SSS (Subsurface Scattering)#
The SSS material is a special material simulating the effect of light penetrating a translucent surface that is scattered multiple times by the material before exiting the surface. It is typically used to simulate the translucency component of wax, skin, marble etc... The material approximates a homogeneous volume on which properties are defined from the entry point and the SSS parameters.
This material is not meant to be used with thin/flat geometries that doesn't define any volume such as leaves, grass blades etc... For these type of geometries, you'll get better results with diffuse backlighting.
Using Subsurface Preset attribute you can use presets of real world materials that are based on physical measured values assuming scene units are in centimeters (cm).
The Subsurface Model attribute allows you to choose between two techniques to approximate subsurface scattering:
|Diffusion Kernel||Diffusion Kernel aggregates light paths around the entry point of the surface and integrate their contribution using the specified diffusion profile set by Subsurface Diffusion.|
|Random Walk||Random Walk path traces light paths inside the surface making it a better approximation for thin surfaces which is more expensive and slower to compute than Diffusion Kernel.|
Diffusion Kernel is an efficient technique which works really well when mean free path is smaller than the geometric details of the geometry. This technique is faster to render than Random Walk.
The Subsurface Diffusion attribute defines the diffusion profile used to model the attenuation of light with distance due to subsurface scattering.
|Gaussian||The Gaussian profile is a blurry phenomenological model.|
|Cubic||The Cubic profile is sharper and a more accurate approximation.|
|Empirical||The Empirical profile is a good fit based on actual physical SSS measurements.|
Random Walk approximates more accurately the scattering inside the medium and captures more realistic fine geometric details of the geometry that are missed by the Diffusion Kernel. Since it requires more samples, it is also more expensive and slower to compute than Diffusion Kernel.
Max Bounces sets the maximum number of bounces a light path may travel in the geometry during Random Walk exploration before it exits the volume. The higher the max number of bounces the more realistic and slower the renders get.
Difference between Diffusion Kernel and Random Walk#
If we compare the Diffusion Kernel render to the Random Walk one, they looks pretty similar except in areas where Diffusion Kernel fails to capture the fine details of the geometry.
In the renders above Diffusion Kernel looks slightly softer around thin areas while Random Walk looks sharper and more realistic. Note that the Random Walk render was twice as long to render.
The difference between the two techniques becomes even more striking: if we increase the mean free path even further, the Diffusion Kernel render starts to look unnatural and too soft.
Optimal Mode defines which set of attribute is used to control the optical properties of the material.
|Volume||The material properties are defined using actual optical properties of the volume using Volume Albedo and Volume Mean Free Path attributes where the albedo of the material is the resulting color of the light path when it exits the surface after it has been attenuated and scattered in the medium.|
|Surface||The material properties are defined through an artist-friendly WYSIWYG set of attributes: Diffuse Albedo and Diffuse Mean Free Path. In this mode, the interface of the material has been simplified so that you can directly input the desired albedo (reflected color) of the surface and the radius of the underlying material.|
|Hybrid||The material properties are defined using a mix of Volume and Surface set of attributes provided for compatibility purposes with previous version of the path tracer.|
Mean Free Paths#
Mean Free Path (MFP) attributes define the average distance the light is traveling through the surface so that the bigger the mean free path the more translucent the material is. MFP is decomposed per wavelength (RGB).
Volume Mean Free Path, defines the MFP between scattering events on a light path whereas the Diffuse Mean Free Path defines the overall MFP of the light path that travels under the surface.
The SSS material also offers a couple of attributes to globally apply a scale and an exponent on the input mean free paths values: Mean Free Paths Scale and Mean Free Paths Exponent. These attributes both affect Volume and Diffuse Mean Free Path.
Mean Free Paths Scale#
Mean Free Paths Exponent#
The Anisotropy attribute controls the anisotropy of the medium scattering. By default, light is scattered isotropically (by the same quantity in all directions).
Positive Anisotropy values bias the scattering forward in the direction of the light while negative ones backward toward the light.
When using Diffusion Kernel, Anisotropy is an approximation trying to emulate the effect as much as possible. While backward scattering works rather well, forward scattering fails to bring illumination because of the nature of the technique. For best results it is recommended to use forward scattering with Random Walk.
Using the Surface Transmission attribute, it is possible to choose the model of the boundary surface when the light path exits the geometry from the inner medium. By default, the boundary surface is defined as Lambertian but it can be set to Dielectric in which case the light path gets refracted following the value defined by Index of Refraction.
Surface Color allows you to define a surface color tint at the front/back surface point. Using a Side Switch Texture it is possible to specify 2 distinct colors/textures on either side of the surface.
Please note than modifying this value when using Diffusion Kernel breaks the physical plausibility of the model.
The Subsurface Group attribute enables the user to specify the name of the group among which illumination propagates between SSS materials.