Mike Williams wrote:
> Wasn't it Richard Kline who wrote:
> 
>>When I attempted to render the gravitation equipotential surface of two 
>>bodies (i.e the surfaces of the double planet in Robert Forward's 
>>"Flight of the Dragonfly"). The iso_surface object behaved oddly with 
>>these three behaviors:
>>
>>1) If the contained_by shape is larger than the true surface for the 
>>given threshold value, the entire contained_by shape (either box or 
>>sphere) is rendered as part of the surface.
>>
>>2) If parts of the surface extend beyond the contained_by shape, you get 
>>the difference of the contained_by shape and the true surface. (this is 
>>shown in POVRay Example 1 below)
>>
>>3) If the contained_by shape is made large enough to contain the true 
>>surface, camera, and light_source, you get the correct results for the 
>>iso_surface, but it takes a long time to render since every point in the 
>>scene has to be tested.
>>
> 
> 
> I confess to not having tried all your examples, but what you describe
> sounds exactly like rendering the isosurface inside out.
> 
> See <http://www.econym.demon.co.uk/isotut/insideout.htm>
> 
> The "inside" of an isosurface is the region where the function minus the
> threshold is less than zero and the outside is where it's greater than
> zero. For some functions this may be different from what common sense
> tells you should be the inside and outside.
> 
> Try using "isosurface {function {0 - Roche(x,y,z) }" and 
> "threshold -2.0" to flip the thing right side out.
> 

Negating the function and threshold worked.  Thanks.  Also to the other 
respondents, I did add 0.0001 to the denominator of the function to 
avoid having a singularity.  I just didn't mention it up top.

Richard Kline

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