"Thorsten Froehlich" <nomail@nomail> wrote:

>
> You need to consider the purpose of light groups: They exist to allow additional
> local lights for objects, a technique that is frequently used in computer
> animations to help make certain objects appear lit "better" -

That's basically what the documentation says, although the behavior of shadows
vis-a-vis a light_group isn't discussed there-- which seems like an important
topic, as shadow behavior may be 'unexpected', in an intuitive sense. (Well, at
least for me-- and maybe Thomas as well.) My immediate intuition when reading
the docs is basically this: that a light_group should *only* light the objects
in the group, AND *only* those objects should cast shadows from that light.
Obviously, it doesn't work according to my intuition...  ;-) I admit that I
haven't completely thought through the ramifications of my idea; there may
indeed be some logical conundrums with it. But its present behavior seems like a
conundrum (to me.)
>
> What you are looking for is something like "shadow groups", though I
> suppose you might want to rethink what you actually want to render.

That's very possible (and I could probably find a work-around to the scene
problem I encountered, using no_image and/or no_shadow as Thomas mentioned.) My
current scene-- which prompted me to post about this-- is an animation of an
expanding  'explosion' on the top of a tower (like the old A-bomb tests.) I
wanted a VERY bright fading light to flare on the ground (and only there), from
a position at the center of the explosion, so I made a light_group with the
ground-- but found that the explosion sphere and the tower were casting unwanted
harsh shadows... which I didn't expect (based on my... intuition...)

I think my past use of a light_group was always with a carefully-positioned
spotlight, so I probably never noticed the unexpected shadow behavior from other
scene objects. (Actually, I do recall once making a scene where I wanted a light
*underneath* the ground, to artifically light up some above-ground objects-- and
I did have to add no_shadow to the ground. But I thought that was because I had
made some kind of dumb mistake in my complex code.)

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