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For this tutorial we assume that you have followed a least some of the other tutorials and know how to assign a texture. The tutorial start with a textured object.

 

For this tutorial we start directly from a model with a Color and Bump textures assigned.

The Coatings are enabled as well, with Color and Edgeting values of respectively RGB 40,40,40 and 80,80,80 in this case, feel free to adjust to your own leaf/plant and lighting.
The only important thing to do right is to lower the Coatings Color and Edgetint sufficiently so the reflection intensity is not way too strong as to occluding entirely the leave color itself. 

BRDF attributes
Fresnel IOR 1.51 0.0
Falloff 46.5 16
Color
Edgetint
Roughness | % 100
Bump | % 10
Coatings
Enable
Thin-film
Fresnel IOR 1.51
Color
Edgetint
Roughness | % 15
Thickness | nm 250 1000

 

Next go to the Transmittance and Sub-Surface Scattering rollouts and enable both. Also enable 1-sided (also called Single-sheet in older versions of Arion) in the Sub-Surface Scattering rollout (NOT in the Transmittance rollout!).

ALL the other parameters for Transmittance and Sub-Surface Scattering are now going to be ignored by the engine, as they don't apply to this kind of material.

Transmittance
Enable
1-sided
Abbe 63.5
Atten. | cm 25
Color
Sub-Surface Scattering
Enable
1-sided
Coeff. | 1/m 4
Asym. | % 0
Color

 

Probably the first thing you will notice is the leaf material is overall darker than previously, and that is normal (and correct).

So what happened?

What happens is something called energy conservation, if a material let some light pass and transport it to the other side, it means the reflected (not transmitted) light is attenuated, which is what happens here.
In Arion Render for single-sheet SSS, it is always assumed a 50-50 ratio for reflected and transmitted light, which is then modulated by the color of the object itself. Dark colors transmit less and reflect more and vice versa.

In the real world when you see a leaf, this phenomenon has already happened and the color you see or can capture with a camera or scanner is the already attenuated color, which is why in our leaf material based on a real-world leaf color capture, it looks darker than expected.
If Arion did not do that, there could be cases where the engine would stop being energy-conservative which would result in fireflies. The real world is tricky.

But no problem, we will fix that simply by adjusting the texture's RGB modifiers and pumping up its brightess to recover its real color.

Open the texture's properties and increase the Brightness parameter to 25%.

RGB modifiers
Invert
Gamma | % 0
RGB | % 0 0 0
Clamp | % 0 100
Brightness | % 25
Contrast | % 0
Saturation | % 0

 

Now the leaf looks translucent. Note that with single-sheet SSS, it's totally unnecessary to use a map in the Transmittance or Sub-Surface Scattering color channels, as they are ignored. Only the surface color is evaluated.

If we duplicate the plant in the scene until some leaves cast shadows onto other leaves, we can see the effect more clearly.

Without Single-Sheet ScatteringWith Single-Sheet Scattering

 

The information given in this tutorial is true of every use of Single-Sheet Scattering, however each case being specific (different leaf texture, color etc.), you need to adapt.
But the basic is simply 'this'. The brighter your color texture will be, the more light the material will let pass and the more translucent it will appear.

 

Let's check our material against a physical-sky lighting situation:

 

Good, I think we're done here.

 

 

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