Translucency is the superset of Transparency and Subsurface Scattering.
With that in mind, in order to eliminate some conceptual confusion, it is necessary to include Opacity into the equation. Opacity (mass density of matter) is also inclusive on Translucency. So it goes like Opacity>Translucency>Transparency...
There is an inverted relationship between Opaticy and Translucency/Transparency and Opacity is the dominant one here. We need to decrease the Opacity in order to achieve Transparency. Vice versa is not true here. Transparency has no effect on Opacity. Transparency simply works under the Opacity channel with its own parameters like Refraction, IOR, TIR etc.
Let's see it with simple examples from Cinema 4D Material Editor:
In C4D, Opacity is called as Alpha.
In the right hand we see that the parameters like Color, Diffusion, Transparency, Reflectance, Glow , Bump are activated. We can see the result at the top preview sphere.
Alpha/Opacity is not introduced to the equation so it is disabled.
In the left hand we simply activated Alpha/Opacity with value of zero or black. As a result everything disappeared. No reflection, refraction nothing. So what is that? Simple 0 Opacity means 0 density means matter is not exist. If there is no matter, there is no properties of matter.
Introducing Opacity is just an option depending on software.
After all this confusion, let's get back to our main topic.
Translucency
Translucency is also referred to as Back-Lighting in some softwares.
Logic is that: the light tries to penetrate a thin, low density object which has some consistent micro irregularities and these irregularities will break the linearity of light and scatter it before light exits the object. As the result of scattering, while some rays return as reflection, some other will exit from other side as refraction but scattered and we get blurry image.
Usage areas and the some objects obvserved are paper, fabric, leaves, some plastics etc.
Paper example. Due to scattering hand is just lost a lot of details.
But, because of the closeness of the hand to the paper, hand will occlude most of the incident rays coming from all angles and this will allow only the most linear rays can pass thru which will decrease the scattered ray amount and as result we can see the outline of the hand and some colors.
If the hand gets away from the paper, a lot of incident rays will interfere between hand and paper so resulting complete blurry non recognizable image.
In full Transparency distance of hand doesn't matter at all. We can clearly see every detail thru the glass.
Subsurface Scattering
Lets call it as SSS since all softwares uses that way and it is a torture to write Subsurface Scattering all the time.
Now, if we understand Tlanslucency which we can consider as 2D effect as we see in the paper, SSS is just the 3D verison of it.
In logic : Light tries to penetrate a Translucent but high depth, thick object. When light enters to that object, rays will start to scatter very intensely due to density provided by depth. Considering that density of object is not consistent in all areas, most of the rays will be scattered, returned and absorbed in more dense areas like object center while other rays can escape from less dense areas like edges and exits from object. In this process escaped rays will change color due to absorption of most wavelengths.
As we see in the picture SSS doesn't work that simple like basic Translucency. Light coming from back will be absorbed and returned back from most dense areas, in this instance, center of the fingers. This density will not allow that light penetrate the object center so no rays will exit from center.
If density starts to get lower, light will start to penetrate object and will exit from back. We can see it at the edges of finger. While rays exit notice that all wavelengths either absorbed or returned back except around red color so we see the incoming rays as red.
Earlier on we explained that the image reaches our eyes as Diffuse or Specular Reflection. Skin is an ideal and common example for SSS behavior. Only 4% of the light reflected from the skin is direct Diffuse reflection, while the remaining 96% is indirect Diffuse reflection that passes through the skin and returns by SSS.
Deriden yansıyan ışığın sadece %4 ü doğrudan Diffuse yansımasıyken kalan %96'sı deriyi geçip SSS e maruz kaldıktan sonra geri dönen dolaylı Diffuse yansımasıdır.
Common examples of SSS are skin, fresh meat, candles, wax, milk, ketchup, marble! etc...
Lets list some parameters in softwares:
General Amount = Overall effect of SSS.
General Radius Scale = Determines how deep the light can penetrate the object.
Layers
We need to explain layers. Imagine a sphere or a head.
Layer 1 is the initial interaction point of light with the object.
Layer 1 Radius defines an area starts from outer surface of object to maximum depth which can be penetrated by light.
And the Layer 1 Color defines which color will be returned from this area. If you define red, then red you will see.
Notice that if it is called Absorbtion Color this means given color will be absorbed and the combination of the rest of wavelength will be returned. If it is called Scattered Color then we are back to the red.
Layer 3 is inverse of Layer 1. It is the center of density. Radius will define an area starting from center. Color goes same way.
And the Layer 2 is the area in between of 1 and 3.
You maybe tried to assign some colors to layers and radiuses and did get a weird result.
The main reason for this is the transition between the layers. One can not simply draw the borders for layers. And the colors will be merged in additive way and we will get an unintended result. Exception is the Layer 1 which starts with outer surface. Layer 1 can keep its color more accurately compared by the other layers because of it is most ray dense area. Lets see how Layers, Radius and Colors works in software:
Focus on the bottom two.
Left one's Layer 2 color is pure Blue but due to addive mix with Layer 1's Green we get overall color like dark cyan or aqua. Compare it with the M in text.
At the right one we see same addition is just more amplified with adding the Red as Layer 3 color. With the combination of Red and Blue we get overall color like dark Magenta.
Now notice that in both example, how Layer 1 protects its Green color to some degree. In both image Green is noticable at the edges. We can of course break this by increasing radius of Layer 1.
While the layer logic is called as 1,2,3 in many softwares and rendering engines, it may be called as Shallow, Medium, Deep in some other.
Some softwares may have sample value which defines the quality of SSS.
In next chapter, we will try to examine what are the Shader, Material, Texture, Map and what they are not.
Stay safe.
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