Volume Mediums offers the ability to apply color ramps independently to each of absorption, scattering and emission. The Octane Volume Medium can be found in the Octane Medium category of the Hypershade window.
Rendering volumes using Octane for Maya will require a plug-in in order to render VDB format volumes. This is because Maya does not support the VDB format. The OpenVDB project (http://www.openvdb.org/) offers scripts that can allow for importing VDB files which can then be rendered in Octane for Maya. Other applications such as Octane standalone, Houdini, and Lightwave do support VDB files without the need for plug-ins or scripts.
In order to make use of a ramp, users must have a color specified for the corresponding channel. For example, to use the absorption ramp, users must select a color texture for absorption (Figure 1).
Figure 1: Ramps are required for rendering colored volumes.
This specifies the color texture for absorption
The absorption color ramp that defines the colors range
The absorption ramp takes as input the grid value. In the color gradient, the colors near “0” on the left are used for mapping low grid values to some custom color. Higher grid values are mapped to colors on the right of the color gradient. Bear in mind that less saturated colors will cause there to be less pronounced absorption. Emission and scattering ramps operate in the same way.
This inverts the absorption color so that the absorption channel becomes a transparency channel. This helps visualize the effect of the specified color since a neutral background shining through the medium will appear approximately in that color.
This is the scattering cross section. This channel defines how much light is absorbed over the color range.
The phase function also affects a volume as it would affect a medium node. The Phase function controls the direction of the light as it is scattered in the volume. A value of zero results in light being scattered equally in all directions, positive values result in forward scattering, where the photons continue in roughly the same direction they were going when they entered the surface. Negative values result in backwards scattering where the light moves through the surface in the direction roughly opposite to the angle at which the enter the volume. (Figure 2).
Figure 2: A comparison of volumes rendered with different Phase settings.
This controls the volume emission. For emission, the Medium map can have either a blackbody emission map or a texture emission map.
This is the emission color ramp.
Volume step length
The “volume step length” parameter on the volume map may need to be adjusted depending on the volume. The default value for the step length is 4m. Should the volume be smaller than this, you will likely need to decrease the step length. Please note that decreasing this will reduce the render speed. Increasing this value will cause the ray marching algorithm to take longer steps. Should the step length far exceed the volume’s dimensions, then the ray marching algorithm will take a single step through the whole volume. Most accurate results are obtained when the step length is as small as possible. For simplifying workflow, the volume step length should be set first to an acceptable value (Figure 3).
Figure 3: A comparison of volumes rendered with different Volume Step Length settings.
Also bear in mind that it is important to ensure the volume is not too dense. It is recommended that you reduce the volume step length to an acceptable performance and accuracy level, and then reduce the volume density. Otherwise you may risk rendering a solid object at a high step length, giving deceiving results sometimes.
Volumes may also have their own scattering, emission and absorption. These colors influence the appearance of a volume significantly. The phase function also affects a volume as it would affect a medium map, and modifying the scale value of the volume scales the density values of the volume linearly. This will also potentially increase emission as absorption values are also used as particle density
Volumes are rendered in an unbiased way, and therefore are able to scatter multiple times and cause self-shadowing effects. Should you wish to reduce the maximum scatter events in a volume, reduce the Diffuse Depth in your kernel node.
Video Tutorial: Rendering Mediums with Lighting