Which Intensity Determines Reflection Parameter in Materials?

[i313lluminance]

Hello Coronauts,

Does anyone can tell "what kind of reflection intensity determines Reflection parameter in corona materials? The one we see looking frontally at the surface (F0) or the one we see at grazing angles, so called 100% intensity?

Thanks in advance for help!

i313

[Juraj]

The Specular reflectance in Corona (or Vray,etc..) is the grazing one. Changing it to anything else than 1.0 255/255/255 is clamping it therefore.

IOR determines frontal reflectance. Default IOR of 1.52 equals to 4perc. frontal reflectance.


So, in theory, it really is the same as in any other PBR renderer, except the controls are inverted. But in practice, you won't get the same results for few reasons..

[maru]

Already replied on Facebook but pasting link here too as it may be useful for someone:
https://forum.corona-renderer.com/index.php/topic,11024.msg70304.html#msg70304

[i313lluminance]

Thank you Juraj for the great and super quick explanation. So going further with this, is it a true statement that any texture in the reflection slot will give then non-physically correct results?

The other thing I have realized is that Albedo value is a sum of diffuse + reflection intensities. Can we estimate then, that a white plaster wall which albedo is meant to be 80 (according to "Neufert") should have:

DIFF=202 RGB (sRGB 229) which is basically 79,2% of brightness, 0.79,2*255=202
REFL=1.0
IOR=1.52

which will result in exactly 80% albedo at the front angle with value of 204RGB (0.8*255=204)?
Would these settings be true and give correct results in your opinion? Of course if we consider plaster to have IOR of 1.52 in this case ...

[mtanasiewicz]

Neufert - You mean Ernst Neufert? He wrote something useful for rendering in his book? It looks like I will need some review :D

[Juraj]

Neufert - You mean Ernst Neufert? He wrote something useful for rendering in his book? It looks like I will need some review :D

Every architect's bible :- D

[Juraj]

Thank you Juraj for the great and super quick explanation. So going further with this, is it a true statement that any texture in the reflection slot will give then non-physically correct results?

It would be physically correct as Corona will always adhere to energy preservation law, but it could be questionable in terms of physical plausibility as all the material modelling resources say otherwise.

But I honestly don't know.... Keeping full specular in every PRB renderer for rough materials give me correct look, but not in Corona. The material literally shines more than I would expect... so I do in fact lower reflectance for rough materials.
At least in Corona. I don't do so in Unreal4.

Thank you Juraj for the great and super quick explanation. So going further with this, is it a true statement that any texture in the reflection slot will give then non-physically correct results?

The other thing I have realized is that Albedo value is a sum of diffuse + reflection intensities. Can we estimate then, that a white plaster wall which albedo is meant to be 80 (according to "Neufert") should have:

DIFF=202 RGB (sRGB 229) which is basically 79,2% of brightness, 0.79,2*255=202
REFL=1.0
IOR=1.52

which will result in exactly 80% albedo at the front angle with value of 204RGB (0.8*255=204)?
Would these settings be true and give correct results in your opinion? Of course if we consider plaster to have IOR of 1.52 in this case ...

I agree with this, same line of thinking as I understand the concepts.

[mtanasiewicz]

Neufert - You mean Ernst Neufert? He wrote something useful for rendering in his book? It looks like I will need some review :D

Every architect's bible :- D

That's what I thought. I just didn't know there was anything about materials reflecting light :D

[dubcat]

But not in Corona. The material literally shines more than I would expect... so I do in fact lower reflectance for rough materials.

This !

I've been doing some Reflectance/Angle of incidence tests for "IOR 1.5" and "Iron" in V-Ray, since V-Ray has the same GGX look as Quixel/Substance.
I started with 1.0 Glossiness and worked my way down to 0.0 with 0.5 increments.

My test results show that it's not enough to just reduce the "Reflection Level" in Corona, you have to reduce the IOR too.
I'm going to compile a list for "IOR 1.5"/"Iron" and post it in my Quixel thread later.

Glossiness 0.0 is the hardest one to match with the current Corona GGX, but I think my test results got it pretty close.
If you have a material with IOR 1.5 and Glossiness 0.0. You need to change IOR to "1.27" and "Reflection Level" to "0.62" (These are not perfect, I will be making falloffs later).

This is going to make my Quixel life a lot easier!



Edit:
Here are the test results for IOR 1.5. Glossiness 1.0 to 0.0 from top to bottom.
I'm not adjusting any IOR or "Reflection Level" here, this is what glossiness does.

[Juraj]

Another perfect research by Dubcat :- )

So Corona is basically always wrong with reflection with exception of purely 1.0 glossy specular diffuse material. And it gets progressively worse the rougher the material gets.

I did your correction by simple guessing only, I have no brain or patience to do any kind of actual comparisons of anything. But too kept lowering IOR esp. for horizontal rough materials (soaped wood floors,etc.. ) and clamping the level.

Shown so to my students in Venice this week too. It kinds of irks me when I tell them "simple rule to live by is...ABC, but oh yeah, when rules doesn't work because of... I don't know what... please break the rule". Breaking rule for artistic reason is absolutely great, but not when the rule doesn't work in first place.

Would getting rid of the glossy curve solve this ? Or would this simply extend the options to go lower than current 0 but keep the incorrect reflection level ?

Let's get this thread back into General section, this needs more attention that other stuff.

[dubcat]

So Corona is basically always wrong with reflection with exception of purely 1.0 glossy specular diffuse material. And it gets progressively worse the rougher the material gets.

Yes. F0 is always 59 RGB and the gradient is getting somewhat darker down to 0.6. 0.5 and bellow actually becomes brighter again!