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POLYFILL / Z-Buffer Bug ?

I've noticed that when I use the Z-Buffer to store several moving
polygons, that if any polygon has only one vertex within the data
window, then using the /Pclip option causes the POLYFILL routine to
crash with the following diagnostic:

% POLYFILL: No valid points, must have at least 2 distinct points

This happens specifically when the polygon in question is just about to
move outside the data window.

Has anyone else observed this and if so, is there a fix for it?

Interestingly this dosent happen if I'm NOT using the Z-Buffer!

I've also found another anomaly with POLYFILL in that if the polygon has
vertices far outside the data window (beyond some cutoff limit), then
POLYFILL fills the COMPLEMENT of the intersection of the data-window and
the polygon (with clipping), rather than the intersection itself!!

Again, has anyone else noticed this. The fix that I use consists of
simply scaling the distant vertices (that violate the empirically
determined cutoff criteria) back along the lines joining them and their
corresponding "visible" neighbor vertices. As you can imagine, this
tends to slow down a dynamic simulation quite a bit!

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