Downloads

• Zap Andersson's SSS skin tutorial (1.5M pdf)
• Follow Zap's tutorial with these application independent files (3.7M zip).
• A mel script replacement to automatically hook up SSS light maps in Maya 6. Install Zap's createAndAssignShader.mel in scripts/startup/
• A Maya 6 specific globulus-after.mb (under 900K) scene file to go with Zap's tutorial materials. Requires relinking the texture images.

Concepts for non-physical SSS

• Lightmaps hold incoming light
  • diffuse irradiance
  • used to simulate scattering
• Front and back
  • front - surface facing camera
  • back - from back side of object
• Not required
  • photons
  • GI, caustics
  • raytracing

The object holds incoming light (irradiance) data in light maps. When looking at the object from the camera's point of view, we separate the light into a front and a back side. The light map actually stores the diffuse irradiance and z-depth for each side, i.e., the incoming diffuse light and the distance from the camera. The front side captures light for scattering that happens beneath the surface facing the camera, while the back side captures light that scatters through the object from the back. A good example of back scatter is an ear lobe, or the thin skin between your fingers lit from the back. Image courtesy Barry Dempsey

Simple Scattered Layers

Image courtesy Zap Anderson - mental images

The layers in the simplest form of non-physical SSS Phenomena include diffuse (surface, not-scattered), specular (surface, not-scattered), front scatter, and back scatter light contributions. For this, one may use the misss_fast_simple_phen material Phenomenon, or, in Maya, both the misss_simple_maya and misss_fast_lmap_maya color Phenomena.

A weight factor is included for each of these contributions to more easily control the 'mix' of these layers.

Scatter Radius

For each of the scatter components, a radius parameter controls how far the light will scatter. The front radius controls distance along the surface. The back radius, and an additional depth value control how much light will reach the front surface that you are looking at.

Skin

Image courtesy Barry Dempsey	A more advanced skin model, used in the misss_fast_skin_phen material Phenomenon, adds a middle scatter layer to the above simpler SSS shader, as well as some specular color enhancements specifically tailored for skin. The picture to the left shows a render using all the defaults of the misss_fast_skin_phen material.
The front scatter layer in the following diagram is labeled epidermis with an epidermal scatter radius; the middle scatter layer, dermis with a subdermal scatter radius. Image courtesy Zap Anderson - mental images
In addition, there are two levels of specularity modeled for this skin shader, in addition to a glossy reflection contribution.

Skin parameters separated and depicted by layer

Layer	Image	Turntable Movie
Diffuse diffuse_color = 0.95 0.95 1 diffuse_weight = 1.0 (default) 0.3		320x240 640x480
Front SSS front_sss_color = 1 0.85 0.6 front_sss_weight = 0.5 front_sss_radius = 8		320x240 640x480
Middle SSS mid_sss_color = 0.95 0.5 0.2 mid_sss_weight = 0.4 mid_sss_radius = 25		320x240 640x480
Back SSS back_sss_color = 0.7 0.1 0.1 back_sss_weight = 0.5 back_sss_radius = 25 back_sss_depth = 25		320x240 640x480
Primary Specularity edge_factor = 5.0 primary_spec_color = 0.75 0.9 1 primary_weight = 0.3 primary_edge_weight = 0.8 primary_shinyness = 5.0		320x240 640x480
Secondary Specularity secondary_spec_color = 0.9 0.95 1.0 secondary_weight = 0.3 secondary_edge_weight = 0.0 secondary_shinyness = 33.0		320x240 640x480
A full composite of the above values
Other parameters	Image	Animated Value Movie
Falloff Controls the light falloff curve from the incident point to the scatter radius distance. Sharpen the edge falloff at the radius distance by increasing the value from 1.0 to 10.0. For large values, light contribution weight remains the same throughout the disc surface area defined by the radius. Smooth the falloff by decreasing the value from 1. to 0.1. You may need to increase the scatter radius as you smooth.		320x240 640x480
Lightmap gamma Controls the apparent directional strength of the light stored in lightmap. If less than 0, light flattens out from the incoming light direction; if greater than 1, it narrows to the incoming light direction. Technically, it is the gamma curve of the light stored in the lightmap, and a value of 1 represents a normal Lambertian distribution curve.		320x240 640x480
Scatter Bias Controls the appearance of forward or backward scattering. A value of 0 is even. -1 is the maximum backward scatter, and 1 is the maximum forward scatter. Use small amounts of bias between -0.2 and 0.2.		320x240 640x480

Finally, don't forget to check out the SSS skin tutorial pdf and the app independent mi files designed to help you follow along. In response to some of your questions on how to use these in Maya 6, we have also created this supplementary .mb scene file, showing Globulous in an "after" shot.