Poilished or Brushed Steel IOR ?

[stilgarna]

Hello Vlado,

I'm always using Fresnel Reflections in VRayMtl.
I'm currently using a hight value for simulating polished steel (IOR : 15). This is only a guessed value, nothing physical or mathematical, maybe 5 or 6 is enough.

What is the best IOR value (for VRay world) for :

Polished Steel ?
Brushed Steel ?

Best regards.

[Wobi]

You know this site?:

http://refractiveindex.info/?group=METALS&material=Vanadium

I found it quite helpful looking at the reflection curves when scrolling down the pages. Though there is no "steel" material in there, but maybe this helps...

[joconnell]

Yep - metals don't have fresnel, they've an odd curve instead which doesn't obey the usual laws. As a rough rule it's normally flat for the majority of the front on angle until it reaches about 75 degrees, then it dips downward for a bit and for the final section nearing 90 degrees it shoots back up to near 100 percent reflectivity. The curves at the bottom of that page show that type of behaviour in the reflectance calculator section. Also of note is that the strength of reflection is different for each colour hence gold or copper reflecting yellow colours more. A high fresnel or even a non fresnel simple colour value is probably good enough, otherwise try making a curve like the ones graphed using the output curve of the falloff map and see if it looks any better to you.

[stilgarna]

You know this site?:

http://refractiveindex.info/?group=METALS&material=Vanadium

I found it quite helpful looking at the reflection curves when scrolling down the pages. Though there is no "steel" material in there, but maybe this helps...

Thank you, very interesting and useful.

Yep - metals don't have fresnel, they've an odd curve instead which doesn't obey the usual laws. As a rough rule it's normally flat for the majority of the front on angle until it reaches about 75 degrees, then it dips downward for a bit and for the final section nearing 90 degrees it shoots back up to near 100 percent reflectivity. The curves at the bottom of that page show that type of behaviour in the reflectance calculator section. Also of note is that the strength of reflection is different for each colour hence gold or copper reflecting yellow colours more. A high fresnel or even a non fresnel simple colour value is probably good enough, otherwise try making a curve like the ones graphed using the output curve of the falloff map and see if it looks any better to you.

I think, I'm already close to your answer. But I only want to known Vlado's point of view, from the inside.

I know it works, but maybe Vlado will said : Don't go over 10.

I know we are not talking about real life, but about the best way to simulate a texture without going too far with VRay rules.

VRay is our Matrix, so if I want to fly I need to know the rules

[joconnell]

Yep - I'd love to know how to interpret that curve data - the bottom one seems to be based on the viewing angle to the surface but I'm not sure if it's a case of just replicating one of the three coloured curves as a simple version, or there's a better way to use the other data above to do something more accurate.

[joconnell]

Heya Stilgarna,

Just to follow this up with you, I did some digging into this and found out a good bit more about fresnel curves. For lower fresnel values the curve is generally very flat and low for the majority of the surface, and then it starts to turn upwards at 60 degrees until it reaches 100% reflective at the 90 degree angle. If you try to use very very high fresnel values (10 or more), the facing values are very reflective, but then around the 40 / 50 degree angle they take a very sharp dip downwards until maybe 75 / 80 degrees and again shoot back up to 100% reflective at 90 degrees. This isn't very natural and it definitely doesn't match the type of reflective behaviour you get with metals.

I got in touch with Mikhail, who writes refractiveindex.info and asked him a few questions on the curve data that he has. His graphs calculate the reflectivity of a metal at certain wavelengths of light and shows you how intense the reflection is between 0 and 90 degrees across a surface. So what we can do with this is at the top of the page you can type in what wavelength of light you want to see a curve for. So if is you use the reflection calculator at 0.475um, 0.510um and 0.650um they're the wavelengths that correspond with blue, green and red light and you'll get an accurate curve that you can recreate per metal.

In terms of actually implementing this there's a few things to take into account.

1. The falloff curve won't let you use different red, green and blue curves in the same map so you'd have to do some kind of custom setup blending 3 different falloff maps. HOWEVER what you can do instead is use the gradient ramp map and set the gradient type to "mapped" using a default falloff map in the map source - this will make the gradient map use the black to white transition of the falloff map to decide which part of the gradient gets used. If you maybe put in 6 different flags on the gradient evenly spaced, you could take the r, g and b values from the refractiveindex.info curves at 0,15,30,45,60,75 and 90 degrees and get some pretty accurate values. Annoying to do but for each metal you only need to do it once and you have a reusable map for ever.

2. For all of the metals, the curve normally stays flat between 0 and around 60 degrees - while the amounts of red, green and blue are different,each of the r, g and b curves are flat until 60 degrees before turning upwards. In a lot of cases, the amount of a curved surface that is visible to the camera and between 60 and 90 degrees is a lot smaller than the section between 0 and 60 degrees, so if you wanted to be a bit lazy but still fairly accurate, you could ignore the curve and only take the reflectivity values at 0 degrees and put a solid colour value into your reflection slot instead.

I asked Mikhail was there any way to get the data that makes his curve as a text file so I could automate loading in the curves into something like Bercon gradient but I think he was bored of me asking stupid questions at that stage

Either way it's pretty useful stuff and might give you enough information to go on so you can construct more useful curves for your materials.

[snivlem]

Interesting post joconnell, I saw on his site you can download txt or csv files though?

http://refractiveindex.info/tmp/META...mium_sopra.txt

Taken from here:

http://refractiveindex.info/?group=M...erial=Chromium

[joconnell]

Yes, but what I'd like is text from the graph below instead which is our actual reflection strength that we could write into a gradient. The numbers that you've downloaded could probably be used too but I'd need to find out the equation to change those numbers into the curve below to get something directly useable. Nothing back from Mikhail yet but I've got a while to figure things out on it. Having the equation makes more sense since otherwise we'd be limited to the range of metals given on the site.

[elivnA]

Hey joconnell,

I've been trying to produce what you describe in your post, and am slightly confused about the execution, namely the falloff map. Isn't it so that you can use the colour map in the output tab of the falloff map? this bit here:



Instead of doing the gradient ramp thing.. or am i completely off here?

edit: ignore that, i think i have it figured out..

[joconnell]

Yep - I don't know if the falloff map is the correct way to do it, what I was thinking was using a gradient ramp map in "mapped" mode with all of the colour values of the metal set from left to right, and then putting a default falloff map into the "mapped" slot of the gradient ramp to drive the incidence.

Edit - sorry, the only reason I was looking to do it this way is that the output curves in max can't be accessed by maxscript, and I was looking at a way of automating the setup of these curves, what you're looking at seems like it should work - just might take a bit of time!

[henning]

I found this: http://www.raytracing.co.uk/?p=17
Maybe it helps a bit.

[joconnell]

That is VERY handy - it means I can get in touch with that person to find out the equation they used to plot this curve. Nice find!

http://www.raytracing.co.uk/wp-conte...2/01/au_nk.png

[Deflaminis]

I am so interested in this. Trying to follow you guys, but not quite there yet. Would this conversion table help for this? Or am I off base? http://www.ledtuning.nl/cie.php

[joconnell]

That's part of it alright - I've sent Ben who wrote the page on raytracing a mail, but it might be a bit too big an ask for a reply for someone that doesn't have a great grounding in physics. The nice thing about this is that we all don't have to know how this is done or keep redoing it, it's literally just setting up a gradient ramp once, and then having that gradient downloadable for everyone.

[Deflaminis]

The physics I understand until planetary (switched majors at that point), but it's the application into vray that confuses me. (I do pretty well with Maxwell renderer, for example.) But I'm floundering getting the right effect in vray.

If you guys do plan to make a preset for people for various metals, I would be more than happy to participate and offer my time. I'd just need a single one to study.

I've gotten a gold color based on the previously posted RGB curves (normalized) image.. it's a bit archi in it's color but it's there. Should the mix curve be part of this? Maybe the Extinction coefficient? I pretty sure I'm going about this wrong but here is what I have. I tried to incorporate the mix curve and extinction coefficient (very roughly) but I'm not convinced I'm doing the right thing. Maybe it's my lighting here, but it seems to lack some orange. Just a simple HDRI and a plane light.

Thanks for this thread, so interesting.

EDIT: In addition, on this website: http://refractiveindex.info/?group=METALS&material=Gold , there are 2 different options. Johnson and Christy and Palik. I wonder the difference in the application to vray.