It's not complicated what it does. It's basically a trick they used on Finding Nemo, described in some Siggraph notes, but using the camera's pov, rather than the light's pov. (see: http://graphics.stanford.edu/facecourse/facecloning05/Presentations/scat.pdf )
The lightmap contains the interleaved illumination of the front and back surfaces (+depth of back) as seen from the camera, then the fast_sss shader simply blurs the image samples based on distance, for each sample lookup. This is why this trick won't work in a wraparound mode. If you froze the lightmap and then orbited, the SSS effect would look quite funky where you were viewing perpendicular to what the original camera's sampling position was.
The only decent way to do it would be something along the lines of what Alan describes.
- ½
-----Original Message-----
From: owner-xsi(at)Softimage.COM [mailto:owner-xsi(at)Softimage.COM] On Behalf Of Alan Jones
Sent: 27-May-2008 10:55
To: XSI(at)Softimage.COM
Subject: Re: rendermap / lightmap SSS shader
On Tue, May 27, 2008 at 3:20 PM, peter boeykens <peter_b(at)skynet.be> wrote:
> For "unsupporting" SSS - this seems lame - as it wouldn't take all
> that much to get it to work.
> The downfall is with the lightmap being camera dependent.
> I dont think that's a limitation of lightmaps - as color sampler
> lightmap is working with UV space.
The fast SSS shaders don't do anything akin to an ordinary lightmap in UV space.
They use lots of strange trickery behind the scenes to do their work and Zap is very tight lipped on their secret sauce used. Don't forget that Halfy is an exceptionally intelligent guy, if there was something on his end he could do to resolve this it would already be done.
There's nothing stopping you from writing/employing someone to write your own SSS shader which could achieve similar speed using lightmaps by doing some nice trickery to store to a kdtree/bih instead of a UV space texutre and then looking that up within the main shader. It'd be nice and fast and you'd avoid the camera dependency that makes fast SSS so unhappy.
Though if you use SSS a lot grab a copy of 3Delight - their SSS is absolutely gorgeous, though I've not look at whether they support rendermapping in XSI, though I'm sure Aghiles would be more than happy to provide customers with something that does this within XSI in order to get 3Delight into a new facility.
Cheers,
Alan.
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