Get to know the setup and requirements for Volume Shader in Redshift for Cinema 4D

In the series of volumes in Reshift for Cinema 4D, we have explored Volume Object, Volumetric Scattering and Fog. Today, iRender will get to know the final topic which is Volume Shader in Redshift. In the first part, let’s see how to set up and the requirements for the scene, as well as volumetric multiple scattering.

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. OpenVDB organizes its voxel data into grids, which are usually referred to as channels. Redshift also uses the channel terminology when referencing a VDB grid in a volume shader.

Unlike your average material, the volume shader in Redshift requires more careful consideration than you think. Make sure you meet all the following prerequisites before any volume rendering can even occur:

• • • • Set at least one channel from the volume objectto drive a volume’s scattering or emission.
      • Have at least one light in the scenewith volume contribution set to a value greater than 0 (by default lights have a volume contribution of 1).
      • Connect the RS Volumenode 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.

* Note: You must store volume objects on the GPU’s memory in their entirety and they are not handled by Redshift’s out-of-core system. Therefore, if a scene uses too many different volume objects or if these volume objects are of a particularly high resolution and do not fit on the GPU’s memory, rendering will be aborted with an out-of-VRAM error message.

Cinema 4D 2023.1 includes a Pyro Volume material preset that creates a volume shader with the default pyro ‘density’ and ‘temperature’ grids predefined for the scatter and emissive channels. This allows you to quickly get up and rendering with Cinema 4D pyro simulations and Redshift.

You can come to Create > Redshift > Materials > Pyro Volume to create a new Redshift ‘Pyro Volume’ material. Then you can apply attributes like Scatter channel, Emission channel to the Pyro object in your scene. And it’s now ready for rendering in Redshift.

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, you can control it with the Volume Trace Depth parameter which is in the Render Settings. When the volume trace depth is increased, it allows a light ray to scatter and bounce around more often inside a volume. This leads to brighter and more realistic volume renders, but 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.

Asset Courtesy of Walt Disney Animation
CC BY-SA 3.0 License

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, when you compare trace depth 1 to 11 of the emissive fire, it looks more obvious. The emissive fire is able to travel farther and illuminate more of the smoke from the inside, 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 in the next article, 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.

You can download the demo files (emissive fire and cloud multiscatter) from Maxon.net.

The VDB file: Reshift demo volume VDB

The cloud multiscatter file created by Saul Espinosa: Redshift Cloud Multiscatter Demo

Download Contents:

• • • • Scene files for 3ds Max, Blender, Cinema 4D, Houdini, and Maya
      • HDRI dome light texture by Saul Espinosa
      • Example PNG render

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Source and images: help.maxon.net