Hi guys,
Some days ago I posted a little tutorial about multi lyer rendering here. I tought maybe I should post it in here, so here I am. (And sorry for those who already saw it 
)
Here are some explanation about the multi layer (or multi passes) technique that I like to use to render my models.
I’m kind of new to 3d so I spent more that a week on the web trying to find information about rendering and how to make things look real in the 3d world… My quest went for translucency in skin rendering and realism. I found some really cool web pages that you should check out if you are interested:
http://www.cgtalk.com/showthread.php?t=200732&page=1&pp=15 is the first thing that I found and an amazing tut.
http://www.cgtalk.com/showthread.php?t=199389 is another super cool tut from the same guy
http://www.andrew-whitehurst.net/amb_occlude.html is a very interesting paper on ambient occlusion lighting
http://www.techimage.co.uk/index.php?c=sssdemo&m=1 is a cool page about subsurface scattering
http://www.thegnomonworkshop.com/tutorials/passes_layers/passes_layers.html is about rendering in passes by M. Alex Alvarez himself
and I found a book that is a must to read: http://www.elise.com/lmtstore/1562059548/Digital_Lighting__Rendering.html
Well, multi passes rendering is a powerful tool to control the rendering of a scene and can help a lot with translucency in skin shading.
What I like to do is split the rendering of my model in 7 or 8 passes that will give me 7 or 8 layers to play with in my compositing package (photoshop).
To do so I simply make different kind of Best render out of Zbrush (which is the only 3d package that I use) using different material and light settings.
Here is a picture of the different layers for my fairy.
The flat texture is obtained by Best rendering the scene with my skin material and textures with no light and ambient sets to about 15
The Glow layer is my little contribution to the technique! I use it because I thing it helps with translucency in skin rendering. I back light my model in the center with a glow light (using the color that I need) setting the intensity around 1.3 and the radius around 1.35
for the shadow layer I render my model with a shadow material that I created from the toy Plastic material. That the thing that I’m not completely satisfied with because it doesn’t work that well with all kind of lighting and I often have to tweak that layer in photoshop to get something that like. Maybe you can help me on this. I’m looking for a material that will restitute all shade of shadows (in gray to black scale) letting the lighted parts of the model white…
For the occlusion passes, I use Meats Meier material that you’ll find here: http://www.zbrushcentral.com/zbc/showthread.php?t=24222
Once I’m done, I copy the group of layer and flatten it in photoshop. I then auto level it + auto color it. Then add some blur and composite it in normal mode at a very low level ( around 15 %). It help to soften the skin and (I feel) add a bit of translucency too.
Well that’s about it! It sound complicated but it"s not !! The power of multi passes rendering is that it gives you more control on the final picture and make the rendering time very fast…
Have fun and give me your opinion on this. 
Dizzy Boy
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