Double J, Mac, Image, and VFX

simulating Subsurface-Scattering with Maya/Mental Ray

(btw: It will also work with XSI I think !)

First of all I have to say that there are two ways

(or maybe even more !) to simulate Subsurface scattering.

You could use a real material-volume-shader and a few sampler/light-info nodes and a lot of time for tweaking

and tuning !

pros: maybe a better look

cons: very slow, new calculation for each frame, complex setup

So.... 'I did it my way !'

and this one is pretty fast and easy to setup !

The idea :

Take an empty bottle and a cigaret and blow some smoke into the bottle.

Then take the bottle and hold it under a bright halogen lamp.

But see it yourself !

Okay lets start with some easy modeling ...

Create Poly-Sphere

Give it a radius of round about 5 and 8 by 8 Subdivisions

Get into the component mode and selct all faces

Make sure that you switch off 'Keep Faces together'

(under Modeling > Polygons > Tool Options)

Extrude your selected Faces ...

Local Translate Z : 0.2

Local Scale X/Y/Z : round about 0.4

(simply something like this )

Now Extrude your Faces again and again so that

you will get something like this:

smooth your spiky ball two times

(Modeling > Polygons > Smooth)

Modeling done !

Go to your Mental Ray Render Globals and choose:

'PreviewCaustics' under the Render Quality tab!

Now create a Directional Light source with the following settings:

leave all other settings as they are !

Place the light somehow like this:

Scene-Setup done !

The SHADER-SETUP

(using Custom MR-Nodes)

Create new PHONG-Shader (MAYA defalut one)

and apply the following settings !

if you are done click the Node-button to get to the MR-Shader Setup

(the blue marked above)

Assing a 'PARTI-VOLUME' Node to the MR Volume Shader-Node

Make this settings:

Scatter: 100% RED ... this one controls the Subsurface color.

Extiction: 0.500 ...( I call it : Caustic - Visibility - Attenuation)

this controls the strength of the Caustics

when they hit the Volume

0.1 is HIGH - 1.0 is LOW

Min_Step_Len : 0.02

Max_step_Len : 0.20

Controls the accuracy

(Don't forget this both !!!)

That's all for the PARTI_VOLUME Shader !

Go back to the Mental-Ray Custom-Shader Tab and apply

a PARTI_VOLUME_PHOTON Node to the PHOTon Volume Shader.

Scatter: round about 20% of the slider !

Extiction: 1.500

Min_Step_Len : 0.02

Max_step_Len : 0.20

So your Shader have to look like this :

(next page)

The SHADER-SETUP (PART2)

And you won't believe it ... this is ALL !

Get a liitle bit closer to your spiky ball in the

Persp. View and make a Test-Render ...

You should see something like this:

Rendertime: 84 seconds (AMD Athlon XP 1800)

Some tips ..... and the Maya Scene-File !

(next page)

Tips & Tweaks

You dont really need to turn Refraction ON in many cases.

You can store your Photon-Solution... so you don't need to re-emit photons

for every frame if you do a Camera-Animation for example.

Play around with the Scatter and Extiction values

:::

You could also use a point or spotlight

If you use a Spotlight you should increase the Photon-Energy (Maya/Light Rollout)

to 20.000 or 30.000 in !

:::

If you dont like the rendered alpha-mask

(e.g. you have to put your object onto another background)

You could set a solid Alpha under 'Matte Opacity' of the Phong-Shader !

: : : : :

To colorize your Subsurface Scatter simply add an 3D Texture to the Parti-Volume Node's Scatter Attribute

(the one which is 100% RED )

in this case a color-ramped/remapped Volume Noise (MAYA builtin)

: : : :

If you got area's like the following...

you can get rid of this :

Increase the emitted Photons to approx. 150.000

(in some cases you'll need 2.000.000 Photons)

also increase the

Photon Volume Accuracy

Caustic Accuracy

And tint your Shaders-Diffuse and Specular Color a litte bit towards

the Scatter-Color !

:::

If you would like to see how your Photons are stored

(and how your object is filled up)

you can set the Photon-Volume-Radius to 0.02 - 0.1

and you will see something like this!

(5000 Photons emitted)

:::

Tips & Tweaks (Part 2)

To have more control over the Photons simply assing a

DGS_MATERIAL_PHOTON to the MR PhotonShader-node

:::

There are still 2 methods left for simulating SubSurfaceScattering

the one I talked about at the intro-page (the pixho way )

and the MIB_Raymarcher which is

also a Mental Ray default shader but not really

usable implemented in the current version

of the Maya Mental Ray Translator 1.5

Maybe we could talk about this non-caustic way if

Alias updates its (still buggy) Translator !

So that's all for now ...

Check back in a few weeks

coming next:

'creating realistic volume smoke'

Hope you enjoyed it ! / Gerald Naujoks 2003

! For more images check the old gallery here !

:::

DOWNLOADS:

The Tutorial Scene(sss_tutorial.zip)

MsLiberty.zip

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