Phenomena (predefined shader combinations) come with the SSS shaders to simplify usage. As they are presented in the mental images shader documentation, these are material Phenomena which produce a material result, whereas typical color Phenomena define shader combinations that produce a color result. As noted, the Phenomena for using SSS in your particular distribution may vary. For example, in the Maya 6 Bonus tools, the shader combinations form color Phenomena, which group shaders by their input to the material of the documented material Phenomena.

A material Phenomenon combines component shaders into a material, defining the input connections for material shaders, photon shaders, lightmap shaders, etc. For those who wish to add these directly to .mi scene files, please note that material Phenomena have a different usage syntax than color Phenomena. We show this usage in the sections below.

Physical SSS Phenomenon

This phenomenon implements the physical SSS material:

• misss_physical_phen

Component Shader

• misss_physical

Phenomenon Diagram

This simple material Phenomenon uses its component shader for both its material(surface) shader and its photon shader.

In Maya, simply use the component shader as an input to both the material and photon inputs of the SG node.

Example usage in .mi scene file

Image courtesy Dominika Waclawiak

If we use the material/surface shader directly, make a shader definition, and then connect it to both the material and photon inputs of the material. For example, from the grapes mi scene file that made the picture above, note the max_photons and max_radius parameters in bold. As mentioned in the physical sss usage tips, try setting these to 1 and 1 to see the photons.

shader "misss_physical1"
"misss_physical" (
"material" = "lambert3.outColor",
"transmission" 0.678113 0.835 0.505175 1.,
"ior" 1.33,
"absorption_coeff" 0.001 0.0025 0.01,
"scattering_coeff" 0.7 1.22 1.9,
"scale_conversion" 10.,
"scattering_anisotropy" 0.75,
"depth" 5.,
"max_samples" 18,
"max_photons" 1000,
"max_radius" 20.,
"approx_diffusion" on,
"approx_single_scatter" on,
"approx_multiple_scatter" on,
"lights" [
"spotLight1:spotLightShape1"
]
)

material "misss_physical1SG"
= "misss_physical1"
photon = "misss_physical1"
end material

object "grape:pSphereShape12"
...
end object

instance "grape:pSphere12:grape:pSphereShape12" "grape:pSphereShape12"
material ["misss_physical1SG"]
...
end instance

If we were using the material phenomenon misss_physical_phen, we would make a definition of that, and then use it directly from the instance, without requiring a material block, eg,

shader "misss_physical_phen1"
"misss_physical_phen" (
"material" = "lambert3.outColor",
...
]
)

object "grape:pSphereShape12"
...
end object

instance "grape:pSphere12:grape:pSphereShape12" "grape:pSphereShape12"
material ["misss_physical_phen1"]
...
end instance

To give you a simple mi file small enough to download (about 200K), this grape has been extracted from the grapes scene, and the texture maps have been removed. The file requires mayabase and subsurface libraries.

Non-Physical SSS Phenomena

The following Phenomena implement the above mentioned non-physical SSS materials:

• misss_fast_simple_phen
• misss_fast_skin_phen
• misss_fast_skin_phen_d

Non-Physical SSS Component Shaders

The above phenomena use these component shaders:

• misss_fast_shader
• misss_lightmap_write
• misss_lambert_gamma
• misss_skin_specular
• misss_call_shader

Non-Physical SSS Phenomena Diagrams

The diagrams below show a simplified depiction of the component shader connections. These diagrams also show how the Phenomena relate to each other.

This first diagram shows a simple two-layer SSS model.

The outer box represents the material Phenomenon we see in the mental ray shader documentation. The material Phenomena may also come in the shader library shipped with mental ray standalone distributions at version 3.3 and above. The inner surrounding boxes labeled misss_fast_*_maya represent the color Phenomena that come with Maya 6 Bonus Tools. In Maya, connect the appropriate color Phenomenon into the material or lightmap input of an SG node. (If you are unsure of your Phenomena type, please check your subsurface.mi file.)

The next diagram shows the three-layer skin model

When using the material Phenomena in the standalone version, the misss_call_shader component shader is designed to pass a procedural shader through to the material.

The Phenomenon in the next diagram merely adds a displacement shader input. The above Phenomena are simplifications of this next one.

In Maya, you hook up the environment and the displacement separately, anyway. Note that an SG node in Maya translates into a material in mental ray. From Maya, connect the environment shader, and the displacement shader into the respective SG inputs.

mi Examples to come ...