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The Sub-Surface Scattering (SSS in short) parameters allows you to simulate materials which let light enter and propagate through them, but are made of a more or less dense material which scatters light in random direction and make them appear foggy or even solid.
Some famous examples are: Orange juice, milk, soap, wax... 

SSS is fully compatible with every other parameter of the Arion Advanced Material (BRDF attributesTransmittance...).

The SSS parameters require the Transmittance to be enable to be available. Please refer to the corresponding documentation section for details about Transmittance.

 

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Sub-Surface Scattering increases the number of light bounces necessary to achieve good-looking SSS materials substantially. Indeed, light does not travel in straight lines inside an SSS material, but bounces off many virtual particles of matter. It is mandatory to achieve realistic SSS materials to increase the SSS ray depth in the Arion Globals to relatively high value (100 or more). The examples in this page were using Arion Render's maximum SSS ray depth value (4096) to be as perfect as it can be for the demonstration.

 

 

Color

The Color parameter controls the color of the scattered rays. It is not to be confused with the Transmittance Color, which we advice to use to control the color of the scattered light inside your material instead as it is a more physically-correct way to tint a dielectric material in its mass.

Indeed, light that is scattered inside the material normally receive its color from the absorption of the material. The scattered rays should not be tinted a second time, unless this effect is necessary to achieve the desired result.

The pictures below illustrate the difference between tinting an SSS material using the Transmittance Absorption color and using the SSS Color directly.

 

Absorption color
Attenuation
SSS color


Off

 

Absorption color
Attenuation
SSS color

Off
Absorption color
Attenuation
SSS color

Off
Absorption color
Attenuation
SSS color

Off
Absorption color
Attenuation
SSS color

Off
Absorption color
Attenuation
SSS color

50cm
Absorption color
Attenuation
SSS color

25cm
Absorption color
Attenuation
SSS color

10cm
Absorption color
Attenuation
SSS color

5cm
Absorption color
Attenuation
SSS color

1cm
 

As you can see, not only coloring an SSS material using the SSS color is less predictable, but it also looks less natural and often too dark. Using the attenuation distance on the other hand gives a natural 'creamy' look to the material and with values low enough slowly start to look like soap or wax.

 

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When configuring SSS materials, you will often notice that an attenuation distance that works great for a glass-like material is way too bright once SSS is enabled. This is because light is scattered in random directions inside the object and thus, may (and very often do) exit the object much sooner. Because it exits the object much sooner, it has traveled a shorter distance inside the object which prevents it from being fully colored by the attenuation color and attenuation distance like it would be without scattering.

 

 

1-sided


Single-sided can recreate the soft diffuse transmission of thin objects like paper.


The 1-sided parameter works similarly to the homonym setting in Transmittance. The notable difference is that it is intended to be used on objects without volume such as sheets of paper, leaves or curtains for example.

That being said, it will work just fine on volumes but will not generate realistic results. Where regular SSS may generate hundreds or thousands of light bounces in a complex object, the 1-sided option will bounce once into a random direction and then travel straight until it exits the object.

This parameter ignores entirely the Attenuation color and the SSS color and only considers the BRDF color input. It can be a solid color or a texture.

 

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Although the 1-sided parameter does not require that the Roughness value of the material is 100, it is recommended to use high Roughness values for the material to behave as expected.

 

The pictures below illustrate the effect of 1-sided SSS on a flat piece of geometry.

 

Without 1-sided SSSWith 1-sided SSS
 

Coefficient

The Coefficient parameter indicates to the engine how likely it is for a light ray to hit a particle of matter and changes its direction for a 1 meter distance. This parameter is a probability and the value entered, despite it will affect the density of the SSS material, does not guarantee that for a 1 meter traveled by a ray inside the material, it will encounter the exact same number of particles as defined in the Coefficient parameter. Sometimes, probabilities can make a ray bounces more than the Coefficient, sometimes less, but in average after many many rays have been shot, the Coefficient value will be respected.

The pictures below illustrate the effect of the Coefficient parameter on the rendered image, including the brightening of the color as the Coefficient produces a denser material.

 

Coefficient = 5

Coefficient = 10Coefficient = 50Coefficient = 100Coefficient = 500
 

Asymmetry

The Asymmetry parameter controls the likeliness of a light ray to bounce backward (negative Asymmetry), randomly (neutral Asymmetry) or forward (positive Asymmetry) after it has hit an SSS particle.

The outgoing direction of the light ray is relative to its incoming direction. This parameter tend to make the scattering more directional either backward or forward and thus scatter less as it has a higher positive or negative value for a given Coefficient.

This parameter has generally no real interest for most materials and is present more because the Henyey-Greenstein scattering function (which is widely used in many render engines for scattering) allows for it natively than because it is a requirement. This parameter makes more sense when scattering light through gases like in our atmosphere than matter.

 

Asymmetry = -75

Asymmetry = -50Asymmetry = 0Asymmetry = 50Asymmetry = 75
 

It is interesting to note that since forward scattering (positive Asymmetry) allows incoming light to keep travelling in straighter lines, the Attenuation color is more prominent. While with backward scattering (negative Asymmetry), the incoming light exits the object toward the incoming light more often and quickly, which tends to reduce the impact of the Attenuation color.

 


 

Extra material about Sub-Surface Scattering


SSS bounces

The Sub-Surface Scattering subject deserves some additional information that may make the difference between a good looking and a bad looking material, or the difference between a fast render and a slow render.

As we mentioned earlier in this document, SSS requires many light bounces to render correctly when the Coefficient value is raised. In the pictures below we demonstrate the difference between different SSS bounces values and its impact on the rendered material.

 

SSS bounces = 3
Rendertime =
38sec

SSS bounces = 6
Rendertime = 50sec
SSS bounces = 15
Rendertime = 67sec
SSS bounces = 30
Rendertime =
72sec
SSS bounces = 100
Rendertime =
73sec
 

Despite the render time is quite low for SSS, we can improve that even more by carefully using the Exit Color of the Advanced rollout parameters.

 

SSS bounces
Exit color
Render time

100
None
170sec

 

SSS bounces
Exit color
Render time
3
None
136sec
SSS bounces
Exit color
Render time
3

136sec
SSS bounces
Exit color
Render time
100
None
91sec
SSS bounces
Exit color
Render time
3
None
51sec
SSS bounces
Exit color
Render time
3

51sec
 

Some SSS material examples and their settings


Soap BlueOrange JuiceHoney
Color
Edgetint
 Fresnel IOR
Roughness
Attenuation color
Attenuation distance
SSS color
Coefficient



1.2
15.0

2.0cm

250

 

Color
Edgetint
 Fresnel IOR
Roughness
Attenuation color
Attenuation distance
SSS color
Coefficient


1.35
3.0

1.5cm

100
Color
Edgetint
 Fresnel IOR
Roughness
Attenuation color
Attenuation distance
SSS color
Coefficient


1.484
3.0

4.0cm

10
Strawberries MarmeladePolyurethanePasta
Color
Edgetint
 Fresnel IOR
Roughness
Attenuation color
Attenuation distance
SSS color
Coefficient


1.3
3.0

4.0cm

40
Color
Edgetint
 Fresnel IOR
Roughness
Attenuation color
Attenuation distance
SSS color
Coefficient


1.5
60.0

5.0cm

100
Color
Edgetint
 Fresnel IOR
Roughness
Attenuation color
Attenuation distance
SSS color
Coefficient


1.3
40.0

2.0cm

1000

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