains a prim to always point toward a target.

  • Material Library

    Imports Material VOP nodes as USD material prims.

  • 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.

  • Modify Point Instances

    Modify 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 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.

  • 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.

  • 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 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.

  • attribvop