In an earlier post, Scott Gammans asked a question regarding masking out
objects using something he described as an "alpha_only" property (search
general for "alpha masking").  After a verbal pummeling on why this would be
useless to speed up rendering, someone made the (somewhat helpful)
suggestion of using different colors to "key out" parts of the picture.

This is exactly the scenario that would benefit from using an "alpha_only"
object.  For instance, I am adding animated objects to a live video, and I
need a way to mask out sections of the video foreground so that the CG
objects can go "behind" objects in the video.  I have used color keying
(placing bright green objects in the foreground to mask regions out) but
this causes some artifacts along antialiased edges and causes problems with
reflective surfaces.  An "alpha_only" object would be most helpful here.
Since that feature probably won't be available soon, is there an easy way to
"fake" it using bounding or some kind of surface checking?  For instance, if
a ray hits the masking object, the pixel is automatically set to
transparent?  I have thought about using CSG difference to "remove" portions
of the background objects at the appropriate spots, but I haven't tried it
yet (seems like it might be time consuming getting all the angles to line up
right with the camera).

Any suggestions?  Thanks!

-------------------------
text{ttf "crystal.ttf", "Mike", 0.01, 0 pigment {blue 10}translate
<-1,-0.2,2>}

Post a reply to this message