Subsurface Scattering Phase ->?

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Hey all

I'm playing around with SSS at the moment (H12, Mantra Surface Material) and don't understand what this SSS phase parameter is all about. I read the help text serveral times, but I just don't get it. Could someone of you folks explain it to me in easy words? Maybe also give me an example or a image to look at? I searched the web, but there seems to be hardly nothing out there covering this.

Houdini Help says:

Controls the nature of the scattering. Positive values give forward scattering, 0 gives isotropic scattering, and negative values give backscattering. Range is -1 (full backscattering) to 1 (full forward scattering). The value depends on the type of material you are trying to model. For example, skin is highly forward scattering, while marble is backscattering.

I also found a similar parameter for the Vray fastSSS Shader, here is what it's help says:

Phase function - a value between -1.0 and 1.0 that determines the general way light scatters inside the material. Its effect can be somewhat likened to the difference between diffuse and glossy reflections from a surface, however the phase function controls the reflection and transmittance of a volume. A value of 0.0 means that light scatters uniformly in all directions (isotropic scattering); positive values mean that light scatters predominantly forward in the same direction as it comes from; negative values mean that light scatters mostly backward. Most water-based materials (e.g. skin, milk) exhibit strong forward scattering, while hard materials like marble exhibit backward scattering. This parameter affects most strongly the single scattering component of the material. Positive values reduce the visible effect of single scattering component, while negative values make the single scattering component generally more prominent.

Thx in advance!

[symek]

Well, it pretty much does what it says. Imagine photons traveling inside a substance. Some of them hit the atoms and scatter: some scatter back, some aside, others stay on a track.

Houdini SSS uses so called Henyey-Greenstein phase function, which, for a pair of incoming direction and phase value computes proportion of energy scattered towards some outgoing direction.

You can think of it as a shape around a hit point. For phase equal 0, this shape is close to a sphere - meaning atoms are scattered equally in all directions.

This is from Houdini Help about henyeygreenstein:

The Henyey-Greenstein function scatters light either forward or in reverse depending on the anisotropic_bias provided to the function which must be a floating point value between -1 and 1. A value of 0 will cause isotropic scattering (identical to the isotropic() bsdf) while positive values produce forward scattering and negative values produce reverse scattering. The extrema of -1 and 1 cause all light to be scattered in a single direction, back toward the light for -1 and without any directional change for 1.

Ah, well, see the example.

phase_shape_example.hipnc

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Very intresting! Big Thx for putting this together!

It makes a lot more sense now! Its still a bit hard to control this effect and predict what it will look like, but at least I now know what it is good for and how it works

Is there somewhere a list available where you can look up phase values for different kinds of materials?

Again Thx alot for this quick help and awesome example! I'll continue to experiemnt