Deborah R. Fowler

Houdini Lighting — Solaris/Karma — Render Basics

Posted Dec 2 2019 Updated June 22 2023 Updated April 11 2026

In 2023 I was using Karma to render (cpu) on one of my projects (Candy Corn Scissor Stretches) and it was taking 51 minutes a frame in Mantra vs 15 in Karma-cpu). I was not getting the results I wanted out of Redshift on this particular piece, so I thought I'd give Karma another go. Since I had set it up in mantra I wanted a quick solution and Karma-xpu did not respect the rounding of corners in the shader, but cpu did.

Quick note — Karma motion blur and depth of field are on by default so you may want to turn them off.
Render Basics - Karma
Assigning a Shader
Overview

To assign a shader to your object use can use the assignmaterial node (also called a lop) or the materiallibrary lop.

This assumes you are in the lopnet (or stage) context and you have set up your scene for lighting using USD. You will also need an available material.

materiallibrary

materialibrary - to create materials, dive inside this node - you can copy them from the /mat context or tab create, for example principled or classic.

Option 1: Click Auto-fill Materials. Drag your mesh to the geometry path.
Option 2: Use the assignmaterial by dragging the mesh you want to assign to the primitive and dragging the material from the Scene Graph Tree.

Shader Override: Similar to Mantra, you can override properties of a shader by creating a vexpression and binding it to the parameter. You must promote the parameter first. This works with both the principled shader (use basecolor) and the classic shader (use diff_color).
Material Assign Material Override Promote Parameter
Shadows
Shadow Color
  • you can use shadow color (this really should be defined by the fill lights, but may in some cases want to add a wee little touch of color - for example  (.01,.007, .01)).
  • Parameter was added as of H12.5 and up and exists in H18
  • This parameter is useful for two things:
    • when the lighting is perfect but the shadow is a bit drab or dingy
    • when you are debugging - crank it up to red for example (less useful now that we can easily  use the lightmixer to disable and solo lights.
Shadow Masks — can do this in the lightlinker. ALT drag the lights into the middle to get the shadow linking properties (it will appear purple rather than gray).
Shadow Color
Lightmixer & Lightlinker
Lightmixer

Lightmixer: allows you to manipulate lights easily, including solo and disable - there is also a sheet tab if you have many many lights (similar to the old data tree)

Lightlinker

There is also a lightlinker
In the figure I have two rules, one enabled to turn the light off of the sphere.

see also H18 Launch Presentation at timestamp 28:02

Lightmixer Lightlinker
Render Nodes — Karma Settings
Viewport Preview

For the preview render in the viewport, you can hit d and it will bring up the options. For quick preview set samples to 2.

Saving a Render

For an actual render - note the karma lop (node). Specify where you want to save the image and then click Save to Disk. This will create your rendered image (a process husk.exe is run)

Karma Render Settings