Physically Accurate Subsurface Scattering
| Optical Properties | Algorithm Control | Render Tree Usage
Location: Nodes > Illumination > mental images > Subsurface
Node Names: Jade and Skim_Milk
Shader Type: Surface
Output: Color (RGB) value
In the real world, many materials are translucent to some extent, and do not immediately reflect light at their surface. Instead, light penetrates the surface, and is scattered inside the material, before it is either absorbed or transmitted. This effect, called “subsurface scattering”, greatly enhances the realism of a wide variety of rendered materials, even when used sparingly.
This shader simulates subsurface scattering in a physically correct way by using a simplified version of the real-world subsurface scattering process.
The Jade and Skim_Milk presets are variations of the physically accurate subsurface scattering shader, and each one is pre-configured with the real-world values necessary to simulate a given material. Of course you can modify the parameters of any one of these presets to have it simulate a different material.
For subsurface scattering to work correctly, your scene must meet the following basic conditions:
• Raytracing must be activated in the Render Options and/or View Render Options property editor.
• At least one scene light (ideally a spotlight focused on the shaded object) must be set as a caustic and/or global illumination emitter. The number of emitted photons, as well as the energy value must be set.
• Caustics and/or global illumination must be activated in the Render Options and/or View Render Options property editor.
• The subsurface scattering shader should be connected to the Surface and Photon inputs of the shaded object’s Material node.
• The shaded object should be set as a caustics and/or global illumination receiver and transmitter.
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Color |
Defines the surface color of the shaded object. You can connect other shaders, like surface shaders (Phong, Blinn, etc.) or texture shaders, to this parameter to modify the object’s shading, surface color, and so on. |
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Transmission |
Defines the transmission color that filters light as it enters the object. Connecting 3D texture shaders to this parameter allows you to modulate the way light is filtered as it enters the object. |
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Index of Refraction |
Controls the bending of light through a transparent material. Defines the index of refraction, which varies according to the nature of the material. |
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Absorption R/G/B |
Defines the absorption coefficient, which controls, on a per-channel basis, the fraction of light absorbed per unit of distance as it passes through the shaded object’s volume. Real-world absorption coefficient values for different materials can usually be found in any good optics textbook. You should use these values to accurately recreate the desired material’s surface characteristics using this shader. |
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Scattering R/G/B |
Defines the scattering coefficient, which controls, on a per-channel basis, the fraction of light scattered per unit of distance as it passes through the shaded object’s volume. Real-world scattering coefficient values for different materials can usually be found in any good optics textbook. You should use these values to accurately recreate the desired material’s surface characteristics using this shader. |
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Millimeters to unit |
Specifies the conversion between millimeters (which are generally used to express absorption and scattering coefficient values) and scene units. For example, if your chosen scene unit is inches, the correct setting for this parameter is 25.4 (mm per inch). |
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ScatteringDir |
Controls the degree of scattering, since most materials do not scatter light uniformly. Values for this parameter range between -1 and 1. A value of -1 results in backward scatter only, a value of 1 results in forward scatter only, and a value of 0 results in uniform scattering in all directions. |
The properties on this tab control the quality and complexity of the rendering. The subsurface scattering calculation is broken into three components: single scatter, multiple scatter, and diffusion.
Depending on the optical properties of the material, the contributions of each component may dominate or be negligible. For example, single scattering is dominant under low scattering conditions, whereas multiple scattering and diffusion are dominant under high scattering conditions, such as those where the scattering coefficient is much larger than the absorption coefficient.
Each component can be turned on or off as desired, though in most cases it is advisable to leave all components on.
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Deep Layer Depth |
This shader uses two different ways to scatter photons. After a certain number of scatterings a photon is scattered less precisely, but more quickly. This parameter specifies the number of scattering events done using the precise scattering, also called “shallow layer”. Once this number is reached, the less precise “deep layer” scatterings begin. |
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Maximum Samples |
The maximum number of samples evaluated by the single and multiple scatter components. Typical values range between 10 and 30. |
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Maximum Photons |
The maximum number of photons to sample per lookup. This is similar to the global illumination Accuracy parameter. Typical values range between 100 to 1000. Values up to 5000 are useful for scenes with uneven photon distribution. |
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Maximum Photon Search Radius |
The maximum radius within which a sphere centered at a sample point will collect photons. It is similar to the global illumination Radius parameter. Typical values are a small percentage of the total size of the object. |
Approximation
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ApproxDiffusion |
Turns the diffusion component of the calculation on or off. Usually it should be on. |
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ApproxSingleScatter |
Turns the single scatter component of the calculation on or off. Usually it should be on. |
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ApproxMultiScatter |
Turns the multiple scatter component of the calculation on or off. Usually it should be on. |
This surface shader can be used almost anywhere in a render tree, but is most commonly connected directly to a Material node’s Surface and Photon inputs (as well as Shadow). You can use texture shaders to control the Color and Transmission inputs.
Autodesk Softimage v.7.5