So sorry I didn't get back to you earlier, I don't read this group as
often as others. Following is your scene file re-done. Not sure if it
will make any more sence, I am still puzzled by the behaviour
surrounding it, though it is fixed(?).

//  totally crazy ????
#version 3.1;

global_settings
{
  assumed_gamma 1.0
}

//
// a camera
//
camera
{
  location  <-12, 0, 0>
  look_at   <7, 0, 0>
}
//
// some lights
//
//light_source { <-5,3,-5> color rgb <1,1,1>}
//light_source { <-5,3,5> color rgb <1,1,1>}
//light_source { <-5,-3,-5> color rgb <1,1,1>}
light_source { <-5,-3,5> color rgb <1,1,1>}
//
// A Tube
//
#declare Tube=difference {
        cylinder {-2.6*x,2.6*x,.2}
        cylinder {-2.7*x,2.7*x,.15}
     // pigment {rgbf 1}
}
//
// I put it around a sphere
#declare Machin=union {
        sphere {0,2.5}
        #local Nb=1;
        #while (Nb<9)
                #local Angle=(360/8)*Nb;
                object { Tube
                        rotate 25*z
                        rotate Angle*y
                }
                object { Tube
                        rotate 50*z
                        rotate (360/16+Angle)*y
                }
                #local Nb=Nb+1;
        #end
}
//
// I want to make this object empty
// So I make a CSG difference with a sphere 
//
#declare Machin1=difference {
        object {Machin} // Radius of sphere : 2.5
        sphere {0,1 pigment {rgb<0,1,0>} finish {ambient 1}} // a
smaller radius
      inverse
}

//difference {
 union {
object {Machin // Not empty object
        translate 3*z
        pigment {color rgb 1.5*<.6, .25, .15>}
        finish {ambient 0.4}
     //inverse
}
object {Machin1 // empty object : what about the black circle on his
surface !!!!!
        translate -3*z
        pigment {color rgb 1.5*<.6, .25, .15>}
        finish {ambient 0.4}
     //inverse
}
 }
// plane {x,0 pigment {rgbf 1}} //attempted cut-away looks strange (-x
if inversed above)
   //inverse
//}


bassompierre laurent wrote:
> 
> Sorry, but my english is too poor to understand your explanation. Can you
> post my code modified please ?
> Thanks
> Laurent bassompierre
> lba### [at] ubifr
> 

> >'inverse' used either within the Machin object (used in Machin1) or the
> >'difference' of it and the smaller sphere removes the spot (it is the
> >inner sphere or hollow caused by it you see). I have no explanation for
> >this effect. Appears to cause a hole in the surface.
> >I tried a difference on the 2 test objects for a cut-away view and only
> >the correct one on the left side seems right no matter what I tried. The
> >one on the right (incorrect) did not have the proper interior.
> >Anyhow unless you are going to be viewing the interior the 'inverse'
> >will fix it.
> >Something odd does seem to be going on though, just wish I knew what.
> >
> >

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