Try to render the little animation below and you will see the Ghost of POV
effect caused by union and wood normals (name made up by Uwe Zimmermann I
think). 10-20 frames is enough to get the idea.

I still wonder what actually causes the weird effect.

Can any of the programmers explain it?

camera {
   location y/50
   angle 150
   look_at 0
}

light_source {100*y, color 1}

#declare X = clock;

#declare Y = pow(10,X*2-1);

plane {
   y, 0
   pigment {color 0}
   normal {onion 1.5*Y sine_wave scale 1*Y}
   finish {ambient 0 diffuse 0 phong 1 phong_size 300}
}
// Also try to rotate the effect... It's impossible!

Greetings,

Rune S. Johansen

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"Rune S. Johansen" wrote:
> 
> Try to render the little animation below and you will see the Ghost of POV
> effect caused by union and wood normals (name made up by Uwe Zimmermann I
> think). 10-20 frames is enough to get the idea.
> 
> I still wonder what actually causes the weird effect.
> 
> Can any of the programmers explain it?
> 

You mean those three mysterious light/dark spots?  That's caused by the
algorithm used to find the surface normal.  The algorithm (in the function
Perturb_Normal in the file normal.c for you programmers) uses a pyramid
of points to sample the normal map (here a simple onion pattern) to
determine the surface normal.  This locus of points is chosen at a
specific 'radius' around the sample point.  The 'radius' of this
pyramid of points is chosen by DELTA which is defined just prior to
Perturb_Normal.  By changing the value of DELTA I can make these dark
points move in and out.  Getting rid of them is something I do not know
how to do, though.  And making DELTA smaller or bigger causes other
problems.

-Nathan

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