POV-Ray : Newsgroups : povray.binaries.tutorials : Isosurface Tutorial updated : Re: Isosurface Tutorial updated Server Time
30 Mar 2023 19:12:51 EDT (-0400)
  Re: Isosurface Tutorial updated  
From: Mike Williams
Date: 14 May 2000 14:37:57
Message: <ZTq9ZDAyVhH5EwXn@econym.demon.co.uk>
Wasn't it Chris Huff who wrote:
>In article <6dS### [at] econymdemoncouk>, Mike Williams 
><mik### [at] econymdemoncouk> wrote:
>> An alternative solution is to add the keyword "open" just after the
>> contained_by statement.
>Note that this solution doesn't help if your object contains media or is 
>used in CSG...though "open inverse" should work. I would suggest just 
>using a negative threshold, or declaring the function(a good practice 
>anyway) and using it's negative("-FuncName(x,y,z)").
>Or use "sign -1". :-)

Using a negative threshold isn't a solution, it just makes the surface
that you're interested in larger. MegaPOV still treats locations where
the function is numerically less than the threshold as "inside".

Experiments also reveal that "open inverse" produces a rather different
effect from "sign -1". In CSG, the "open" seems to be ignored most of
the time, so the objects intersect with just the "inverse", i.e. the
positive copy of the target surface inside an inverse contained_by

Another solution relies on the fact that all (or is that "almost all")
the problematic surfaces are those from the "i_algbr" library and
therefore have what I call "field strength" as their first parameter.
Negating the field strength (e.g. function {"Bicorn" <-1,1>}) has
exactly the same effect as adding "sign -1".

The problematic surfaces seem to be those in the i_algbr library where
the origin lies in the region that we would intuitively consider to be
the "outside" of the object. The author of the i_algbr library seems to
have rigged the code so that the point <0,0,0> is always in the "inside"
region, even in surfaces like Torus_gumdrop and Dupin_Cyclid (which are
rather like torusses, with <0,0,0> in the middle of what we would
consider to be the hole) .

Mike Williams
Gentleman of Leisure

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