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titleSubsurface Scattering Parameters

Subsurface Scattering Parameters

Subsurface Model

Select a subsurface scattering model: Jensen Dipole, d'Eon Better Dipole, Burley Normalized, and Multiple Mean Free Paths. The parameters are populated based on the model chosen and are valid for that model. If you change which model you use, your connections may be lost to invalid parameters.

Burley Normalized produces accurate effects while preserving details.

Jensen and d'Eon Dipoles are great for very translucent objects like gummies and extended art direction.

Multiple Mean Free Paths is great for texturing to produce color bleed intuitively. While not necessarily physically correct, its scattering of textured colors works well for art direction while avoiding color shifting.

 

 

Gain

Subsurface scattering weight. Higher numbers increase the visibility of the subsurface scattering.

 

 

Color

Subsurface scattering color.

 

 

Mean Free Path Distance

Subsurface scattering mean free path distance (mfpd). This specifies how far the light travels inside an object and as a consequence how smooth the subsurface scattering is. This gets multiplied by the unit length set in the Properties section. Higher amounts make the object appear less opaque and more translucent as well as increase noise. Small amounts make the surface look diffuse and it may be more efficient to turn off the effect (0.0 Gain) if it's not visually important. Below we change the values from 8 to 16 and then 32. Notice how the sphere and pedestal become "softer" and more translucent. Your render times might also increase at high values due to noise generated by translucent objects.

 

 

Mean Free Path Color

How far the light travels in the Red, Green, and Blue spectra. This is scaled by Mean Free Path Distance. Different colors may spread more or less and provide interesting effects like the red color bleeding into shadow edges on skin. The RGB values correspond to how far the light travels in that color band. For example, and RGB value of 0.8 0.65 0.5 means Red spreads furthest at 0.8, then Green and finally Blue traveling the least distance in the object scatter result. Below we've taken out the center sphere and replaced it with a bright disk light at the back of the outer sphere. Note that the sphere and pedestal share the same material and lighting plays an important role in your result.

 

 

Post Tint

Tint that is applied at the end of the subsurface computation. Below on the left is a normal render and on the right a very light blue tint is added. If we want to apply the tint before the subsurface computation, set Irradiance Tint in the Properties section.

 

Multiple Mean Free Paths Description


Multiple Mean Free Paths operates differently than the others in a few significant ways. For most cases you will be happy with the results of the other models. However, there are instances where the user may want a non-physical way to control the colors of the scattering. The Mean Free Path Color in many models uses the supplied RGB value to determine the color scatter as noted in the parameter description. This means your result may have color shifts that are not desirable. Let's say you supply a textured color of RGB 0.51 0.28 0.31 which is a pink color. The scattering result will also include the green (0.28) and blue (0.31) responses. The Multiple Mean Free Paths model maintains the pink color. Below is an example using this scenario illustrating the differences. The texture is connected to the Color and Mean Free Path Color of the Jensen Dipole and to the Short and Long Colors of the Multiple Mean Free Paths model in addition. Note the color shift on the Jensen model (Left) while the Multiple Mean Free Paths version maintains the colors of the texture at all depths where connected.

 

 

Short Gain

Short subsurface gain or weight. This is only valid for Multiple Mean Free Paths subsurface model.

 


Short Color

Short subsurface color. This is only valid for Multiple Mean Free Paths subsurface model. Above uses Red as the chosen color.


Long Gain

Long subsurface gain or weight. This is only valid for Multiple Mean Free Paths subsurface model. Operates the same as Short Gain.


Long Color

Long subsurface color. This is only valid for Multiple Mean Free Paths subsurface model. Operates the same as Short Color.

 

Short MFP Distance

Short subsurface mean free path scalar distance. Below we go from 16 to 24 and finally 32.

 

 

Long MFP Distance

Short subsurface mean free path scalar distance. Below we go from 16 to 24 and finally 32.

 

 

Diffuse Computation Switch

Switch the subsurface computation to a diffuse computation if the dmfp is smaller than the ray footprint (not visible given the settings and distance). This is an optimization to ignore computing scattering, especially on far away objects or objects where the scattering scale is so low a diffuse computation is visually similar and much faster. This may be easier than manually setting the Gain to 0.0. Lower values increase the chance of rendering a diffuse computation instead of subsurface scattering.


Double Sided

If on, illuminate on both sides of the surface for this subsurface lobe, that is, this will illuminate the surface whose normal is pointing away from the camera as well. If not on, it will not render on the backside.


Trace Control:

Consider Backside

Whether subsurface respects surfaces on the other side. This is for the hit side, not the illuminating side (which is subsurfaceDoubleSided):

  • "Off" - It will ignore surfaces on the other side completely. This is useful to make objects appear thicker than they are.
  • "On" - Normal mode, where the diffusion happens between the front and the first surface behind it.


Continuation Ray Mode

Control continuation ray mode:

"Off" - Simply trace out of the object (default).

"Last Hit" - Ignore internal geometry and jump to the last surface.

"All Hits" - Scatter (collect light) on all hits as the ray leaves the object. This can bring additional brightness, at the cost of additional noise.

Below left we use "All Hits" and you can plainly see the text "SUBSURFACE" embedded in the sphere. To the right it's set to "Off" which causes the embedded geometry to render incorrectly by clamping that falloff.

 

 

Max Continuation Hits

Maximum number of hits to test in all hits mode. This is only valid when Continuation Ray Mode equals All Hits

 

Follow Topology

Controls how strongly normals are considered in the subsurface computation. This may affect visible details created through bump mapping as well.

 

Trace Subset

Specify trace subset for inclusion/exclusion when struck by a ray indirectly.

 

 

Expand
titleSingle Scatter Parameters

Single Scattering Parameters

Single scatter is a simple and inexpensive effect for scattering effects. Below a disk light is placed inside the model and we render using no diffuse color and again with a default middle grey diffuse color.

 

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Gain

Single scatter gain or weight.

 

 

Color

Single scatter color.

 

Image AddedImage AddedImage Added

 

 

Mean Free Path

Single scattering mean free path scalar distance. This specifies how far the light travels inside an object and as a consequence how smooth the single scattering is. This gets multiplied by the unit length set in the Properties section. Larger values are more translucent.

 

 

 

Mean Free Path Color

How far the light travels in the R, G,  and B spectra. This is scaled by Mean Free Path Distance.

 

Image AddedImage AddedImage Added

 

Directionality

Single scatter directionality:

1: forward scatter which is more light on the backside.

-1: backward scatter which is more light on the front side.

0:isotropic (no effect).

  

Image AddedImage AddedImage Added

Refractive Index

Single scatter index of refraction. Below are 1.3, 5.0, and then 10.0

 

Image AddedImage AddedImage Added

 

Blur

Blur strength for single scatter.

 

 

Backside Direct Illum Gain

Gain for direct illumination from the other side.

 

 

Direction Tint

Tinting color for the Backside Direct Illum Gain.

 

 

Double Sided

If on, illuminate on both sides of the surface for this single scatter lobe, that is, this will illuminate the surface whose normal is pointing away from the camera as well.

 

 

Trace Control Parameters (Operate similarly to Subsurface Controls):

Consider Backside

Whether subsurface respects surfaces on the other side. This is for the hit side, not the illuminating side (which is singlescatterDoubleSided):

  • "Off" - It will ignore surfaces on the other side completely. This is useful to make objects apear thicker than they are.
  • "On" - Normal mode, where the diffusion happens between the front and the first surface behind it.

 

Continuation Ray Mode

Control continuation ray mode:

"Off" - Simply trace out of the object (default).

"Last Hit" - Ignore internal geometry and jump to the last surface.

"All Hits" - Scatter (collect light) on all hits as the ray leaves the object. This can bring additional brightness, at the cost of additional noise.

 

 

Max Continuation Hits

Maximum number of hits to test in all hits mode. This is only valid when Continuation Ray Mode equals All Hits

 

 

Direct Gain Mode

Control continuation ray mode:

"First Hit" - Simply trace to the next surface (this is tied to considerBackside).

"Last Hit" - Ignore internal geometry and jump to the last surface.

"All Hits" - Scatter (collect light) on all hits as the ray leaves the object. This can bring additional brightness, at the cost of additional noise.

 

 

Trace Subset

Specify trace subset for inclusion/exclusion when struck by a ray indirectly.

 

Scattering Globals

Irradiance Tint

A tint applied to illumination before being scattered by subsurface or single scatter.

 

Irradiance Roughness

Diffuse roughness to be applied for subsurface or single scatter. A value of 0 represents classic Lambertian shading model. Non-zero values increase the microfacet roughness for the Oren-Nayar shading model. A greater value produces rougher diffuse.

 

Unit Length

Subsurface and single scatter unit length. It is a multiplier on Mean Free Path Distance. Mean Free Path Distance is often measured in millimeters. If the scene is modeled in some other scale, Unit Length should be set accordingly. The default value of 0.1 is appropriate for scenes modeled in centimeters (the default in Maya) and Mean Free Path Distance measured in millimeters.

 

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