Table Of Contents

• Overview
• Shader Graph Setup and Scene Requirements
• Pyro Rendering
• Volumetric Multiple Scattering
• Multiscattering Demo Scene
• Parameters
• Scatter
• Absorption
• Emission
• Advanced

Overview

The Redshift Volume shader is a physically based shader designed to render heterogeneous volume objects like clouds, smoke, fire, and explosions. The shading components of a volume shader are driven by grids of voxel data contained inside a volume object, like an OpenVDB file. Volume grids may also be referred to as channels, particularly in a shading context, and Redshift uses the channel terminology when referencing a VDB grid in a volume shader.

The volume shader is divided into three shading components: Scatter, Absorption and Emission. For a loose approximation of volume shading as compared to object shading you might think of Scatter as similar to "diffuse" shading, Absorption like volume "transparency," and Emission as "incandescence" or "self-illumination." Therefore, if you want to make your volume brighter or darker, you should adjust the Scatter parameters. If you want to make your volume appear more solid or dense, you should adjust the Absorption parameters. Finally, if your volume represents a self-illuminating effect like fire and want to make it brighter or dimmer, you should adjust the Emission parameters.

 

Shader Graph Setup and Scene Requirements

The volume shader requires more careful consideration than your average material. Before any volume rendering can even occur the following prerequisites must be met: 

• At least one channel must be set from the volume object to drive a volume's scattering or emission.

• At least one light must be present in the scene with volume contribution set to a value greater than 0 (by default lights have a volume contribution of 1).

• The RS Volume node must be connected to the 'Volume' input of an Output node.

  • This should occur automatically with the creation of a new volume shader but it's an important distinction to be aware of if you are setting up a volume shader from scratch or troubleshooting a volume that is not rendering.

 

 

Volumetric Multiple Scattering

The look and shape of a volume is heavily dependent on how it interacts with light. When a light photon enters a cloud it makes contact with the different molecules that make up its composition and then scatter around inside the medium. Clouds are mostly made up of water and ice molecules which results in a lot of scattered light, thick cumulus clouds can even reflect up to 90% of incoming light.

Volumetric multiple scattering simulates this phenomenon to create much more realistic volume renders. In Redshift, this is controlled by the Volume Trace Depth parameter which is found in the Render Settings. Increasing the volume trace depth allows a light ray to scatter and bounce around more often inside a volume, this leads to brighter and more realistic volume renders at the expense of longer render times. The impact multiple scattering has on the look of a volume depends on the volume shader parameters.

 

In the example images below, you can see how increasing the volume depth significantly changes the look of a cloud. Clouds have a heavily forward scattering (positive anisotropy) nature which results in more light being driven deeper into the volume rather than reflecting back out, because of this, each successive volume bounce carries more light energy than a volume that is more isotropic which scatters light more evenly in all directions.

 

Smoke, by comparison, tends to scatter far fewer times as it is more isotropic (anisotropy closer to 0) and therefore sees diminishing returns as volume trace depth is increased. The difference between a trace depth of 11 and 31 is minimal with the demo example compared to the cloud images above.

 

However, the emissive fire is able to travel farther and illuminate more of the smoke from the inside when comparing trace depth 1 to 11 as seen below where the only source of light is the volume's own emission.

 

It is important to balance volume trace depth with the rest of the Volume Shader parameters covered in the section below as increasing the trace depth can have an exponential impact on render times. For the most realistic results higher volume trace depths will be required, but if faster render times are more important try adjusting the scatter / absorption coefficient ratio and decreasing the shadow density scale first and then increase the volume trace depth slowly as needed.

 

Multiscattering Demo Scene

A demo scene created by Saul Espinosa that uses the Disney cloud VDB is provided below as a starting point for better understanding the effects of multiple scattering and other volume shader parameters. The final look of the scene requires the photographic exposure post effect to match the example image.

Redshift Cloud Multiscatter Demo.zip (65 MB)

 

Parameters

Scatter

Channel

The scatter channel drives the overall look of a volume objects scattering contribution, this is the primary shading component for clouds and smoke. A VDB grid must be specified here by typing its name into the channel field before any scatter rendering can take place. The most common name for a scatter channel is 'density.'

If a volume object is intended to render only an emissive type effect like a flame with no smoke then emission should be used by itself and the scatter channel can be left blank.

 

Scatter Coefficient

Increasing the scatter coefficient makes the volume brighter by increasing the strength of the scattered light when it hits a volume object. Decreasing this value reduces the strength of the scattered light and a scatter coefficient of 0 will result in a volume that is completely black even if in direct light.

 

Scattering vs Absorption

Scattering works in combination with absorption as they are both controlled by the same volume grid being used to drive the scatter channel. To preserve energy conservation adjustments made to the scatter coefficient should also be made to the absorption coefficient, which controls how much light is absorbed by the volume, in equal amounts.

For example, usually increasing the scatter coefficient makes a volume appear brighter but if you also increase the absorption coefficient by the same amount the approximate intensity of the volume will be preserved while increasing the perceived density of the volume. In the example images below note how the overall brightness of the volume hardly changes despite raising the scatter coefficient since the absorption coefficient is raised the same amount each time. Instead, the apparent density is increased, make note of the changes in particular to the thin areas of the volume in the bottom left of the images below.

 

Ratios weighted more heavily towards the scatter coefficient result in a brighter more airy volume compared to a darker more dense volume when weighted towards absorption coefficient. The larger the difference between the scatter coefficient and the absorption coefficient the more dramatic the effect.

 

Tint

Tint controls the overall color of the volume, by default scatter tint is left white resulting in a grey smokey color.

Scatter Remap Ramp

The Scatter Ramp color gradient allows you to color remap the scatter channel. Low density values are colored by the left side of the gradient, while high density values are colored by the right side of the gradient. By default a black and white gradient drives the scattering color but multicolored gradients can be used for more artistic effects as depicted below. Bright colors result in more scattering and dark colors result in less scattering.

Color Channel

Instead of setting your own Tint or Scatter Ramp, an OpenVDB grid can be specified here for coloring the scatter component of the shader. This is useful when the coloration of a volume has already been determined.

 

 

Absorption

Redshift computes Absorption using the same OpenVDB grid used for Scatter channel.

Absorption Coefficient

Increasing the Absorption Coefficient makes the volume more opaque as more light gets absorbed by the volume. The more opaque the volume the less light will travel through it, high absorption coefficient values generally result in darker volume renders, particularly when the absorption coefficient is higher than the scatter coefficient, for more information please see the Scattering vs Absorption section.

Tint

Use this to adjust the absorption color of the volume. Using darker colors results in reduced light absorption and brighter volumes, this is why the red and blue examples below look brighter compared to the white default.

 

Absorption Remap Ramp

The Absorption Ramp color gradient allows you to color remap the scatter channel. Low density values are colored by the left side of the gradient, while high density values are colored by the right side of the gradient. By default a black and white gradient drives the scattering color but multicolored gradients can be used for more artistic effects as depicted below.

Color Channel

Instead of setting your own Tint or Scatter Ramp, an OpenVDB grid can be specified here for coloring the absorption component of the shader. This is useful when the coloration of a volume has already been determined.

 

Emission

The emission component is used when creating effects such as fire, explosions, or any time you need a volume to emit light. In the images below all examples use the Blackbody emission mode unless otherwise specified.

Channel

The emission channel drives the overall look of a volume objects emission contribution, this is a key shading component for fires and explosions. A VDB grid must be specified here by typing its name into the channel field before any emission rendering can take place. The most common name for a scatter channel is 'temperature.'

 

Tint

Use this to apply an overall color to the emission. Using darker colors will result in a reduction of the emission intensity.

Scale

Use Scale as a multiplier for the emission Channel to increase or decrease emission intensity. A value of 0 results in no emission.

 

Emission Remap Ramp

The Emission Ramp color gradient allows you to color remap the emission channel when the emission mode is set to 'Color Ramp'. Low emission values are colored by the left side of the gradient, while high emission values are colored by the right side of the gradient. By default a black and white gradient drives the scattering color but multicolored gradients can be used for more artistic effects as depicted below.

When using the Color Ramp mode it's up to the user to adjust the Emission Ramp to 'translate' the temperature channel into the appropriate emissive colors.

With the end result looking something like this:

For more realistic emission colors consider using the Blackbody emission mode instead.

Color Mode

This mode controls the results of the Color Channel in combination with the Emission Remap Ramp and Tint. The following options are available:

• Multiply with ramp: The loaded Color Channel is multiplied by the Emission Remap Ramp and Tint.
• Multiply with temperature: The loaded Color Channel gets multiplied with the Emission Channel values, before it gets used with the Tint. The Emission Remap Ramp has no effect on the emission color.
• Replace: The Emission Remap Ramp has no effect on the emission Color Channel.

Color Channel

Instead of setting your own Tint or Scatter Ramp, an OpenVDB grid can be specified here for coloring the emission component of the shader. This is useful when the coloration of a volume has already been determined.

 

 

Advanced

This section offers additional settings to tune the overall Shadow, Density and Emission evaluation

Shadow Density Scale

This parameter controls how dense a volume looks without affecting its overall shape the way making adjustments to scatter and absorption would. In effect it is a "trick" that can help emulate multiple scattering, i.e. the effect of light bouncing around a volume with a less severe impact to render times than actual multiple scattering by increasing the volume trace depth. An increased multiple scattering look may be achieved by setting the shadow density scale to a value below 1.0 while higher values will make a volume look more dense.

Range Remapping for Density and Emission

The values stored in OpenVDB channels can vary wildly depending on the 3d app that was used to author them. This can be true for both density and emission channels.
The range remapping controls allow the adjustment of these values before Redshift uses them. The Old Min/Old Max range is remapped to the New Min/New Max range.

 

Density Remap

If your volume looks blocky it can sometimes be helpful to remap the density ranges to allow for a more gradual falloff in volume shading, frequently this can be achieved by increasing the Old Max value.

 

Emission Remap

A temperature channel might contains values that range between 0 and 100 but the Emission Ramp only accepts inputs ranging between 0 and 1. In this case, the user should use old min=0, old max=100 and leave new min and new max at their default 0 and 1 respectively. In essence, this would remap or 'squash' the 0-100 range into a 0-1 range.