Standard Volume

The Standard Volume shader is a physically based shader designed to render heterogeneous volumes like clouds, smoke, fire, and explosions. It combines an easy to use interface and controls while enabling greater flexibility and artistic control with the introduction of programmable volume shading. The Standard Volume shader accepts external shader inputs allowing for the creation and control of procedural volumes or the modification and displacement of existing volume grids.

The shading components of a volume shader are mainly 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.

Speed and Simplicity

If speed is what you need, the Standard Volume shader supports a special mode when no external nodes are attached making it faster and more optimized. This can be achieved simply by using the built-in Volume Channel and Emission Channel fields to import your volume grids. In this mode, it behaves very similarly to the classic Volume Shader but with more intuitive controls.

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 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.

Multiscattering Demo Scene

An updated demo scene created by Saul Espinosa that uses the Disney cloud VDB and the Standard Volume shader is provided below as a starting point for better understanding the effects of multiple scattering and other volume shader parameters.

Redshift Cloud Multiscatter Demo.zip (65 MB)