Information on Translucency Colour and Transmitted Colour Please

Please tell me how these colours show up in the render. I'm not asking for too much technical detail, more how they affect what we see.

Thanks

Comments

  • PatroklosPatroklos Posts: 502

    Is there anyone who could give me a rather less technical explanation ?

  • JonnyRayJonnyRay Posts: 1,743
    edited May 2019

    I'll give it a shot. :)

    Transmission

    Let's start with Transmission because it's easier. Whenever light strikes an object, it can do one of three things. It can be absorbed, it can reflect, or it can be transmitted. Transmission color affects how the light passing through an object will be tinted as it goes through.

    You didn't ask, but transmission depth says how far the light has to go through an object before it changes color. So some materials may only tint the color of the light if the light hits at a low angle and passes through a lot of the material. If the light shines straight on, it might go through without any color change. Think of a piece of glass with only a slight tint to it. If the light goes straight through you may not notice, but tilt the glass at an angle, sending the light through more material, and now you see the color of the glass in the light.

    Translucency

    Now, for some surfaces the transmission factor isn't quite as clean. If you shine a really bright light like a bright flashlight or laser pointer at your skin, some of the skin around where the light strikes your hand will appear to "glow". That is light which is being scattered by your skin. It's not a coincidence that this is part of the "Base" portion of the Iray Uber shader. Translucency is calculated and blended with your diffuse color settings and happens separately from the volume effects like subsurface scattering and transmission.

    In the Iray surface, Translucency has a few properties which have to be considered together.

    • Translucency Weight - Is factored together with the Diffuse weight to figure out what percent of the light striking the surface is affected by these parameters.
    • Base Color Effect - Determines what happens to the light that's being modified by this part of the material
      • Scatter & Transmit (default) - Whatever light isn't absorbed by the translucency is passed through to transmission
      • Scatter Only - Basically you only get the "surface glow" effect. No light passes through the surface for transmission
    • Translucency Color - Determines the color filter applied to any light that is being affected by this channel. This affects both the scattering and transmission of the light.

    Interaction

    So, basically, translucency is calculated first as part of the base. It will also effect what light is going past the initial interaction with the surface for further processing by the transmission and other Volume properties. If you have a color for translucency, it will change the light color BEFORE it gets to Transmission; so both filters will be applied to any light that passes completely through the object.

    Please let me know if any of this wasn't clear and you need more information?

    Light-Surface_ReflectTransmit.png
    960 x 541 - 13K
    Light-Surface_ReflectTransluceTransmit.png
    960 x 560 - 14K
    Post edited by JonnyRay on
  • Oso3DOso3D Posts: 14,572

    There are several different volume effects of greater/lesser complexity.

    Translucency is basically a highly simplified transmission of light through a surface. It's good when something is still pretty opaque. Translucency color tints light going through the item. Darker translucency will effectively lowers the translucency effect (since the 'light' is darker/ ie less).

    Refraction Color does pretty much the same thing, but with the more accurate refraction system. (by default there's an option for 'Share Glossy Inputs'; this replaces things like refraction color with glossy color. If you are doing anything with refraction you probably want it off)

    This behavior shifts a bit depending on whether Thin Shell is on or off. When it's on, the effect looks pretty much like you'd imagine an egg shell or blown glass to look. When off and the mesh doesn't have gaps, it's more like a liquid or mass of glass or flesh or whatever.

     

    Now there's two funkier bits... transmission color and subsurface scattering.

    Subsurface scattering scatters light (duh) as it passes through the object. If refraction index is 1 and weight 1, normally you end up with an invisible object. Give it some SSS, and it starts looking like haze or fog, because light, as it passes through, gets scattered in all directions.

    SSS direction accentuates the effect at an angle to the light. A positive value means light scatters more forward; if the light is shining on the camera and the object is in the way, it will look extra foggy. If you are off to the side or the light is behind you, it will look more transparent.

    A negative value scatters light back toward the source, so the effect is opposite of previous.

     

    Transmission color adds color based on distance into the object. So if refraction color is white and transmission color is blue, you might have an object that looks blue in the middle but not the edges (again, depending on transmission distance, which basically scales the effect).

     

    So, let's say you want to make some ocean water out of a cylinder. You opt for realism, with Refraction Index 1.33 (the index of refraction for water). You set refraction color to a very very very light blue. Then transmission color to dark blue-green, and tweak the transmission distance appropriately. You can probably skip SSS because the effect of darker color and refraction probably will hide the depths, but you could always add a bit if it was too transparent.

    I hope that was clear enough. Heh

     

     

  • PatroklosPatroklos Posts: 502

    Thank you all!

    I had no idea this would be so complex.

    I have taken copies to refer to in future.

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