How to proper use the IOR values for Metals?

[axelts]

I'm studying how to implement in the physical correct way the metal materials in Octane for Blender, but unlucky there's not so much documentation of the official online guide about it.

I've tried to follow the official guide about Octane for C4D on YouTube. In C4D there's a field for the K and N value that you can find at this page: https://refractiveindex.info/ (Look from the Shelf - 3d selected data for Artists)

In Octane for Blender, in the Universal Material, we have three slots for Metallic IOR, Metallic IOR (green) and Metallic IOR (Blue). What are the X, Y, Z values refer to? I've supposed the X was the wave length, Y the n value and Z the k value. Unlucky, when trying to fill the date in this way, I not get the correct results on how I was supposed.

Example 1

Copper IOR

Here I've tried to create a Copper Material using the values from the refractiveIndex.info and the supposition of X,Y.Z like I've written above. It turned out like a sort of blue metal.

Example 2

Gold IOR

Here I've tried to create a Gold material. If I use all the three fields of IOR, I was getting again a blue kind of metal. Using only the IOR blue value I got a sort of weak Gold color.

Example 3

Gold HEX color

Here I've used the "Default" selection on the universal material node and used a HEX color value on the Albedo color. The HEX color I got from this post on Chaos Group Blog where they talk about Metalness workflow and there's a table of RGB / HEX values for Metals: https://www.chaosgroup.com/blog/understanding-metalness

At this point, I would like just to know how to use in the right way the IOR of the metal in Octane for Blender to get a good (and physically correct) metal material.

[grimm]

There is no information on these in the Blender Plugin manual, but in the stand alone manual there is this...

Metallic Reflection Mode - Changes how OctaneRender® calculates the Metallic material's reflectivity.

Artistic - Uses the albedo color.
IOR + Color - Uses the albedo color and adjusts the brightness using the IOR.
RGB IOR - Uses the three IOR values for 650, 550, and 450 nm, and ignores albedo color.
Metallic IOR - Complex-valued Index Of Refraction (n-k*i), which controls the Fresnel effect of the Metallic material's specular reflection. For RGB mode, this serves as the Index Of Refraction for the red light (650nm).

Metallic IOR (Green) - For RGB mode, this is the Index Of Refraction for the green light (550nm).

Metallic IOR (Blue) - For RGB mode, this is the Index Of Refraction for the blue light (450nm).

I'm assuming that the mode values are 0 = Artistic, 1 = IOR+Color, 2 = RGB IOR, and 3 = Metallic IOR

So I think you will need to change the mode to 2 and use the values you have?

Jason

[axelts]

There is no information on these in the Blender Plugin manual, but in the stand alone manual there is this...

Metallic Reflection Mode - Changes how OctaneRender® calculates the Metallic material's reflectivity.

Artistic - Uses the albedo color.
IOR + Color - Uses the albedo color and adjusts the brightness using the IOR.
RGB IOR - Uses the three IOR values for 650, 550, and 450 nm, and ignores albedo color.
Metallic IOR - Complex-valued Index Of Refraction (n-k*i), which controls the Fresnel effect of the Metallic material's specular reflection. For RGB mode, this serves as the Index Of Refraction for the red light (650nm).

Metallic IOR (Green) - For RGB mode, this is the Index Of Refraction for the green light (550nm).

Metallic IOR (Blue) - For RGB mode, this is the Index Of Refraction for the blue light (450nm).

I'm assuming that the mode values are 0 = Artistic, 1 = IOR+Color, 2 = RGB IOR, and 3 = Metallic IOR

So I think you will need to change the mode to 2 and use the values you have?

Jason

Thanks for the reply and to point me out about the guide for the Octane Standalone. I am referring myself more on the C4D guide that I found more detail in the explanations compared to the Blender one...

So reading the information from the guide above I've supposed it almost correctly. And I've set already my material to RGB IOR. At this point is just to understand what the X,Y,Z value stand for. I suppose the X isn't for the wave length since it's given that the first slot is for the red 650nm, the second for the green 550nm and the third for blue 450nm. In the refractiveindex.info I find only the values of "N" and "K", I miss the "i" that is written in the above formula (n-k*i)

[grimm]

You don't need to worry about i, it's for the incident ray. N and K are needed though and I noticed that the Blender plugin is missing the two fields for this, unlike stand alone. Probably a bug, you should put a report for it. You might try to subtract K from N and put that value in the fields?

[grimm]

Ack, ok I really need to take a look at the software itself instead of the manual. So for both metallic and universal shaders there is no mode setting and each IOR values have x,y, and z values. I have no clue as to how that is supposed to work, sorry.

[axelts]

You don't need to worry about i, it's for the incident ray. N and K are needed though and I noticed that the Blender plugin is missing the two fields for this, unlike stand alone. Probably a bug, you should put a report for it. You might try to subtract K from N and put that value in the fields?

Ok, I've figured out doing some tests now: X value is for "N", Y value is for "K" and Z can stay just "0".

In this way the materials works perfectly:

Gold

Gold IOR correct

Silver

Silver IOR correct

Copper

Copper IOR correct

[grimm]

Awesome, but that is not intuitive at all.

[axelts]

Awesome, but that is not intuitive at all.

I totally agree with you and was good that putting together our brains we figured it out! Thanks for the help!

[ChrisH]

No, the inputting of metal IOR in Octane Blender isn't the most intuitive. I think it's mostly due to some limitations in Blender's node system (no "tuple" type, etc).

I while ago I got tired of collecting the IOR values and entering them into the inputs etc, so I made a "library" of it: https://chris.hindefjord.se/resources/rgb-ior-metals/ (The values from Chaos group for v-ray are in in "byte"/"integers", while Octane takes the full floats)

[grimm]

Thanks Chris, that is a very handy chart.

Jason