With the following difference object, should I set my own bounded_by or will
POV bound the object to the box? I'm using megapov, but if there is a
difference I would be curious to know the answer for both standard and mega.
Thanks.

#declare sphere_slice =
sphere
{
  <-90.75, 7, 0> // center of sphere <X Y Z>
  90       // radius of sphere
}

difference{
  box{
    <-1, -4, -1>  // one corner position <X1 Y1 Z1>
    < 1,  8,  1>  // other corner position <X2 Y2 Z2>
  }
  object{sphere_slice}
  object{sphere_slice rotate y*90}
  object{sphere_slice rotate y*180}
  object{sphere_slice rotate y*-90}
  pigment{White}
}


--
Tom Melly
tom### [at] tomandluforce9couk
http://www.tomandlu.force9.co.uk

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On Thu, 23 Mar 2000 21:23:02 -0000, Tom Melly wrote:
>With the following difference object, should I set my own bounded_by or will
>POV bound the object to the box? I'm using megapov, but if there is a
>difference I would be curious to know the answer for both standard and mega.

An intersection takes the intersection of the bounding boxes of all of the
included objects (where they exist; see the recent discussion of intersecting
with a plane.)  It ignores objects that are inverted.  Difference is stored
internally as an intersection with inverses.  Thus, most differences are 
bounded by the bounding box of the first object in the difference (This does
not apply to differences that include inverted objects.)

Thus, your difference is bounded by the box and the spheres are ignored.  If
you can bound it more accurately, you should.

-- 
These are my opinions.  I do NOT speak for the POV-Team.
The superpatch: http://www2.fwi.com/~parkerr/superpatch/
My other stuff: http://www2.fwi.com/~parkerr/traces.html

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Ron Parker <ron### [at] povrayorg> wrote in message
news:38dbcb75$1@news.povray.org...

> ..... Thus, most differences are
> bounded by the bounding box of the first object in the difference (This
does
> not apply to differences that include inverted objects.)
>
> Thus, your difference is bounded by the box and the spheres are ignored.
If
> you can bound it more accurately, you should.
>

That's what I expected - ie. the differenced object can not be larger than
the first object in the difference so apply that objects bounding. What's an
inverted object?

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On Mon, 27 Mar 2000 12:49:56 +0100, Tom Melly wrote:
>
>Ron Parker <ron### [at] povrayorg> wrote in message
>news:38dbcb75$1@news.povray.org...
>
>> ..... Thus, most differences are
>> bounded by the bounding box of the first object in the difference (This
>does
>> not apply to differences that include inverted objects.)
>>
>> Thus, your difference is bounded by the box and the spheres are ignored.
>If
>> you can bound it more accurately, you should.
>>
>
>That's what I expected - ie. the differenced object can not be larger than
>the first object in the difference so apply that objects bounding. What's an
>inverted object?

One with the 'inverse' flag.  That is, 

  difference {
    sphere {0,1}
    sphere {x,1 inverse}
  }

is actually bounded by a tighter box than the one around the first sphere.
That's because it's actually an intersection of the two spheres, of course, 
but more complicated objects might have dozens of components in the difference
where only one or two are inversed like this.  In that case, the intersection
of all their bounding boxes is the bounding box of the object.  Also, you can
always rewrite such a difference unambiguously without the inverses:

  difference {
    object {A}
    object {B}
    object {C inverse}   
    object {D}
  }

can be rewritten as
  
  difference {
    intersection {
      object {A}
      object {C}
    }
    object {B}
    object {D}
  }



-- 
These are my opinions.  I do NOT speak for the POV-Team.
The superpatch: http://www2.fwi.com/~parkerr/superpatch/
My other stuff: http://www2.fwi.com/~parkerr/traces.html

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Ron Parker <ron### [at] povrayorg> wrote in message
news:38df68d8@news.povray.org...
> On Mon, 27 Mar 2000 12:49:56 +0100, Tom Melly wrote:
>
> One with the 'inverse' flag.
> ......

Many thanks - never clocked this flag before, I'll have a play with it.

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On Wed, 29 Mar 2000 10:34:45 +0100, Tom Melly wrote:
>
>Ron Parker <ron### [at] povrayorg> wrote in message
>news:38df68d8@news.povray.org...
>> On Mon, 27 Mar 2000 12:49:56 +0100, Tom Melly wrote:
>>
>> One with the 'inverse' flag.
>> ......
>
>Many thanks - never clocked this flag before, I'll have a play with it.

I've only come accross 'inverse' when using two planes in a scene, I got
a message saying that the camera was inside a non hollow object, to 
get rid of that message I had to make one of the planes inverse. 

-- 
Cheers
Steve              email mailto:sjl### [at] ndirectcouk

%HAV-A-NICEDAY Error not enough coffee  0 pps. 

web http://www.ndirect.co.uk/~sjlen/

or  http://start.at/zero-pps

 11:46am  up 12:35,  3 users,  load average: 1.07, 1.08, 1.07

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Steve <sjl### [at] ndirectcouk> wrote:
: I've only come accross 'inverse' when using two planes in a scene, I got
: a message saying that the camera was inside a non hollow object, to 
: get rid of that message I had to make one of the planes inverse. 

  Well, that's one way of doing it. The correct way would be reversing the
normal of the plane. The normal should always point outside the plane.

  There's one use for the inverse that I use very often: If you need a
real skysphere (ie. a sky sphere made with a real sphere):

sphere { 0,10000 inverse texture { Sky } }

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Warp <war### [at] tagpovrayorg> wrote in message news:38e2024d@news.povray.org...
>
>   There's one use for the inverse that I use very often: If you need a
> real skysphere (ie. a sky sphere made with a real sphere):
>
> sphere { 0,10000 inverse texture { Sky } }
>

Why not

sphere { 0,10000 hollow on texture { Sky } }

Is there a difference or is it just preference? (Actually, I like your
method - the idea of a sphere who's inside is the rest of the universe is,
well, bizarre and intriguing)

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Tom Melly <tom### [at] tomandluforce9couk> wrote:
: Why not

: sphere { 0,10000 hollow on texture { Sky } }

  Because that's a misuse of the hollow keyword. Also the normals get
inverted.
  It's mostly a preference, but I like to be precise. It's more intuitive
(to me) to define well the inside and the outside of the object. Some
features work differently depending on whether we are inside or outside.
  If I wanted, for example, a media outside the sky sphere (which is
actually the inside of the object), it would be possible. In this case
the sky sphere would have to be inversed and hollow.

  I'm a hopeless perfectionist.

  Btw: Any hope of getting the 'hollow' keyword changed to something better,
like 'allow_media' or 'container' or 'media_container' ?

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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On 29 Mar 2000 10:47:21 -0500, Warp wrote:
>  If I wanted, for example, a media outside the sky sphere (which is
>actually the inside of the object), it would be possible. In this case
>the sky sphere would have to be inversed and hollow.

Possible, but not useful.  Media effects only get calculated when the
ray LEAVES the object.

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