John Allers wrote:
> 
> Is there any way to project a shadow onto an invisible plane?
> 
> Here's what I'm trying to do:
> I've created an "alien" for our graphics class.  I want to be able to put
> the alien into a real image.  Ex. I may want to show the alien walking
> behind me in a photograph.  The only problem I'm having is being able to
> project shadows.  If I put a white box under the alien for the shadow to
> fall on, the white box gets included in the alpha channel.

So, you want the shading due to the shadow with alpha < 1,
while alpha should be = 1 where there is no shadow and no alien...

You may want to put a plane (not a box) with a transmit value near 1 (such as
.9) so that the shadow can somehow manifest itself while keeping some alpha.
But it won't help you to extract the silhouette for the composition...
 (but then, this is a problem of 2D images software)

I'm afraid you're taking a wrong route for your problem.
it seems that you want to get the perception of the flux of light in the environnement
of the alien. So instead of trying to show the shadow on a support, why not
try to use a media (scattering type), so that the part of the air in shadow
looks dimmer than the part in full light. 
Or you should model more of the original photograph (at least the wall & floor)
into the alien scene... 
But anyway, no automatic silhouette...
-- 
Non Sine Numine
http://grimbert.cjb.net/
Puis, s'il advient d'un peu triompher, par hasard,

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