Volume

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.

Displacement

The Standard Volume supports displacement by using the Position Offset parameter in a Volume Scalar Attribute or Volume Color Attribute node, this allows you to move existing voxels in a grid around the scene.

A basic example node graph depicting displacement and programmable volume shading is pictured below. It uses:

• A Volume Scalar Attribute node to load a density channel and directly drive the Standard Volume's density input. The density node is displaced by a Maxon noise for added detail.

  • The Density Channel built into the Standard Volume is left blank so it cannot mask the displaced density voxels.

• A Volume Scalar Attribute node to load a temperature channel and directly drive the Standard Volume's temperature input. The Output Max is set to 6500 to set the temperature appropriately.

  • The Emission Channel built into the Standard Volume is left blank so it cannot mask the displaced temperature voxels.

• A Maxon noise remapped by a color ramp to create a colorful pattern throughout the volume.

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)