Volume Medium


The Volume medium applies independent AbsorptionDefines how fast light is absorbed while passing through a medium., ScatteringDefines how fast light gets scattered when traveling through the medium., and Emission color ramps. To use a ramp, you must specify a Color for the corresponding parameter. For example, to use the Absorption ramp, you must select a Color for Absorption.

Figure 1: Volume medium parameters


Volume Medium Parameters

Density - Refers to the Absorption and Scattering scales.

Volume Step Length - This parameter determines the average distance between points where OctaneRender® samples the Absorption, Scattering, and Emission textures. The default value is 4.0. If your Volume is smaller than this, decrease the Step Length. Decreasing this value reduces the render speed. Increasing this value causes the ray marching algorithm to take longer steps. You get the most accurate results when the Step Length is as small as possible. For simplifying your workflow, set the Volume Step Length to an acceptable value.

Vol. Shadow Step Length - Similar to Volume Step Length but defined for Shadows.

Use Vol. Step Length - Will Set the Vol. Shadow Step Length to the value of Volume Step Length.

Sample Position DisplacementThe process of utilizing a 2D texture map to generate 3D surface relief. As opposed to bump and normal mapping, Displacement mapping does not only provide the illusion of depth but it effectively displaces the actual geometric position of points over the textured surface. - Allows a texture to control a volume's sample position displacement.


Figure 2: A comparison of Volumes rendered with different Volume Step Length settings

The Volume can't be too dense. We recommend reducing the Volume Step Length to an acceptable performance and accuracy level, and then reducing the Volume Density. Otherwise, you risk rendering a solid Object at a high Step Length and get deceiving results.

Volumes can also have their own Scattering, Emission, and Absorption ramps. These colors make a strong influence on the Volume's appearance. The Phase function also affects a Volume like a Medium node, and modifying the Volume's Scale value scales the Volume's Density values in a linear fashion. This can also increase emission as Absorption values are also used as particle density.

Figure 3: A comparison of Volumes rendered with different Phase settings

Volumes are rendered in an unbiased way and can scatter multiple times. They also cause self-shadowing effects. To reduce the maximum scatter events in a Volume, reduce the DiffuseAmount of diffusion, or the reflection of light photons at different angles from an uneven or granular surface. Used for dull, non-reflecting materials or mesh emitters. Depth in your Kernel node.

Figure 4: A comparison of Volumes rendered with different Diffuse Depth settings

Absorption - Specifies the color texture for absorption.

Absorption Ramp - Defines the color range. Absorption Ramp takes the grid value as input. In the color gradient, the colors near 0 on the left side map low grid values to a custom color. Higher grid values map to colors on the right side of the color gradient. Desaturated colors create less-pronounced absorption. Emission and Scattering ramps operate in the same way.

Figure 5: Absorption ramp parameters


Figure 6: A render of a Volume using an Absorption ramp


Figure 7: A graph of the Volume medium with incoming connections


Invert Absorption - Inverts the Absorption color so that the Absorption channel becomes a Transparency channel. This helps visualize the specified color's effect since a neutral background shining through the Medium appears close to that color.

Scattering - This is the scattering cross-section. This channel defines how much light is absorbed over the color range. It can accept a value, Color, or Texture map.

Scattering Ramp - Acts similar to the Absorption ramp, but instead it maps colors to the light as it scatters within the volume.

Phase - Affects a volume as it would affect a Medium node, and modifying the Volume's scale value scales the Volume's Density values in a linear fashion.

Emission - Controls the Volume's emission. For Emission, the Medium map can have either a Blackbody emission or a Texture emission.

Emission Ramp - This is the Emission color ramp.