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Contents 1 Volumetric Rendering in Houdini using Mantra 1.1 Uniform Density 1.1.1 How To... 1.2 Varying Density 1.2.1 RayMarching 1.2.2 Tutorials 1.3 Dedicated Volume Renderers 1.3.1 Commercial Renderers 1.3.2 Proprietary Renderers

Volumetric Rendering in Houdini using Mantra

Volumetric Rendering is the technique of calculating the resultant lighting and opacity in a 3 dimension region filled with uniform or varying density.

Uniform Density

Rendering simple uniform or a rolloff of density based on height can be rendered in Mantra by applying a Linear Fog or Layered Fog Shader to your scene. These are quick and easy ways to get the sense of distance in a scene but they will not support the lighting of the scene; they are artistically applied. Since they don't respond to lighting, neither do they render shadows. They simply apply a colour and a density based on the depth of the sample.

How To...

• In the SHOPs, create a VEXUniformFogSHOP
• In the Objects, create an AtmosphereObject and point to the new shader in the SHOP Volume Shader field.
• Now you may render your scene using Mantra and the shader parameters will affect the colour and opacity of your fog.

Please note that you can create multiple Atmosphere Objects in one scene.

Varying Density

A common technique of rendering lighting in clouds of varying density is to employ a method called RayMarching.

RayMarching

In this technique, small steps are taken from the viewers position and "marched" along the viewing direction through the scene. The lighting is calculated at each step based on the density of the volume at that location in space. This density can be defined procedurally or accessed from a predefined (stored) density. Houdini reads and writes a volumetric format called i3d - or Image3D. There is a VEX context in Houdini for defining the density values in an i3d file as voxels. Often times the basic density is precomputed into i3d files and the higher frequency (finer) detail can be added on at render time.

In the ShadingContext, you'll find a VEX volume shader called VEXLitFogSHOP which allows you to render i3d files.

Tutorials

• od[force] Image3D Tutorial
  • http://www.odforce.net/tips/i3d_intro_page1.php

Dedicated Volume Renderers

Here is a list of known volume renderers that can be be used to render from Houdini.

Commercial Renderers

Jig by Steamboat Software. http://www.steamboat-software.com

• Jig is a volumetric and a general renderer that allows bucket rendering of smoke volumes. It support hair rendering using voxels too. It will also support various surfaces types.

5x5 Voxel Renderer by Myrtle Software. http://http://www.myrtlesoftware.com/

• From the web site - "Myrtle Software's 5x5 Voxel Renderer is the world's first commercial voxel renderer capable of producing all the intermediate images required for production quality visual effects."

Proprietary Renderers

Storm by Digital Domain. http://www.d2.com

• Storm is a SOP and a standalone renderer that allows the modeling, compositing and rendering of high-resolution voxel buffers. It earned a Technical Achievement Academy Award in 2005. It also goes by its original name, VoxelBitch and is the successor to CloudBastard.