Houdini 20.5 Nodes LOP nodes

Karma Sky Atmosphere

Creates or edits a Karma Sky Atmosphere.

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Since 20.5

Overview

Karma Sky Atmosphere describes a volumetric planetary atmosphere for rendering and integrating objects into a plausible and art-directable sky.

Creating vs. editing prims

This node belongs to a class of nodes that create or edit USD prims directly. These nodes operate in Create mode or Edit mode. This is controlled by a Create primitives checkbox or a Create/Edit popup menu. In create mode, the node creates new prims. In edit mode, the node changes the attributes on an existing prim. The Edit mode has two variations. Edit will not modify primitives which have a houdini:editable attribute set to false. Force edit will modify a primitive regardless of the existence or value of this attribute. This attribute can be set on a primitive using the Configure Primitives LOP.

Parameters that correspond to a USD attribute have a pop-up menu to the left that controls how the node authors the attribute.

In addition to that, any connectable USD attributes (i.e., the ones in the inputs: namespace) will have menu items that allow disconnecting them from their sources.

Pop-up menu item

Meaning

Set or Create

Sets the attribute to the given value, whether it previously existed or not.

Set If Exists

Only set the attribute to the given value if it previously existed.

Use this mode to make sure an attribute is only set on primitives of the correct type. For example, only UsdGeomSphere primitives are likely to have a radius attribute.

Block

Makes the attribute appear to not exist, so it takes on its default value. (If the attribute doesn’t already exist on the prim, this does nothing.)

Disconnect Input

Deletes the attribute input connection to its source. Input connections take precedence over attribute values, so disconnecting an input allows the attribute value to take effect.

Do Nothing

Ignore this parameter, don’t create or change the attribute in any way.

Tips

  • Make sure that the scene’s Camera LOP far Clipping Range value is large enough to encompass the entire atmosphere. Otherwise the atmosphere may get clipped and the sky may appear dim.

  • Consider a Light Linker LOP to link the atmosphere primitive against significant light sources (e.g. the Sun). This will avoid wasting computing time on lights that have minimal impact on the sky’s overall appearance.

  • Unlike Houdini’s Dome Light LOP, the atmosphere is not a direct light source: objects won’t reflect the sky and appear dark if bounce limit is set to 0 on its BSDF components.

Note

  • Only one Karma Sky Atmosphere primitive is allowed per scene.

  • While the atmosphere contributes to color values of deep camera map samples, it is not capable of generating deep samples itself. For deep a compositing workflow, you must render each foreground element with atmosphere turned on. Furthermore, the sky as a backdrop has to be rendered out as a flat image and composited separately.

  • Indirect rays from atmospheric samples do not pick up emission (unless it’s turned into geometry light) or reflected light from other objects in the scene.

Parameters

Rayleigh

Rayleigh scattering models light interacting with air molecules to give Earth’s sky its signature blue color at daytime and orange color at sunrise/sunset.

Effect of Rayleigh scattering with change in Sun’s elevation.

Scatter

This parameter is multiplied by Scale and creates a Rayleigh scattering coefficient for each color channel representing the amount of light that is scattered by pure air.

Scale

Multiplier for Rayleigh Scatter.

Distribution

Controls, how fast the air density falls off with increasing altitude. This parameter is measured in kilometers and multiplied by Unit Scale. For example, with the default of 8 km and a Unit Scale of 1000, air density is 37% at 8000 units above sea level.

Distribution uses Nishita’s exponential decay model (referred to as “Scale Height” in literature) for calculating air density.

Mie

Mie scattering/absorption models large aerosols in air such as water or dust particles creating smoggy haze and haloing around light sources such as the Sun.

Without and with Mie scattering.

Scatter

This parameter is multiplied by Scale and creates a Mie scattering coefficient for each color channel representing the amount of light that is scattered by aerosols.

Scatter Scale

Multiplier for Mie Scatter.

Absorption

This parameter is multiplied by Absorption Scale and creates a Mie absorption coefficient for each color channel representing the amount of light that is absorbed by aerosols.

Absorption Scale

Multiplier for Mie Absorption.

Distribution

Distribution uses Nishita’s exponential decay model (referred to as “Scale Height” in literature) for calculating air density.

Phase

Phase ranges from -1 to 1 and covers full back scatter to full forward scatter. Scattering represents the atmosphere’s anisotropy. Aniostropy means that rays don’t propagate evenly in all directions.

Ozone

The ozone layer is an additional absorption layer that contributes to the deep blue tint of the Earth’s sky, particularly near sunrise/sunset.

Without and with ozone absorption.

Absorption

This parameter is multiplied by Scale and creates an ozone absorption coefficient for each color channel representing the amount of light that is scattered by the ozone layer.

Scale

Multiplier for ozone Absorption.

Altitude

The maximum ozone density/absorption is reached at the parameter’s value given in kilometers measured from the Earth’s surface. The value is multiplied by Unit Scale. For example, at an Altitude of 8 km and a Unit Scale of 1000, the final value will be 8000.

Width

This value indicates the total height range in kilometers at which ozone absorbs (sun)light. Absorption reaches its maximum at the Altitude parameter’s value. Note, that Width is multiplied by Unit Scale. For example, when Width is 10 km and Unit Scale is 1000, the final value will be 10000.

Ground

Without and with ground albedo.

Ground Albedo

The ground reflectivity that affects the sky color (when multiple scattering is turned on), as well as its appearance in primary and bounce rays.

Ground Visibility

Controls visibility of the ground depending on different types of rays. For instance, you may not want the ground to be visible in the camera, but still want the sky and indirect bounce rays from underside of clouds (and other geometry) to be affected by the ground color. You can use one of the following expressions

*

Visible to both primary and indirect bounce rays.

primary

Visible to primary rays only.

-primary

Visible to indirect bounce rays only.

Geometry

Planet Center

This parameter is multiplied by Unit Scale and represents the position where the planetary atmosphere is centered around. Planet Center is measured in kilometers.

Sea Level

This parameter is multiplied by Unit Scale and represents the distance from the Planet Center in which the atmosphere starts. Sea Level is measured in kilometers.

Unit Scale

Multiplier to every parameter in Karma Sky Atmosphere that represents position or distance.

Sampling

LPE Tag

Custom label assigned to atmospheric volume for use with light path expression.

Minimum Stepsize

This parameter is multiplied by Unit Scale and represents the minimum distance between samples when marching through atmosphere. Reducing this parameter can help eliminate banding artifacts when modeling a particularly thick atmosphere, but at extra cost.

Maximum Stepsize

This parameter is multiplied by Unit Scale represents the maximum distance between samples when marching through atmosphere. We recommend leaving Maximum Stepsize at 5 to 10 times the value of Minimum Stepsize.

Multi Scatter Limit

Specifies, how often an atmospheric ray can propagate through the atmosphere.

Note

Increasing depth will impact render times. A depth of 1 or 2 is typically enough for the realistic appearance of a clear sky on Earth, with diminishing returns beyond that. A limit of 1 or greater is required if Ground Albedo should have an effect on the color of the sky.

Advanced

Internal Color Space

Scatter and absorption coefficients are interpreted in the OpenColorIO color space specified by this parameter, and transformed into scene_linear (i.e. working space) when computing transmittance. Leave it empty to apply no transformation. This can help in preserving the look if the lookdev for the atmosphere was done in a different color space than the project’s working space (however, due to the nature of RGB rendering, it can never be an exact match).

LUT Resolution

X and Y resolution for the transmittance table. The X axis corresponds to the light angle, and the Y axis corresponds to the viewer’s altitude. With appropriate settings, there is no need to deviate from the default settings. An exception is if there are huge differences between the Distribution or ozone Altitude parameters. In this case it can help to increase the Y resolution to avoid bias.

Ground Offset

Distance between ground level and sea level. Ground is modeled as a sphere with radius calculated as Sea Level - Ground Offset. You can set this parameter to values greater than zero to lower the ground and avoid discontinuity artifacts from objects crossing ground level. As a consequence, you’ll see a vacuum between the atmosphere and the ground which may be noticeable near horizon. We recommend leaving this parameter at 0 and keeping the rest of the scene above Sea Level instead.

Ground Receives Shadows

Allow ground elements to receive shadows (i.e. cast shadow rays).

Render Backside Atmosphere

Allows rendering of the atmosphere on the other side of the planet.

Solid Alpha in IPR

Set alpha to 1 in IPR mode. Turned on by default to avoid misleading appearance due to compositing against light viewport background.

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.