RE: [Algorithms] Blending Spherical harmonics with traditionnal lighting

[Tom F. <tom...@bl...>]

It's worth mentioning that animated objects can be made to work, but the
lighting model may not change according to the animation. So in the
example of a person wearing a baggy shirt, the folds and wrinkles in the
shirt will still do the right thing, because locally the animation
doesn't change how they react to light very much. But the more global
effects - such as the arm shadowing the torso in certain poses - they're
much harder to represent.

Probably the most important feature from a perceptual standpoint is the
face. The really good thing about the face is that although it does
animate, it doesn;t animate all that much. Probably the major animation
is the jaw moving up and down, and that accounts for most of the
changing in lighting (how much it shadows the neck). Things like
eyebrows moving, smiling, etc - none of those change the lighting values
very much, and the most prominent feature as far as lighting is
concerned - the nose - doesn't have any moving parts very near it,
except maybe bits of the bottom lip. So in fact the lighting on the face
can use really cool stuff like PRT, and you simply ignore the fact that
the lighting is not perfect, because all the low-hanging fruit (the BRDF
of skin, subsurface scattering effects, the "halo" caused by whiskers
and hairs when the light is behind the subject, the shadowing of the
nose) - all that still works fine.

So when Peter-Pike says "PRT doesn't work for animated objects", he's
being too modest. It works just fine - it doesn't produce random junk or
anything scary like that. It just doesn't change it behaviour according
to the animation. But in many cases it's still better than nothing at
all. And of course you can happily combine it with conventional
techniques like stencil volumes or stencil buffers to add those
large-scale self-shadowing effects back in.

TomF.

> -----Original Message-----
> From: gda...@li... 
> [mailto:gda...@li...] On 
> Behalf Of Willem H. de Boer
> Sent: 28 August 2003 17:14
> To: gda...@li...
> Subject: RE: [Algorithms] Blending Spherical harmonics with 
> traditionnal lighting
> 
> 
> If you are using SH to represent irradiance environment
> maps, like Ramamoorthi et al.'s "An Efficient Representation
> of Irradiance Environment Maps" then it scales very well
> to deformable meshes. Just think about it, you have pre-
> computed the convolution of the lighting function L(s)
> with the cosine-transfer A(s), and you are feeding it
> normals. So the only thing that would need to be re-calculated
> are the normals of every vertex on the mesh.
> 
> If, however, you are using SH for precomputed radiance
> transfer, as in Sloan et al.'s "Precomputed Radiance Transfer
> , [...]" then it gets a bit tricky. For diffuse surfaces,
> it would imply recalculating the visibility function V(s)
> for every vertex of the mesh (if you are storing a coeff.
> vector per-vertex that is). Considering the naive approach
> of calculating V(s) in the first place is stochastic
> ray-tracing, I wouldn't recommend it ;)
> 
> I believe that there exist some papers on using PRT with
> deformable meshes, but titles of papers elude me.
> 
>   >My main concern is 
>   >to how well SH
>   >lighting blends in with traditionnal dot lighting (or I 
>   >should say how
>   >traditionnal lighting blends in with SH lighting).
> 
> Tom Forsyth did a talk at this year's GDCE on this
> very problem. Also, there have been lots of discussions
> on this list involving Tom, that talked about this.
> 
> HTH,
> 
> Willem
> 
> 
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