Almost the very first image I rendered consisted of a simple sphere lit from
three different directions with red, green and blue light sources.  When the
sphere is smooth, the different colours of illumination blend together very
smoothly.  The trouble is, when I add a bumpy texture with "normal { bumps
0.8 scale 0.2 }" I can see a definite sharp edge to the illumination that
should not be there.  What is going on?  The sharp edges do not follow the
contours created by the texture - they are just circular lines going around
the sphere.

Thomas

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"Thomas White" <her### [at] removebtinternetcom> wrote in message
news:39f5fce1@news.povray.org...
> Almost the very first image I rendered consisted of a simple sphere lit from
> three different directions with red, green and blue light sources.  When the
> sphere is smooth, the different colours of illumination blend together very
> smoothly.  The trouble is, when I add a bumpy texture with "normal { bumps
> 0.8 scale 0.2 }" I can see a definite sharp edge to the illumination that
> should not be there.  What is going on?  The sharp edges do not follow the
> contours created by the texture - they are just circular lines going around
> the sphere.

Common misconception about what a texture normal is versus a surface
displacement (of which only MegaPOV is currently capable of in any way
whatsoever).
The texture doesn't perturb or affect the physical surface at all, much the
same thing would be seen if you painted a ball in such a way so as to make it
appear to deform from a perfect sphere with shading and the like.  Lights shone
upon it would show the same thing as in a POV-Ray render, it's all a trick of
illumination and shading.  POV-Ray won't flex the surface to fit a texture,
quite the opposite could be said (not empirically however).

Note to Warp:  this doesn't seem to be in your VFAQ.  Might make a good
inclusion.

Bob

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"Bob H."
<per### [at] aolcom?subject=PoV-News:%20&body=Relating%20to%20POV-Ray:>
wrote in message news:39f6060c@news.povray.org...
> "Thomas White" <her### [at] removebtinternetcom> wrote in message
> news:39f5fce1@news.povray.org...
> Common misconception about what a texture normal is versus a surface
> displacement (of which only MegaPOV is currently capable of in any way
> whatsoever).
> The texture doesn't perturb or affect the physical surface at all, much
the
> same thing would be seen if you painted a ball in such a way so as to make
it
> appear to deform from a perfect sphere with shading and the like.  Lights
shone
> upon it would show the same thing as in a POV-Ray render, it's all a trick
of
> illumination and shading.  POV-Ray won't flex the surface to fit a
texture,
> quite the opposite could be said (not empirically however).

I've noticed that bumpiness isn't affecting the overall shape of the sphere,
but I can definately see a sharp edge to the illumination that isn't there
when the bumpiness is commented out.  When I reduce to just one light source
it's made even more clear.

Apologies if I'm just digging myself further into a newbie-pit here but I
don't understand where the sharp edge is coming from.

Thomas

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"Thomas White" <her### [at] removebtinternetcom> wrote in message
news:39f608de@news.povray.org...
>
> I've noticed that bumpiness isn't affecting the overall shape of the sphere,
> but I can definately see a sharp edge to the illumination that isn't there
> when the bumpiness is commented out.  When I reduce to just one light source
> it's made even more clear.
>
> Apologies if I'm just digging myself further into a newbie-pit here but I
> don't understand where the sharp edge is coming from.

I see now what you were actually trying to find out about is why a texture
normal causes the sharp shadow/light boundary on a object.  That I don't know,
I simply live with the fact.  But, I think you are seeing the faked high parts
mostly and perhaps the low parts in the normal aren't as obvious, leaving some
places with a expected terminator (shadow line) and not other parts.  The
surfaces finish makes a difference in how it appears as well, so if a highlight
is on there (esp. with second, third lights) it's all the more noticeable... or
unnoticeable.  Depends.
Anyway, it comes down to the same general thing I first said.  The faked high
spots aren't really higher up than their surroundings and thus only a
undisturbed shadow line shows up.
Once again, MegaPOV can do actual surface perturbations so maybe you'll want to
try it.  Check out isosurface and pigment function (pigment pattern equals any
normal pattern of the same type).

Bob

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Thomas White wrote:
> 
> The trouble is, when I add a bumpy texture with "normal { bumps
> 0.8 scale 0.2 }" I can see a definite sharp edge to the illumination
> that should not be there.  What is going on?  The sharp edges do not
> follow the contours created by the texture - they are just circular
> lines going around the sphere.

This is because the texture doesn't create any actual contours.  It
merely fiddles with a property of the surface called the normal, in
such a way as to make the surface appear rough.  Away from the edge of
the shadow this looks fairly convincing, but at the edges the
appearance gives the method away.

The sharp edge is the terminator between the lit and shadowed sides of
the sphere.  It remains circular, instead of becoming jagged, because
the actual shape of the sphere hasn't changed.

Indeed, the texture effect causes spots immediately adjacent to the
terminator to appear brighter than the same place, smoothly textured,
so the sharpness of the terminator becomes pronounced in spots.

Regards,
John
-- 
ICQ: 46085459

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Actually, it's not as simple as you state.

  The normal modifier DOES indeed affect the shadow line of an object (ie.
the line that is between the part of the surface that faces the light source
and the part that faces the opposite direction, ie. the line where the
shadowed part of the object starts).
  To see how the normal modifier affects this line, try making your object


  What happens is that the sphere surface shadows itself. When a point is
in the "dark side" of the sphere but due to normal perturbation the normal
vector of that point faces the light source, there's a surface between that
point and the light source and thus it gets shadowed.
  This can be fixed by making the object shadowless, but then you lose all
shadows from that object, which is usually not wanted.

  I have suggested some work-arounds for this problem long ago, but no-one
seems interested enough to make any.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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I added an extensive description of the problem to the VFAQ.
  It can be found at:

http://iki.fi/warp/povVFAQ/TheShadowLineArtifact/

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):_;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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"Warp" <war### [at] tagpovrayorg> wrote in message
news:39f6cac1@news.povray.org...
>   I added an extensive description of the problem to the VFAQ.
>   It can be found at:
>
> http://iki.fi/warp/povVFAQ/TheShadowLineArtifact/

Thanks.  That's just the information I was looking for.

Thomas

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"Bob H."
<per### [at] aolcom?subject=PoV-News:%20&body=Relating%20to%20POV-Ray:>
wrote in message news:39f613cd$1@news.povray.org...
> "Thomas White" <her### [at] removebtinternetcom> wrote in message
> news:39f608de@news.povray.org...
> I see now what you were actually trying to find out about is why a texture
> normal causes the sharp shadow/light boundary on a object.  That I don't
know,
> I simply live with the fact.  But, I think you are seeing the faked high
parts
> mostly and perhaps the low parts in the normal aren't as obvious, leaving
some
> places with a expected terminator (shadow line) and not other parts.  The
> surfaces finish makes a difference in how it appears as well, so if a
highlight
> is on there (esp. with second, third lights) it's all the more
noticeable... or
> unnoticeable.  Depends.
> Anyway, it comes down to the same general thing I first said.  The faked
high
> spots aren't really higher up than their surroundings and thus only a
> undisturbed shadow line shows up.
> Once again, MegaPOV can do actual surface perturbations so maybe you'll
want to
> try it.  Check out isosurface and pigment function (pigment pattern equals
any
> normal pattern of the same type).

Thanks.  I've downloaded MegaPov 0.6 and I'm now getting acquainted with it.

Thomas

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Warp wrote:

>   Actually, it's not as simple as you state.
>
>   The normal modifier DOES indeed affect the shadow line of an object (ie.
> the line that is between the part of the surface that faces the light source
> and the part that faces the opposite direction, ie. the line where the
> shadowed part of the object starts).
>   To see how the normal modifier affects this line, try making your object


It's nice that it works, unfortunatly the shadows are also important. What I
tried to do was place an identical sphere bounded by the light source to get my
shadow back, and it creates the shadowline artifact again... grrr.
--
Josh English -- Lexiphanic Lethomaniac
eng### [at] spiritonecom
The POV-Ray Cyclopedia http://www.spiritone.com/~english/cyclopedia/

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