Masking IOR for 2 different values in one CoronaMtl

[Tanner]

Hi everyone,

I've created a model originally with mental ray and have since converted it for Corona.  Its a simpler model but with a complex series of diffuse, spec, gloss, opacity, bump, and emmisive maps.  There are parts that I would like to have an IOR value of 2.2 and other parts (metal, mirrors, highly reflective strips) to be 20.0 or more.  I've created a simple black and white mask to separate the different elements and plugged it into the mask slot of a mix map.  Using 2 falloff maps set to Fresnel (2.2 and 20.0), I tried plugging them into the two layer slots but 2.2 from a falloff map doesn't create the same effect as 2.2 using CoronaMtl's native value (Fresnel 20 works but not as well as the native 20.0 value).  Is there a way around this using a mask?  I can't separate elements of the model for two different IOR's and if it comes to that then I'll just stick to 2.2 across the whole model.

Thanks for any help with this!
Tanner

[romullus]

It's actually very simple - just replace your fallof maps with output maps with according output amount and it will work as you're expecting. If you want to use fallof map to guide reflectance, then you should set IOR to 999 and plug fallof into reflection slot.

[Juraj]

2.2 from a falloff map doesn't create the same effect as 2.2 using CoronaMtl's native value

Fallof IOR model is computed differently than Corona's native. Corona native's is more correct (but still faulty regarding to reflectance dimming but 1.5 will solve that). Putting 999 into IOR still yields a little clamp that further accents custom IOR driving maps (according to Dubcat's excellent guide, have a read : https://forum.corona-renderer.com/index.php/topic,11377.0.html )

Alternatively, best solution here is to use CoronaBlend material and use the mask to separate two different Corona materials. This is easiest, physically most correct way to separate metal from non-metal. You don't have to use tons of mix maps, hacks and can use material specific features like anisotropy on metals..

[Tanner]

Thank you, both of you.  I ended up using a CoronaLayerMtl for it but I agree Blend would have worked as well.

[Juraj]

That was just me having poor short-term memory, by 'CoronaBlend' I meant CoronaLayeredMtl" :- ) not native 3dsMax Blend. Although at two mats, it doesn't make any difference at all.