SSS, Bump and Reflections with Mentalray/Maya


#1

May be, this is worth a discussion:

When working with the misss_fast_simple_maya node and trying to ad bump and reflections, I’m always ending up with strange results or horrible render times.
The misss node has a tripple input for bump maps - this indicates the ability for dealing with color.
In tutorials I learned that here an image has to be connected directly - not using a special bump node, what results in an one channel connection. For having a reflective and scattered surface, you should pipe the misss shader into a glossy shader before connecting to the shading group. This gives very strange results:

As you can see, the normals are going crazzy (light should come from the back in the upper left) and the bump (procedural cloth in this case) looks somewhat truncated.
And this is the shader tree:

When connecting a pregenerated normal map the result is much better, but still not what is expected.
I tried networks using mib_color_mix, mib_passthrough_bump_map, the two mib_bump_maps and combinations with different Mentalray materials, but the results didn’t get much better and render time increased.
Then I came up with the idea to connect the misss to a maya phong (incandescence), which I plugged then into the maya material port of the shading group. It looks like this:

And the miracle happens - the lightmap is still intact and gives the correct output:

What are your experiences? Is there a better way to get this combination working?
Please let me know what you’re thinking about this.

Cheers
Martin


#2

Hey man, I love this solution!

You tried Francesca’s Mix_Layer?


#3

Hey Joie,

where can I find Francesca’s Mix_Layer ??
I’m very interested in …

Thanks
Martin


#4

The triple input rather indicates the ability to deal with vectors (XYZ), not colors. The bump2d or bump3d have a proper output for this, so try plugging one of them into the bump slot of the misss_fast_simple.

Get the latest mix8layer from here.


#5

Thanks Floze,
bump works, but composing reflection on top still sucks …

I double checked where the bump information came from:
It’s on the Gnomon DVD “Mental Ray: Lighting and Shaders” in the chapter about misss_fast_simple …


#6

Keep in mind that misss_fast_simple has a default specularity built into it.

What happens if you change it to misss_fast_skin?

The other option is to make your own Phenomenon by copying misss_fast_simple and modifying the specularity. Its defined in the subsurface.mi file in the include directory.
I wouldn’t change that file, I’d make a new mi file with the copy, and your name for it, in it.


#7

It really should work.

And the scene:

http://individual.floze.de/fileadmin/files/cgtalk/misss_donut.zip

Do they really suggest to plug a plain texture into the bump slot? If the do so, maybe drop them a mail…?


#8

bgaboy: It’s just specular - no reflections!

Keep in mind that misss_fast_simple has a default specularity built into it.
What happens if you change it to misss_fast_skin?

floze: Yes - they did!

Do they really suggest to plug a plain texture into the bump slot? If the do so, maybe drop them a mail…?

Unfortunately the Mentalray docs coming with Maya are as thick as XSIs … this makes you rely on anything else you can get …


#9

I’d really drop them a mail to correct that… that’s no offense, but it’s sort of nonsense to plug a rgba image into a XYZ vector input, specially since a bump shader typically takes a scalar (single value) as an input - of course my statement would be nonsense if the misss_fast_simple had some fancy supder duper bump shader implemented, but it doesnt.

However, even if you’re using a normalmap, it takes further steps to get it right: http://www.headus.com/au/samples/norm-MR/
The terminal vectorProduct in the shown graph would go into the bump input of the misss_fast_simple as well.


#10

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