Houdini 20.5 Solaris and Karma Karma User Guide

Karma User Guide Geometry

On this page

Previous Getting Started

Overview

Karma can render all of the geometry primitives defined by USD. One departure USD makes from traditional workflows is that many geometry properties are not renderer-specific, but are part of the schema defining the primitive. For example, subdivision surfaces are a built-in property of the Mesh schema.

Note

Karma’s log messages or documentation sometimes refer to objects; these are primitives transmitted from USD via Hydra, to Karma.

Properties are often shared between primitive types, as their schema definitions inherit from one another. For example, meshes, curves, and points are “points-based”, and have the points attribute.

Creating prims and setting properties

The three most common nodes for controlling render properties on geometry are:

Edit Properties

This node makes it convenient to edit primitives. There are several tools based on this node for creating or editing specific primitive types (Mesh Edit, BasisCurves Edit, and so on)

Render Geometry Settings

This node provides renderer-specific attributes and settings. Any third-party renderer can add their own spare parameters to this node, alongside Karma’s properties.

Nearly all properties can be applied to any primitive type. Geometry and Light properties are specific to those prim types.

Configure Primitive

This node lets you set the visibility and activation of primitives.

When lighting and rendering in Solaris, you might need to create or modify geometry primitives or attributes. This document highlights some prim-specific nodes, but you can also use these general nodes:

SOP Create LOP

Creates new geometry from SOP networks. SOP primitives are converted into their USD equivalents. See Importing SOP geometry into USD for more information.

SOP Modify LOP

Edits geometry using SOPs. Useful for in-line modifications when rendering with Karma and Solaris.

Attribute Wrangle LOP

Alters geometry attributes using VEX snippets. Use the Run on Elements of Array Attributes parameter to run the snippet on each point of a mesh.

Note

Advanced USD pipelines may have custom Prim Adapters, which should work with Karma, as long as your plugin paths are configured properly.

Basic primitive shapes

Tip

While these primitives behave like parametric solid shapes, what USD sends through Hydra to Karma is a polygonal mesh. This is why rendered spheres often don’t look perfectly round, and pinched poles are visible at certain angles.

Capsule LOP

Create or edit capsule primitives.

Cone LOP

Create or edit cone primitives.

Cube LOP

Create or edit cube primitives.

Cylinder LOP

Create or edit cylinder primitives.

Sphere LOP

Create or edit sphere primitives.

Meshes

Polygonal surfaces are represented by Mesh primitives in USD. USD supports both right- and left-hand oriented meshes, though SOP geometry is left-handed by default. Meshes can also have primvars, surface-wide attributes you can use to drive shading graphs.

Mesh LOP

This node’s true power lies in its Edit mode, for changing mesh properties. You can define subdivisions here, or with SOP attributes.

Geometry Subset VOP

Useful for adding geometry subsets to USD meshes. You can also define Subsets in SOPs, using attributes or primitive groups.

Note

Mesh prims do not have to be contiguous, nor are they required to be watertight.

GeomSubsets

GeomSubsets appear in the scene graph tree hierarchy, but are not actually primitives. They represent face sets for material assignments. When pieces of geometry don’t need to independently transform or change visibility, you can consolidate them using GeomSubsets to boost performance.

Subdivs

catmullClark subdiv
catmullClark subdiv, sharp creases
catmullClark subdiv, sharp corners

Meshes become subdivision surfaces when their subdivisionScheme property is set to one of the supported schemes. Any authored normals attribute values are ignored when the subdivision surface is rendered. Karma supports both edge and point creases. The Dicing quality property adjusts how much Karma subdivides the surface.

Scheme

Creases

Description

none

n/a

No subdivision scheme is applied; this is the default for Mesh prims.

catmullClark

Yes

Industry standard subdivision surfaces, good for smoothing quad-dominant meshes.

loop

Yes

Good for smoothing triangulated meshes.

bilinear

No

Subdivides faces into quads, without any smoothing.

Dicing

  • When dicing instances, Karma sets the dicing based on the nearest instance to the camera. Animated camera moves can thus sometimes cause flickering in the dicing, if the instances and camera distances are changing slightly between frames.

  • A separate dicing camera can be specified to control the frustum for dicing.

Curves

Curves in USD are of the type BasisCurves. Each primitive represents potentially many thousands of individual strands. The size of the curveVertexCounts property is number of individual curves on the primitive.

BasisCurves LOP

This node can create BasisCurves primitives, but it’s true value lies in its Edit mode, for changing curve properties

There are two types of BasisCurves: Linear and Cubic. Most of the properties defined by the schema apply to cubic curves.

Tip

If the default Non-periodic linear curves look almost right but you want a smoother curve, use Pinned cubic B-splines. These two flavors of curves should handle the vast majority of cases.

Property

Purpose

Curve Basis

When Curve Type is Cubic, this parameter controls how the curve is interpolated.

  • Bezier - Curve is smoothly interpolated near the vertices, and has strict vertex count requirements.

  • BSpline - Curve is smoothly interpolated near the vertices.

  • CatmullRom - Curve is smoothly interpolated through the vertices.

Wrap

Defines how the ends of each curve gets resolved:

  • Non-Periodic - Curves end without special consideration; cubic curves may end up with shortened curves, depending on the number of CVs

  • Periodic - Loops the curve’s end back onto the beginning

  • Pinned - Karma will insert vertices as needed, to ensure cubic curves are valid, and that they start/end as they were modeled; use this for close-up hair such as eye-lashes

Different curve types require different numbers of vertices for a strand to be valid. Incorrect topologies can cause curves to disappear, or look strange, when rendered with Karma.

Curve

Vertex Count (N)

Non-Periodic Linear

N>=2

Periodic Linear

N>=3

Periodic Bezier

N*3

Non-Periodic Bezier

4+3*N

All Other Cubics

N>=4

Karma renders curves as rounded tubes by default, but can also render ribbons and oriented ribbons. You can also override the curve basis at render time.

Rounded Curves
Oriented Ribbons
Flat Ribbons

Tip

Scenes with lots of hair/fur tend to resolve better with Path Traced convergence, and with Karma XPU. Relying extensively on secondary samples to clean up noisy fur gets very expensive in these situations.

Karma renders curves as rounded tubes by default, but can also render them as ribbons and oriented ribbons (using normals to orient the curves). The curve basis can also be overridden at render time.

Karma can provide a nice texture space for curves. Flat and oriented ribbons get textures automatically. Adding normals to rounded curves lets karma compute textures around the curve.

UVs on Rounded Curves
UVs on Oriented Ribbons
UVs on Flat Ribbons

Particles

Sphere Points
Disc Points
Oriented Disc Points

Particles or point clouds are represented as a Points prim in USD. Karma renders points as sphere by default, but you can change their appearance in Render Geometry Settings. Points-based primitives can have different point counts across time, as particles are born and die. The existence of the Points prim must be consistent across time in the stage, but within the prim itself, individual points can vary.

Note

In past releases, Karma provided some basic procedurals for instancing geometry onto points. These have been deprecated, because they current don’t provide performance/memory benefits compared to Point Instancers.

Points LOP

Create or edit points primitives.

Instances

Original OpenUSD definition

USD provides instancing in two forms:

  • Native instancing is a property that tells USD to generate a single "prototype primitive” in order to efficiently represent many copies of an asset.

  • Point Instancer is a special schema that allows for representing vast copies of geometry efficiently.

Both types of instances support transforming the top-level of each primitive and they also allow you to author primvars (on the native instance individually, or on the point instancer primitive) to set per-instance material properties. Native instances are actual transforms on each instance, and each instance is represented in the scene as unique namespaces. They are easily promoted from instances to hero (you just turn their “instanceable” property off).

Point Instancers are fast, but the transforms of all instances are set as array attributes on the point instancer primitive. Promotion involves mutating the scene. Point instances should be preferred mostly based on the number of instances that will exist.

Instancer LOP

Creates Point Instancer or native instances using SOP points.

Volumes

USD volumes are represented by multiple primitives, and the volume data is not actually stored in USD.

Volume

Represent the 'whole' volume, including materials.

Houdini Field Asset

Houdini volume primitives from SOPs or .bgeo files.

OpenVDB Field Asset

VDB volume primitives from SOPs or .vdb files.

Field3D Asset

Field3D volume primitives stored in .f3d files.

The Volume primitive represents the volume as a whole, while field primitives represent individual grids, such as density or temperature. As children of their parent volume, Field primitives inherit the volume prim’s transform, but can also transformed individually.

Houdini has nodes for creating the volume-field combination:

Volume LOP

Create or volume and field primitives from files on disk.

SOP Import LOP

Create volumes of VDB or Houdini field primitives from SOPs.

By default Karma expects the density field, in order to sample a volume. You change this using the Volume Sampling Field property (see below).

Uniform Volumes

You can render Surface geometry as a uniform volume using these render properties. You can control the color using displayColor. You can also bind a basic volume shader to uniform volumes.

Xforms and scopes

Karma does not directly render xforms or scopes. In fact, Hydra does not send them to Karma. Karma only receives the gprims that will actually be drawn/rendered.

Xform LOP

Create or edit xform primitives.

Scope LOP

Create or edit scope primitives.

Scopes and xforms can help organize the scene graph, and can hold render properties or other primvars. Because xforms can be transformed, they can also be used as Coordinate Systems. You can then use them in shaders to transform data from one space to another.

Render Visibility

Rendering primitives in Karma respects the visibility attribute found on nearly all USD prims. In addition to visibility, Karma-specific properties exist to determine the prim’s render visibility.

Render Visibility: * (default)
Render Visibility: primary
Render Visibility: primary|shadow
Render Visibility: -primary

How to

To...Do this

Edit a primitive from the Viewer or Scene Graph Tree

  1. Select a primitive in the Viewer or Scene Graph Tree

  2. on the primitive in the Viewer or Scene Graph Tree, go to Edit Primitive ▸ New node to edit primitive…

  3. This will create an Edit node that corresponds to the primitive type selected.

Tip

Instead of selecting New node… you can choose Jump to node… and re-edit the Houdini node that either created or edited the primitive in this scene.

Edit a specific property from the Scene Graph Details pane

  1. Select a primitive in the Viewer or Scene Graph Tree

  2. In the Scene Graph Details panel, on the property or properties, and click Edit Property.

  3. This will create an Edit Properties in your node graph, configured to edit the selected properties on your primitive.

Edit many primitives via their inherited class

Multiple primitives which inherit from a single class prim can be edited at the same time. Assets made using the Component Builder inherit from a class prim by default.

  1. Select a primitive in the Viewer or Scene Graph Tree

  2. on the primitive in the Viewer or Scene Graph Tree, go to Edit Primitive ▸ New node to edit primitive…

    This option only shows up if the prim already has an inherits setup.

  3. This will create an Edit node, setup to edit the primitive via the inherited class primitive.

Next Lights

Karma User Guide

Appendices