reflective IOR values for dialectics and conductors

[sharktacos]

I've been reading in various papers on physically based rendering that the reflective IOR for most dielectrics is 1.5 (which was not surprising), and that the reflective value for conductors (i.e. metals) is between 10 and 1000. For example some say gold would be 50. Others say gold would be IOR: 1000.

What makes things more confusing is that http://refractiveindex.info says the reflective IOR for gold is 0.27 which just leaves me scratching my head since this is obviously not the value we would want to put in a VrayMaterial. The other IOR values it lists for dielectrics (like plastic) or glass seem as expected, but the IOR values for conductors are all quite low.

So my question is what would be the recommended value for the IOR of metals (conductors), and is there a good source that we can look at to get these values?

thanks!

[sharktacos]

As a followup to this, my understanding is that the way fresnel is calculated in the Vray Material works for dielectrics, but for conductors the fresnel is more complex. This is reflected on http://refractiveindex.info where the IOR values for dielectrics are represented as n, but with conductors they are represented with n & k. Vlado wrote an OSL texture using this complex fresnel model for conductors that can be used in place of the VrayMaterial fresnel. So using that is certainly an option. My question is, if we are simply using the regular fresnel in a VrayMaterial what would the recommended value of, say gold, be?

[joconnell]

What happens with most metals is rather than being a curve that gradually goes from no reflection at the front to full reflections at the glancing angle, they tend to have a high percentage of reflection at the front which stays consistent until roughly 50 degrees off angle and then curves upwards towards 100% reflective at the 90 degree angle. The faces from 0 50ish degrees may take up the majority of the pixels of your object compared to the faces going towards 100% reflectivity, so if you only have one value to use you might be better inputting a value for the consistent reflection on the more front facing parts, which is pretty much just a solid colour and no fresnel at all. If you want to go a bit fancier, you can use two values using a ramp and a facing ratio with your solid colour in the very left swatch, another solid colour swatch about 55 / 60% along (have a look at the refractive index curve to see the shape of it) and then a white swatch at the very right. This should get you something pretty similar. If you try to use the regular single number fresnel with a really high value, it's got some strange dips in it that aren't too realistic so it might be best avoided.

Other bits that can be handy are in theory, metals don't have any diffuse to them, it's just varying degrees of roughness that gives them the appearance of some kind of diffuse. If you want a slightly rougher looking metal, you can use some glossiness and then use the same colour or ramp from your reflection slot in the diffuse slot, just much much darker. Also set your energy conservation mode to monochrome so your coloured reflections don't tint your diffuse some manky colour. You can also use a few different levels of glossiness blended together in a vray blend material, using monochrome values as your blend amount colours to again prevent any tinting from the energy conservation.

The above ideas aren't mine by the way, they came from an old fxphd course done in mental ray by the very talented robert harrington who was head of 3d in framestore commercials. He did the proper curve bits for reflections using individual curves for the red, green and blue mixed and then just used a simple colour for a comparison, and there wasn't a huge amount of difference in the final look so he came to the conclusion for short run jobs like commercials, a solid colour and no fresnel was a good solution.

Lastly, the refractive index info is taken from scanned samples so it's very accurate, but only to the look of that exact sample. You might find that typing in their numbers for your gold value results in something that's more orange or more yellow than you really wanted and that's down to the data being exact for their sample of gold, and not yours. You can treat their data as a starting point and an overall shape of a reflection curve for gold but might have to take some artistic decisions with the hue of the colours it generates to get something like what you want.

[sharktacos]

Thanks Joconnell, very helpful as always .

set your energy conservation mode to monochrome

I see where this is in Max. Does Maya have this for the VrayMtl? It's not mentioned in the docs.

Also I'm curious of you've worked with the Artist Friendly Fresnel OSL, and what you thought of it. http://forums.chaosgroup.com/showthr...-in-Vray/page2

[joconnell]

I can't find it in the maya docs like yourself. The next thing to do would be use a blend material and only use grey vales in the blend amount swatches so there's no tinting. The new GGX brdf might get you all the way there in one material for starters, and if not it shouldn't be too much slower if you're only blending a simple diffuse base with your ggx for reflections.

On the OSL bits, I haven't tried any of them yet since I haven't had any major metal jobs recently but I've a car crash shot coming up soon so I'll be using this and the new vray speckle maps that chaos have been working on (if the car has flakes!)

[svetlozar.draganov]

Thanks Joconnell, very helpful as always .



I see where this is in Max. Does Maya have this for the VrayMtl? It's not mentioned in the docs.

Also I'm curious of you've worked with the Artist Friendly Fresnel OSL, and what you thought of it. http://forums.chaosgroup.com/showthr...-in-Vray/page2

This option is not available in the default VRayMtl in Maya but you could use BRDFVrayMtl from V-Ray plugin which has it and it is basically the same material:

[joconnell]

Actually that's a thing too - your cloth shader looks great, do you have any links for intro to osl stuff or did you just jump in and start messing with other peoples examples?

[sharktacos]

This option is not available in the default VRayMtl in Maya but you could use BRDFVrayMtl from V-Ray plugin which has it and it is basically the same material:
[ATTACH=CONFIG]25060[/ATTACH]

Thanks for the workaround. Is there a reason the Maya VrayMtl is different in this regard from the VrayMtl in other software (Max, Rhino, Modo, etc)? Are there any plans to add it in with a future release?

[sharktacos]

Actually that's a thing too - your cloth shader looks great, do you have any links for intro to osl stuff or did you just jump in and start messing with other peoples examples?

I decided, for the sake of people searching the forums later, that it would be better to answer that over on the thread with my OSL fabric shader. Hope you don't mind:
http://forums.chaosgroup.com/showthr...326#post659326

[svetlozar.draganov]

Thanks for the workaround. Is there a reason the Maya VrayMtl is different in this regard from the VrayMtl in other software (Max, Rhino, Modo, etc)? Are there any plans to add it in with a future release?

There are some very small discrepancies in VRayMtl and other V-Ray features between different 3D Platforms and this is one of them.
I don't know the reason because of which this options does not exists in the default VRayMtl in Maya but if it is rather important to you I could make a feature request about it.