"Kenneth" <kdw### [at] gmailcom> wrote:
> This plane needs to be made 'hollow' for the media to appear:
>       plane {z,  2... }
>
> That's because the spheres are located 'inside' it. (The same effect could be
> achieved by using 'inverse' instead.)
>
> Whereas, the plane{y, -2...} doesn't need the hollow keyword, as the spheres
> are 'outside' it.
>

I just discovered something interesting, that I didn't know until now.

Given your enclosed 'universe' of two planes, where the media spheres (and
camera) are inside one plane (z) but outside the other (y):

If I add 'hollow' to the z-plane as mentioned, the media shows up. HOWEVER, if I
also 'inverse' the y-plane-- thus making the enclosed space 'inside' from *both*
plane's perspectives-- the media disappears again. Apparently, the space is now
sort of a mix of hollow and non-hollow. I have to add 'hollow' to the inverted
y-plane as well, to get the media to reappear.

Naively, I would have assumed that the z-plane's sole 'hollow' would have
sufficed to make the space 'safe for media' (ha), and that it would somehow
override the non-hollow y-plane space. But not so!

I'm not so sure that using planes that enclose small media objects is a good
idea anyway, but for a different reason: AFAIU, every camera-ray/pixel shot into
such a scene would have to be tested for the presence of media-- not just by
testing the small spheres themselves, but over the entire camera view. Lots of
unnecessary computational overhead, in other words. But I could be wrong(?)

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