I was trying to make a rounded rectangle pane of glass by union { } of some
cylinders and boxes.  That of course is a bad idea because the interior
surfaces.  Replaced it by merge, and got a very ugly result.  It appears that
the ray tracing on a merged object gets very confused if the ray strikes a
surface that is coincident on more than one element.

I can cure this by intersecting the merged object with a box that's slightly
smaller, to slice off the original surfaces that coincide.  The test file below,
adapted from sample file iortest.pov, shows the problem.  The plus in the middle
has nasty artifacts (multi-color mottling) in the center of the plus.  The one
on the right shows the workaround.

--paul

// This work is licensed under the Creative Commons Attribution 3.0 Unported
License.
// To view a copy of this license, visit
http://creativecommons.org/licenses/by/3.0/
// or send a letter to Creative Commons, 444 Castro Street, Suite 900, Mountain
View,
// California, 94041, USA.

// Persistence Of Vision raytracer sample file.
// This file demonstrates the use of the file "ior.inc" and a few other
// interesting and useful tricks.  It can take a bit of time to render,
// (he said, understatingly), because of the transparency and because of
// the 7 element light bank (flourescent tube?).  Eliminating some of the
// lights (and adjusting the brightness color, "Watts", accordingly)
// will help quite a bit.
//
// -w320 -h240
// -w800 -h600 +a0.3

#version 3.6;

global_settings {
  assumed_gamma 1
  max_trace_level 20
  }

#include "colors.inc"
#include "shapes.inc"
#include "textures.inc"
#include "glass.inc"
#include "consts.inc"              // Index of refraction constants

camera {
   location <0, 5, -20>
   angle 57
   right   x*image_width/image_height
   look_at <0, 1, 0>
}

// Assemble a bank of lights here, on the ground...
#declare Watts = color 0.5;
#declare Light_Distance = -50;

union {
    light_source  { <-6, 0, Light_Distance>  color Watts  }
    light_source  { <-4, 0, Light_Distance>  color Watts  }
    light_source  { <-2, 0, Light_Distance>  color Watts  }
    light_source  { < 0, 0, Light_Distance>  color Watts  }
    light_source  { < 2, 0, Light_Distance>  color Watts  }
    light_source  { < 4, 0, Light_Distance>  color Watts  }
    light_source  { < 6, 0, Light_Distance>  color Watts  }

    rotate 60*x           // ... and hoist 'em up into the air

    pigment { White }  // Doesn't do anything but suppresses a parser warning
}


//
plane { z, 100
   pigment {
      checker color Gray20 color Gray80
      scale 10
   }
   finish {
      ambient 0.1
      diffuse 0.6
   }
  rotate <45, 0, 0>
}

// Set up a default texture for all objects that follow that don't already
// have a texture of their own
#default {texture {pigment {color Clear} finish {F_Glass1}}}

#macro plus(m, c)
  #if (c)
    intersection { box { <-3.1, -3.1, 0.01> < 3.1, 3.1, 0.99> }
  #end
  #if (m)
    merge
  #else
    union
  #end
  {
    box { <-2.5, -1, 0> <2.5, 1, 1> }
    box { <-1, -2.5, 0> <1, 2.5, 1> }
    #if (c)
  }
    #end
      interior { ior Diamond_Ior dispersion 1.1
 fade_power 2.0 fade_distance .5 fade_color rgb <0, 1, 0.5> }
      rotate <-20, 45, 0>
  }
#end

object { plus (0, 0) translate <-6, 1, 0> }
object { plus (1, 0) translate y }
object { plus (1, 1) translate <6, 1, 0> }

// end of file mergetest.pov

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