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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.
Post a reply to this message
Attachments:
Download 'normals_and_shadowing.jpg' (160 KB)
Preview of image 'normals_and_shadowing.jpg'
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> 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.
>
As long as the planet don't cast any shadow on another one, or anything
else. Also, if you use any media for the athmosphere, it will get
wrongly illuminated on the night side.
Alain
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Alain <kua### [at] videotron ca> wrote:
>
> As long as the planet don't cast any shadow on another one, or anything
> else. Also, if you use any media for the athmosphere, it will get
> wrongly illuminated on the night side.
>
True on both counts.
I'm currently animating a little space scene, and the Moon is way off in the
distance (in a light_group as well), so it has no effect on the other closer
scene elements, and vice versa. No media either; I didn't want anything to mess
with those normals. ;-)
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Kenneth wrote:
> 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.'
Actually for a moon-like object a sharp shadow line would be more
appropriate. The problem is of course that it should be a sharp line
following the relief and not the perfect spherical surface, which
requires a mesh or iso_surface (a displacement map feature would
be ideal for this but is not available in pov).
The effect you get here with no_shadow looks to me more like what
you would get with an area_light (possibly with area_illumination).
More costly but also more controlled and works for other shapes.
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Christian Froeschlin <chr### [at] chrfrde> wrote:
>
> Actually for a moon-like object a sharp shadow line would be more
> appropriate.
Probably true. But somehow it just doesn't look *cool* enough--from a
science-fiction movie perspective, anyway ;-)
Post a reply to this message
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