OctaneRender® for Blender

Absorption is controlled with the Absorption medium, which defines how fast light is absorbed while passing through a medium. The absorbed light will not continue through the surface and it's absorbed energy is converted to the opposed color from the color specified in the Absorption attribute (figure 1).

Figure 1: The Absorption Medium parameters as seen in the Shader Editor with Invert Absorption deactivated.

The color resulting from the absorption is dependent on the distance light travels through the material. With increased distance, it gets darker, and if the absorption is colored, it becomes more saturated. It works in a subtractive manner in that the scattered color is the compliment of the color designated in the parameter (figure 2).

Figure 2: Complimentary Colors 

Absorption Parameters

Compatibility version - The Octane version that the behavior of this node should match.

Density - Controls the absorption density. A higher value will absorb more light (figure 3).

Figure 3: Increasing the Density parameter.

Volume Step % - Depending on the surface, you may need to adjust this parameter as it is specified as a percentage of the voxel size. The default value is 100%, but if the volume is smaller than this, you need to decrease the value. Decreasing this value decreases render speed, and increasing the value causes the ray marching algorithm to take longer steps. If the Volume Step % exceeds the volume's dimensions, then the ray marching algorithm takes a single step through the whole volume. To get the most accurate results, keep Volume Step % as small as possible.

Volume Shadow Ray Step % - Step length percentage that is used by the shadow ray for marching through volumes.

Use Volume Step Length for Volume Shadow Ray Step Length - Check box for using the Volume Step Length for the Volume Shadow Ray Step Length as well.

Sample Position Displacement - Allows a texture to control a volume's sample positions displacement.

Volume padding - Expands the volume bounding box by the given percentage in all 6 directions, but only if sample position displacement is being used.

Absorption - Determines the absorption value of the material by assigning a texture or altering the default color swatch. This can be either a grayscale or color texture. When using greyscale values, 0 (black) means that there is no absorption. Values greater than zero determine how quickly the medium absorbs white light (figure 4).

Figure 4: Using a grayscale value to control absorption.

Absorption can also be controlled using color values. The observed color is the complementary color (opposite color) of the specified color value if Invert Absorption is deactivated (figure 5).

Figure 5: Using color values to determine absorption.

Invert Absorption - Inverts the absorption characteristics so that the absorption color specified is the same color seen in the surface. For example: Red results in red, otherwise, red would result in green (if this check box is inactive).