> If the first ray intersects
> both objects, one being inside the other, and the other ray intersects
> only one of the objects, which conclusion should be made from that?

Well, in any event, that would be a start.
Naturally, as you say, there would be SOME error, say, in "corners" 
where 2 objects touch or intersect and there is adjacent empty space or 
a more distant surface of one of the objects - but that would likely 
only lead to a few extra pixels in the thickness of the drawn outline at 
that point.

 From the perspective of pixels, "close enough is good enough".  At 
least for the proof of concept.

I also remember seeing somewhere that there was a "POV Outline" or 
somesuch thing that someone wrote on the web...  does anyone have any 
experience with that?

Let's say you had 3 objects - you could shoot a Trace() ray in order to 
see what it hits.  Do so for all 3 objects, and compare the results - of 
the same ray trajectory.

If one object is in front of the other, then it will get hit with the 
trace, if not, it will pass right through.
In the actual rendered scene, if one object is INSIDE the other, then 
the outer object gets hit.

So a positive result from both trace and the scene, would say that there 
is an edge.

It occurs to me (to answer a question that I posed earlier) that the 
smallest resolution one would need to test is based on the displayed 
pixels in the final render.  Divide up the rendered scene into pixels, 
do the same for "POV units", and define the step of any loop to 
increment the number of Pixel-equivalent POV-units.

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