I was playing around with a height_field today, having some fun, adding some
ground fog, with the camera hovering over the enlarged HF surface. Things were
looking good. On a whim, I decided to scale the HF negatively in y (to reverse
it's direction and create a deep chasm instead.)

Suddenly, the fog disappeared, and POV rudely announced: "Camera is inside a
non-hollow object. Fog and participating media may not work as expected."

????

Putting on my thinking cap and doing some experimenting, I discovered this
interesting little fact (which the POV documentation doesn't mention): The
infinitely-deep space under a HF is considered to be "inside" it. Very similar
to the behavior of a plane, but the volume of space is confined to the x/z
dimensions of the HF. (By scaling it -y, I had inadvertently "flipped" this
space to be *above* the HF, and my camera was now inside it.) I had always
known that the inside of a HF SHAPE was considered "inside" --and had assumed
that the HF object itself was confined to a 1X1X1 cubic volume and nowhere
else--so this was a new discovery.

No problem though; adding inverse or hollow on to the HF fixes things, and the
fog or media reappears. (BTW, adding water_level-- before the negative
scaling--doesn't fix it. I thought it might "trim away" everything under the
HF, but no go.)

It should also be noted that, when making a normally-scaled HF hollow (with
pigment {rgbt 1}) and adding interior{media....} to it, the media does NOT
extend into the infinite space underneath. No great discovery there; we're all
familiar with that behavior.  But it points out that there seem to be two
different "types" of insideness to a HF object.

I had never run into this situation before, so I thought I would pass on the
info--for the betterment of mankind. :-P

Ken W.

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