Hi,

I'd like to make a suggestion for the next version of POV-Ray with regard to
alpha channel transparency support.

Right now, if alpha transparency is enabled, all areas of the image where
the background is partly or fully visible will be partly or fully
transparent. So far so good.

I have a complex animation where two highly-detailed and textured spaceships
swoop back and forth from the point-of-view of the camera in such a manner
that sometimes spaceship "A" is obscuring parts of spaceship "B", and
sometimes spaceship "B" is obscuring spaceship "A".

Because each of these spaceships take a LONG time to render by themselves
(and even longer when combined together in a scene), I want to render each
spaceship on a seperate computer using POV-Ray's alpha transparency feature,
and combine the frames together in post-production along with a third
background layer for the scene background. However, because the spaceship
that is "on top" or "in front" changes during the length of the animation, I
can't always layer the frames from spaceship "A" on top of the frames from
spaceship "B"... sometimes "B" is on top/in front.

Here's what I'm suggesting: a new POV-Ray object modifier called
"alpha_only". When applied to an object, "alpha_only" would cause any light
ray striking that object to be rendered as transparent in the alpha channel.
The way I would use this feature is as follows: On computer #1 I would
render spaceships "A" and "B" together, but spaceship "B" would have the
"alpha_only" modifier applied. As the invisible spaceship "B" moved in front
of spaceship "A" during the animation, it would mask out the areas on
spaceship "A" where "B" blocks it from the camera. I would then reverse the
process on computer #2: spaceships "A" and "B" would repeat exactly the same
animation script, only this time spaceship "A" would be "alpha_only" and
would mask itself when obscuring spaceship "B".

Since an object with "alpha_only" applied would not have to have its
textures, reflections, etc. computed, it seems that there would be a minimal
overhead to such an object to a scene. It would certainly make the
post-production step of combining the frames simpler... *especially* if
there are dozens of spaceships (or whatever) in a scene vs. only two.

Does any of this make any sense? Is it doable/feasible/desirable?

--sg

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