 | | | shadow pass? _sc | shadow pass? _sc 2005-03-19 - By Matt Lind
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Basically, you're looking for a light that only casts shadows where a shadow
ray would have a clear path from the cube to the table top. This can be
done in a render, but only with a custom light shader that could probe where
the shadows would go. You're not going to be able to get the effect through
the render tree with the existing shader library as the effect would need
significant tweaking each time the light changes position or intensity
relative to the cube, and would never be exact.
This issue could be readily solved in a compositor without much trouble.
You'll need to render at least 3 passes to pull it off: sphere shadow matte,
table without any shadows, cube casting shadows, plus whatever else you
need. Applying simple boolean logic on the sphere's shadow/alpha matte, you
can selectively composite the cube's shadow where needed. Using the
shadowless pass as the background, you can use the pixel parser to choose
which other pass to lay over the top based on the sphere's shadow matte.
example:
if ( pixel == sphere_shadow ) {
result = shadowless_background;
} else {
result = cube_shadow_pass;
}
If you only care about the regions where the sphere shadow and cube shadow
overlap, then simply render a cube shadow matte pass and modify the above
expression to take it into account.
Matt
-- ---- ---- ---- ---- ------
Matt Lind
Animator / Technical Director
Softimage certified instructor:
Softimage|3D
Softimage|XSI
Mantom, Chicago
Matt(at)Mantom.net
Re: shadow pass? _sc
Date : Sat, 19 Mar 2005 17:47:37 -0500
To : XSI(at)Softimage.COM
>From : Sam Cuttriss <samc(at)quietman.net>
Subject : Re: shadow pass? _sc
here is an example:
You can see the shadows are calculated on without taking into account the
discontinuity settings of the mesh.
the unrendered part shows a smooth falloff whereas the shadow pass render
does not.
sorry about the incidence question.
i missed the illumination check box.
the shadowpassfixed image shows the improvement i was after so it tapers off
to 0000 when the surface is perpendicular to the light. (i cant believe i
missed the illumination check box... )
now all i need to do is figure out a way to clip shaddows with an object, as
far as i can tell shadows continue to infinity regardless of what they
encounter.
_sam
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