POV-Ray : Newsgroups : povray.binaries.images : Glass/water interface Server Time: 30 Apr 2017 03:13:16 GMT
  Glass/water interface (Message 1 to 2 of 2)  
From: Cousin Ricky
Subject: Glass/water interface
Date: 9 Apr 2017 13:57:44
Message: <58ea3dd8@news.povray.org>
After years of procrastinating, I finally did it: I compared various 
models of glass/water interface.

Five models are attached.  After I started work, it occurred to me that 
the demo would be clearer if I used colored glass and beer.  However, I 
do not know the IOR of beer, so I'm calling it unsweetened green tea, 
which is essentially water.  (Besides, bubbles and foam would just 
complicate the demo.)  The colors are modeled entirely by interior fading.

The leftmost image is a naive construction with coincident surfaces.  No 
more need be said.

In the second from the left, the water overlaps the glass. 
Surprisingly, this killed the coloring of the water--totally 
unacceptable.  It left weird highlights that don't show up in real life, 
and whose render blocks took a long time to complete.

The middle image has a tiny space between the water and the glass.  This 
is a vast improvement.  However, there are some unrealistic double 
reflections, and dubious highlights around the top surface of the water.

The second image from the right uses a solution which I have vaguely 
imagined for years, but never managed to hammer out until now: clipping 
the section of the glass where it contacts the water.

The rightmost image uses the technique that Le Forgeron published in the 
wiki.  The technique makes sense on paper, but there are a few problems 
with the resulting image that I cannot explain:
  - The green of the glass shows strongly at the top surface of the
    water.  This seems unlikely given how little green shows through
    the actual glass.
  - Where the straw enters the water, it appears bent at a sharper
    angle than in the other images, as if POV-Ray were using the IOR of
    the glass instead of the water.
  - More subtly, the glass above the water is less green than in the
    other images, and the reflection of the straw in the glass is much
    weaker.
These problems suggest that cutaway_textures does not work on interiors 
the way I (and presumably Jerome?) would expect it to.

The wiki example has an unspecified material for the air.  I have 
omitted one because an interior would seem to be redundant, and a 
texture would defeat the cutaway_textures.  If these assumptions are 
wrong, please let me know.

----------[BEGIN CODE EXCERPT]----------
// +kkff5
#version 3.71;

#ifndef (Test) #declare Test = frame_number; #end

#include "ior.inc"

#declare RGLASS = 0.6;
#declare RWATER = 0.55;
#declare YTOP = 2;
#declare YSURFACE = 1.4;
#declare YBOTTOM = 0;
#declare YFLOOR = 0.15;
#declare RSTRAW = 0.075;
#declare EPSILON = 0.001;

#declare Straw = cylinder
{ 0, 3 * y, RSTRAW open
   pigment
   { radial color_map
     { [0.3 rgb <0.7, 0, 0.07>]
       [0.3 rgb 0.8]
     }
     frequency 4
     rotate 0.35 * 90 * y
   }
   finish { diffuse 1 ambient sotd_c_Ambient }
}

#declare t_Clear = texture
{ pigment { rgbf 1 }
   finish
   { fresnel conserve_energy
     reflection { 0 1 }
     specular albedo 1
     roughness 0.0001
   }
}

#declare m_Glass = material
{ texture { t_Clear }
   interior
   { ior iorGlass
     fade_color rgb <0.1, 0.9, 0.5>
     fade_distance 0.5
     fade_power 1000
   }
}

#declare m_Water = material
{ texture { t_Clear }
   interior
   { ior 1.33
     fade_color rgb <0.95, 0.75, 0.05>
     fade_distance 0.5
     fade_power 1000
   }
}

#switch (Test)
   #case (1)
     #declare Glass_of_water = union
     { difference
       { cylinder { YBOTTOM * y, YTOP * y, RGLASS }
         cylinder { YFLOOR * y, (YTOP + EPSILON) * y, RWATER }
         hollow
         material { m_Glass }
       }
       cylinder
       { YFLOOR * y, YSURFACE * y, RWATER
         hollow
         material { m_Water }
       }
     }
     #break
   #case (2)
     #declare Glass_of_water = union
     { difference
       { cylinder { YBOTTOM * y, YTOP * y, RGLASS }
         cylinder { YFLOOR * y, (YTOP + EPSILON) * y, RWATER }
         hollow
         material { m_Glass }
       }
       cylinder
       { (YFLOOR - EPSILON) * y, YSURFACE * y, RWATER + EPSILON
         hollow
         material { m_Water }
       }
     }
     #break
   #case (3)
     #declare Glass_of_water = union
     { difference
       { cylinder { YBOTTOM * y, YTOP * y, RGLASS }
         cylinder { YFLOOR * y, (YTOP + EPSILON) * y, RWATER }
         hollow
         material { m_Glass }
       }
       cylinder
       { (YFLOOR + EPSILON) * y, YSURFACE * y, RWATER - EPSILON
         hollow
         material { m_Water }
       }
     }
     #break
   #case (4)
     #declare Glass_of_water = union
     { difference
       { cylinder { YBOTTOM * y, YTOP * y, RGLASS }
         cylinder { YFLOOR * y, (YTOP + EPSILON) * y, RWATER }
         clipped_by
         { cylinder
           { (YFLOOR - EPSILON) * y, YSURFACE * y, RWATER + EPSILON
             inverse
           }
         }
         hollow
         material { m_Glass }
       }
       cylinder
       { YFLOOR * y, YSURFACE * y, RWATER
         hollow
         material { m_Water }
       }
     }
     #break
   #case (5)
     #declare Glass_of_water = difference
     { union
       { cylinder
         { YBOTTOM * y, YTOP * y, RGLASS
           hollow
           material { m_Glass }
         }
         cylinder
         { YFLOOR * y, (YSURFACE + EPSILON) * y, RWATER
           hollow
           material { m_Water }
         }
       }
       cylinder // air
       { YSURFACE * y, (YTOP + EPSILON) * y, RWATER + EPSILON
         hollow
       }
       cutaway_textures
       hollow
     }
     #break
#end

union
{ object
   { Glass_of_water
     photons { target collect off reflection on refraction on }
   }
   object
   { Straw
     translate -RSTRAW * x
     rotate degrees (-atan2 (2 * (RWATER - RSTRAW), YTOP - YFLOOR)) * z
     translate <2 * RSTRAW - RWATER, YFLOOR, 0>
   }
}
-----------[END CODE EXCERPT]-----------

Notes:

1. The environment is provided by my personal prefab render rig, thus,
    that code is not shown.

2. The assumed_gamma for this scene is 1.0.

3. The identifier sotd_c_Ambient is declared by my render rig, so the
    soda straw will not parse as-is.  Since I used radiosity for this
    render, the value is irrelevant, but it was calculated to be rgb
    0.26735.

4. There is a tiny space between the glass and the checkered plane.
    No sense in having the render rig create its own coincident
    surface!

5. Photon spacing was set at 0.002.

6. The sole reason for #version 3.71 was finish-level fresnel.  For
    similar effects in 3.7.0, you will need to use different textures
    for the glass and the water.  Move the fresnel keyword inside the
    reflection block, and set specular albedo 0.053 for the glass and
    specular albedo 0.026 for the water.

   6a. If you are using 3.6.*, delete the #include "ior.inc" line, use
       specular 67 and ior 1.5233 for the glass, and use specular 32
       for the water.  (Better yet, upgrade.)


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Download 'glass+water_montage.jpg' (124 KB)

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glass+water_montage.jpg


 

From: Alain
Subject: Re: Glass/water interface
Date: 10 Apr 2017 02:46:51
Message: <58eaf21b@news.povray.org>
Le 17-04-09 à 09:57, Cousin Ricky a écrit :

The first 3 are clearly not correct.

The clipped glass looks the best.

The last looks like the top surface of the liquid is treated as if it 
was the glass itself : Larger ior and get the glass' fading colour.
cutaway_textures work on tectures ONLY. Interiors are NOT textures and 
not part of textures, so, it should be obvious that they are not 
affected by cutaway_textures, and should not be.


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