 | | | shadow pass? _sc | shadow pass? _sc 2005-03-19 - By Sam Cuttriss
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thanks matt, thats what i was starting to think.
i had almost resigned myself to rendering 3 passes, although i may have
inadvertently stumbled across a render tree solution.
it seems to be unpredictable at best, but if the sphere has a simple
shadow shader in it with a brightness of over %50 grey it seems to clip
the shadow.....
very tempromental though?
see how the pill shape clips the shadow of the cube!!!!! weird.
can anyone else replicate this?
_sam
ps: sorry about the images, they are tiny.. hope thats ok.
Matt Lind wrote:
> 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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