Hi,

Here is a quite simple code:

<<<<<
#include "colors.inc"
#include "glass.inc"

#declare un=1/446;

camera {
          location <0, 0, -5*un>
          look_at  <0, 0, 0>
          angle 30
}

light_source { < 1*un,3*un,-4*un>, color White }
light_source { < 1*un,1*un,-4*un>, color White }

blob {
     threshold .5
     sphere {<0,0,0>,1*un,1}
     texture { pigment { color Green  } }
     sturm on
}

sphere {<0,0,0>,.1*un texture { pigment { color Red  } } }
 >>>>>

A trivial blob enclose a red sphere that shouldn't be visible.

A variable, "un" modify the scale of the whole scene, so that any 
reasonable value of un should return the same output.

For value of un>1/445 that seems to be the case.

For lower values, 1/446 and lower, the blob start rendering a bit odd, 
and in fact we start to see the red sphere. Before un=1/1000 the blob is 
completely gone, leaving just the red sphere.

1/1000 makes floating values around 10^-3 for the different objects that 
should be very reasonable for any scene computation accuracy. So why 
does my blob disappear?

Thanks for any help,

	Yannick

PS: I came on this while working on some spaceship rendering where large 
different scales are at work. I defined:
   #declare km=   1;
   #declare  m=  km/1000;
   #declare cm=   m/100;
   #declare mm=   m/1000;
   #declare um=   mm/1000;
On a single scene, it may likely range from a slightly sub-millimeter 
scale (details of everyday objects) to some hundreds of millions of km 
(planets in solar system), that is around 10^-7 to 10^9 in povray units.
I'm afraid that it wont work for the larger scales, and so that I'll 
have to cheat somehow (rescaling planets and putting them closer, 
maybe). But I wasn't expecting problems around 10-3.

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