SCATTERING MEDIUM

Scattering is a very broad definition. What we are talking about now is actually "Subsurface Scattering". When the light penetrates to a translucent surface, it is scattered in different directions through the particles in the surface. It then comes out again from different regions of the surface. This process, called Subsurface Scattering, depends on the characteristics of the surface, medium thickness, IOR factor, density and phase function of the particles in the surface. All of this is a simplified version of the bi-directional scatter-surface reflectance distribution function we have mentioned earlier in the BRDF topic. Vax, skin, milk and leaf surfaces are made by this process.



If you want to make a real subsurface in Octane, you should use scattering medium. You can also make fake subsurface scatter through Transmission channel, the render time is short, but you never get a realistic and beautiful result. When you use scattering medium, you can see the absorption channel which we explained in the previous section. In this case, the use of scatter and absorption together is important. These two factors all work together in the semi-transparent surfaces of the real world.


Now let's look at the Scattering Medium options. We will only include scattering options and will not include the Absorption options here because we explained it in the previous section.



SCATTERING

This parameter is used to control how quickly the scattering will take place inside the surface. At high values, light begins to scatter as it enters the surface. At low values, the light begins to scatter after a while as it penetrates deep into the surface. The candle is a good example if we are going to give a real world example. In this case, light begins to scatter from the moment it enters the candle, which is due to the fact that the number of particles inside the candle surface is "too much". Thus, as the light enters the candle surface, it quickly goes through and interacts with the particles and scattering. If you are going to make a surface like a candle, you can increase the float value (it is also important to use it with density and other options). In such scenarios, you can control the scatter speed by adding "Float" and "RGB spectrum" to the texture slot. Let's explain how Scattering works in both texture types:

USE OF FLOAT TEXTURE

When you use Float, 0 means that there is no scattering. Values greater than zero means that how quickly the scatter is happen. For example, when you assign "floattexture" to texture slot, you can easily observe this situation. As you can see in the picture below, the scattering rate increases as the values get higher. Or, the number of particles increases if we talk about candle analogy. You can increase or decrease this value according to the material you'll create.

 


USE OF RGB SPECTRUM

You can think of the scatter phenomenon here as wavelength-dependent, as it is in the real world. When you assign an RGB spectrum to a texture slot and you enter any RGB value, the dominant color becomes more scattering. For example, if you enter RGB value 23/200/244 as in the picture below, the mixture of green and blue will be heavily scattered (IOR and fresnel effect is also important). Red will be minimum scattering. If you use it with absorption, you can get a wide range of color effects. In the image below you can see RGB scatter values without and with absorption.

Info: We recommend that you test the properties of the material by a simple light setup if you can. For example, if you are going to create milk material, go get a bottle of milk and make a test shot. Observe how much the light absorbs or how much it is scattering. Of course you can find these examples online, but you can be sure to test something physically that it will motivate you more.



PHASE

It describes the amount of light from the incident light direction that is scattered into the viewing direction (towards the eye). In theory, the scattering direction is Isotropic. That is, they are evenly scattered in every direction. However, as the particle density increases, this scattering is not equal and part of the light is scattered in a certain direction. This is called Forward or Backward Scattering. With this parameter you can determine the direction of scattering. A zero value means that the scattering will be equal in all directions. Also called "isotropic scattering". -1 is "backward scattering" and 1 is "forward scattering". In the picture below you can see the differences.



EMISSION

We will explain this parameter in more detail in the "Lighting / Emissions" section. But here we briefly add that the emission power will emit scattered photons, not the medium itself.