I stumbled onto an interesting and simple trick concerning normals on an object.
I suspect it's well-known, although new to me. (The only mention of it seems to
be at 2.4.8 in the docs, "The shadow line artifact", but that deals exclusively
with smooth_triangles.)

I'll use a smooth sphere as an example--which is about the only kind of object
that seems to work well with this set-up. If the object is made no_shadow (or
its light is made shadowless), an applied normal does not show the typical
'sharp terminator line' from the light source. Instead, the normal effect looks
far more natural on the surface, more '3-dimensional.'

It even works if the sphere is animated (revolving, for example.) I expected
some flaws or inconsistencies to show up, but I don't see any.

The only little problem is that the effect extends *past* the terminator line
somewhat (more than seems 'natural.') But with an appropriately-chosen lighting
angle, it fools the eye.

I also tried it on a POV-Ray-generated spherical height_field; but there are
some flaws (most likely due to the typical situation encountered when lighting
smooth_triangles, as mentioned in 2.4.8)

As it says there, the object's self-shadow is the main reason for a normal's
typical sharp-terminator-line appearance. (I found this out myself by overlaying
an identical sphere--made no_image but *with* a shadow--and the terminator line
re-appeared.) But by scaling down this 2nd sphere a little bit--which reduces
its shadow size--the nice effect comes back.

So the trick has a rather limited use; but it's good for planets and such
things.

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