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Daniel Whitton
02-22-2004, 11:39 PM
Hello everyone.

I have been playing around with mental ray for some time now and I just got the book Rendering with Mental ray. In the back of the book there are rendered images with small descrptions. One of which is a Mercedes SLK that has Volumic layered paint. I caint find anything in the book describing this and was curious if anyone could shed some light on this subject for me.

I am trying to apply a custom MR shder for carpaint with fresnel reflections. I have been using a DGS shader to do this. Is Volumic Layered a different approach?

Jozvex
02-23-2004, 12:20 AM
The 'Volumic Layered' shader was probably custom created by a production company who needed that 'extra something' on their shot. Mental Images probably used it as an example to show off MR's power etc.

I have virtually 0 experience writing MR shaders, so I don't know what sort of a shader it would be or how it would work.

DGS shaders should work fine, Dielectric shaders use Fresnel reflections but I'm not sure how well they work for non transparent materials.

:shrug:

sebast1an2
02-23-2004, 12:53 PM
..in this case, i think "volumic layered" only means that the shader use different layers for the ambience of the base-color and reflectivity.. like dgs for base color and the dieelectric material for the fresnel effect..

so the only question is:

"how to build a layered shader with mr custom nodes?":shrug:

bmcaff
02-23-2004, 04:13 PM
I have created this effect by doing the following.

create a DGS and set diffuse colour
create a dielectric set colour to black and IOR to 1.8 or 2.0
link lights
create a MIB_reflect node and a MIB_colour_intensity node.
link DGS to input on MIB_reflect
link dielectric to MIB_colour_intensity and link MIB_colour_intensity to reflect on MIB_reflect
Adjust untill happy

The colour intensity controls the overall effect so you can play with the level while the dielectric gives the fresnel falloff. It seems to work but physical correctness? I can't say. Anyway I have used this shader many times (even as a car paint shader) and acheived some nice results. You can also plug Dielectric straight into the specular of the DGS I suppose. For more metalic effect you could mix in some glossy reflections as in metallic car paint you generally get glossy reflections from the base paint layer and specular from the top clear Lacquer layer.
This is what I get..
http://www.bmcaff.com/post/fresnel.jpg

sebast1an2
02-23-2004, 04:44 PM
hmm.. thx, but it seems to be that there is something wrong... cause i can't see a real "reflection falloff"... its only reflective with brighter edges... any idea how to create a fresnel expression for tthe reflect-channel of mib_reflect??

bmcaff
02-23-2004, 06:37 PM
Yeah the bright edges are the reflection of a bright background sphere (using the ray type node). Sorry, It is not so visible in the last image. I tweeked it and it should be more obvious now. I have shown a standard DGS for comparison. You need to set the dielectric IOR to the appropriate value to fine tune the effect.

http://www.bmcaff.com/post/fresnel1.jpg

sebast1an2
02-23-2004, 06:51 PM
Originally posted by sebast1an2
...cause i can't see a real "reflection falloff"... its only reflective with brighter edges...

hmm..its still the same, with less edge brightness.. fresnel means ~0% reflectivity in the front and ~80% to the edges..

bmcaff
02-23-2004, 09:17 PM
well it is just a cheat.. anyway this should still work fine for a carpaint shader especially if it's not physical accuracy your after. I also tried this in Max(6 demo) with the falloff map and it looked a little better but still had a slight reflection.

I guess we'll have to wait till someone makes the equivalent for Maya.

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