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I am trying to get better dispersion glass and was exploring different solutions. When I tried to do light addition with Principled BSDF shaders, I got a weird lighting as if it were emissive.

enter image description here

When I did it with glass shaders, it was better but very dark in cavities.

enter image description here

Which I assumed is the good old 'infinite ray depth' issue in glass and can be fixed by adding a threshold for transparency :

enter image description here

The normal glass shader looks like that for comparison.

enter image description here

My questions :

  1. for my knowledge, why is the Principled BSDF exhibiting such behavior?
  2. why are additive glass shaders behaving worse than the original? Is it the randomness of rays in the solver not fully matching for each primary color (R,G,B)? But then how to improve it?

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    $\begingroup$ it's not just the shader, it's also the light and the light path -> how high did you set the transmission/transparent light path? $\endgroup$
    – Chris
    Commented Apr 17 at 10:36
  • $\begingroup$ For the first case, everything is in the snapshot, it is only Princ. BSDF with 100% transmission. For the third with the Light path, I considered the physics of reflections and transmissions occuring naturally in glass, up to infinity. See this image (I don't know how to put images in comments) : researchgate.net/figure/… $\endgroup$
    – Jag JB
    Commented Apr 17 at 11:49
  • $\begingroup$ Complementary comment : the Light path setup considers that after a certain number of reflections/transmissions not so much details are added and that it would require to increase a lot the number of maximum bounces of transmission and glossy, so an arbitrary cutoff is applied after 6 interaction in this case to make the ray transparent instead of becoming dark due to not enough bounce computation. $\endgroup$
    – Jag JB
    Commented Apr 17 at 12:04
  • $\begingroup$ pls provide blend file so we can check it out and improve it. thx. $\endgroup$
    – Chris
    Commented Apr 21 at 10:38
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    $\begingroup$ Yes, it's more theoretical indeed. But your image looks very nice! Happy to discover your node setup anyway. Thanks for your contribution! $\endgroup$
    – Jag JB
    Commented Apr 21 at 12:39

2 Answers 2

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Why does your shader with Add Shader is darker ?

By adding together the 3 colored Glass shader, you are asking from Blender to :

  • Let only red light pass through
  • And let only green light pass through
  • And let only blue light pass through

The result is that no light passes through, unlike in the case of a white glass shader. The only light reaching the shadows will be indirect light bouncing from other surfaces, not refracted light.

Why does adding 2 Principled BSDF is emitting light ?

Disclaimer : I'm not 100% sure about that one.

Principled BSDF also have a glossy component. You could bypass it by adding roughness (then glossiness is 0), but then you'd loose the clear glass refractions you currently have. So by adding the shaders, the object emits 2x more light from glossy lightrays.

Custom prism shader

I've build 2 easy shaders. The first one is simple, it works but RGB will be separate and not really realistic. The second one is a bit more complex, using White Noise texture as a base to blur the IOR (credits to this video).

enter image description here

enter image description here

enter image description here

The mapping node is only to translate the white noise for each channel and be sure we don't have some kind of overlap. I think it's useless because of the different IOR.

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  • $\begingroup$ The first shader is actually exactly the one I mentioned in my question and I also know the second one, but I wasn't asking for shaders proposals. In any case, thanks for the explanations in the first part as that's exactly what I was looking for. $\endgroup$
    – Jag JB
    Commented Apr 25 at 19:52
  • $\begingroup$ So all the shaders of my question behave how they meant to. It's not a bug. $\endgroup$
    – Jag JB
    Commented Apr 25 at 19:53
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    $\begingroup$ @JagJB That's right I don't think it's a bug. Cycles is already some years old now, I would expect that kind of bug to be detected by now. But I'm only 80% sure of my answer. $\endgroup$
    – Lutzi
    Commented Apr 25 at 20:31
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I know it's not the answer, but you wanted my node setup:

here it is:

enter image description here

The "rest" as i wrote in my first comment, are "other" things.

these are my render settings:

enter image description here

light path:

enter image description here

i know, that they don't have to be all 60, but it renders fast anyway.

important are these:

enter image description here

transparent + transparent glass

then i just gave that plane a shader, added 3 lamps and a background hdri.

result:

enter image description here

if you need a blend file, let me know.

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