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AbsorptionDefines how fast light is absorbed while passing through a medium. is controlled with the Absorption texture, which defines how fast the Medium absorbs light passing through it. A setting of 0.0 means no absorption. The higher the value, the faster the Medium absorbs the light.
You can add the Absorption map to a MaterialThe representation of the surface or volume properties of an object. by clicking on the None button next to the Medium parameter.
Figure 1: Add the Absorption texture using the Medium button
The Absorption texture can be a Color, value, or Texture map, and is multiplied by the Density parameter. This allows you to set a wide range of values in an easier way.
Figure 2: Absorption parameters
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 3: A diagram shows that complementary colors are opposite each other on the color wheel
Rendering an Absorption medium requires the Path Tracing or the PMC kernel with a large Maxdepth setting. For media inside 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. transmitting MaterialsA set of attributes or parameters that describe surface characteristics., you can use the Direct Light kernel with Diffuse GI Mode too.
Meshes - You should add participating media to Meshes that define a closed volume rather than planes. Using planes to model leaves of a plant with SSS will not work. Using a single plane as a ground plane will work since it is treated as an endless, deep Material. The Mesh can have opaque Objects nested inside, but nested participating media are not supported.
Absorption - Controlled by Absorption color, which defines how fast a Medium absorbs light passing through it. A 0.0 or black value means no absorption. Higher values result in faster light absorption. The specified color in the Absorption parameter produces its complimentary color in the rendering (Figure 1). The Absorption texture is multiplied by the Density parameter. This allows setting a wide range of values.
Volume Step Length - Depending on the surface, you may need to adjust this parameter. The default value is 4, 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 Length 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 Length as small as possible.
Invert Absorption - 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 appears in that approximate color.
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