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					Compositing Ambient Occlusion and channel based lighting

Compositing Ambient Occlusion and channel based lighting.
By Tahl Niran.




Hi this is another in a series of my shake tutorials. Although I am using big
words in the tut name, what I am really interested in covering here is a
workflow for relighting and tweaking the lighting of 3d CGI in a
comopositing environment. As such I will show you all some of the
‘lighting’ techniques which I rely on in compositing!

A quick note for those reading who don’t have lots of cash and studio
experience.. I am using two fairly popular applications in this tut. Alias’
Maya and Apple’s Shake, but this should be achievable in nearly any 3d
app:

For Xsi users: this should be very easy to implement as the shader ‘Dirtmap’
is compiled and works almost identically in XSI.

For 3dsMax users: ver 7 of max contains a full implementation of this
technique in its new mental ray shaders. Maybe someone will be nice
enough to extend this section as I don’t have a copy of max to check out the
specifics.
This exciting episode we will be covering the following techniques:

Ambient occlusion compositing in shake: This technique is loosely based on
some techniques used by some large studios, it may not be technically the
‘ILM’ way to do things but it gives good result fast! I have whipped up a
quick example of ambient occlusion (for the sake of brevity this referred to
as ambOcc or AO from now on) using Alias’ Maya v6 and metal ray v3.3.

Channel lighting in shake: An old favourite technique which I am using here
to further extend the flexibility of this technique. This is a basic technique
achievable in nearly any 3d/compositing app.

‘Light Wrapping’: One of the most basic image integration techniques, but
one that I see done wrong or ignored all together in a lot of people’s work.
Learn it and then its there for ever. Again this is a compositing technique
which can be used in nearly any app !
and a little bit on

Depth Of field: I got a LOT of requests for more on this after my last tut. I
know there is a huge rift in the shake user community about how to use this.
Here is one technique.
The Scene:




Here is a model I have downloaded from ZJ over at deathfall.com, which he
used for his Depth Mapped AmbOcc in Renderman tuts. His stuff rocks ! It
inspired me to find an out of the box way of doing ambient occlusion in
Maya. This is a compositing tutorial so I will focus on that side of things. I
am making the Maya file available and have placed extensive notation in the
scene. If you want a full rundown on the 3d techniques email me and I will
write something.
The Passes

The Diffuse / Texture Pass :
A pass containing only the texture/diffuse colour mapped on our object.
Rendered using a single ambient light with ambient shade set to 0 to give a
constant shaded render.




The Ambient Environment Pass:
This a render from mental ray, created using the IBL dome feature in Maya
v6, I have place an image in the texture slot of the ibl dome. Using the
‘Dirtmap’ shader I unchecked Probe geometry and checked the Use
Environment setting. Giving us a pass which samples the ambient
environment.
The Occlusion pass:
White isn’t it ! Again output from mental ray using ‘Dirtmap’ Here I have
checked Probe Geometry and unchecked Use Environment. I have also set
the camera’s background colour to white !




The RGB Light pass:
I have just placed a white shader on my object and put in three directional
lights. Colouring each one red, green or blue alternatively. This will give us
variable control of lighting based on colour channels. Again using mental
ray




The Shadow Pass:
A shadow only pass. Pictured here is the alpha channel. This shadow
corresponds to the ‘red’ light in the rgbPass. Rendered using mental ray.
The Depth Pass:
Yes I can here the cries now! This a ‘faked’ depth pass created by projecting
a ‘black to white’ ramp along the camera’s zAxis. I have parented this
projection to the camera and turned the camera’s Background colour to
white. Rendered in mental ray for consistency.




The BG Pass:
A render of the image used in the IBL dome. The ‘beach probe’ from
debevec.org (we owe him so much ) Again a mental ray render !

				
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posted:2/7/2010
language:English
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