A couple of initial (naive?) questions, to help me better-visualize the details:
>
> Assume an orthographic camera.
> Let's say that I have a line segment extending from the top left of
> an image to the lower right.

Is that initial line an actual object in the 3-D scene that appears to extend
from corner to corner, or is it a line only in flat 2-D screen space (like
Screen.inc would typically produce, for example)? I'm guessing it's a 3-D
object, extending into the scene.

> If I draw a line perpendicular to this, then I can use this
> new line as an axis for rotation.

Perpendicular to it in x/y screen space, or pointing 'out' from the middle of
the initial line segment directly into the camera? Or maybe pointing somewhere
else in 3-D space? Sorry, this one is hard to visualize.
>
> If I rotate a copy of the line segment around this axis, it should visually
> remain in line with the original line segment, but the ends would appear to
> contract.

I can sort of 'see' this...depending on whether or not I understand the initial
stuff.

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