Haha!  This is kinda what I was looking for in the first place!

It sums everything up pretty well ;)

-Ian

>   I like to think of CSG operations as to how they perform physically in
> an operation. My idea of more better analogies would be:
>
> union        = gluing
> intersection = cutting
> difference   = scooping
> merge        = melting and gluing
> clipping     = cutting and hollowing
> inverse      = inside outing
>
> There ! That should give you something to cry in despair over for a while
:)
>
> --
> Ken Tyler
>
> See my 1000+ Povray and 3D Rendering and Raytracing Links at:
> http://home.pacbell.net/tylereng/index.html

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Ron Parker <par### [at] fwicom> wrote:
: It's also
: possible that POV automatically ignores objects that are entirely
: further away than the closest intersection so far.  If it doesn't,
: it should.

  How can it know if the object is further or not without testing the
ray-object intersection?

-- 
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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Ken <tyl### [at] pacbellnet> wrote:
: intersection = cutting
: difference   = scooping

  I have always thought about the difference as cutting parts out from the
main object.

: clipping     = cutting and hollowing

  Is clipping considered CSG? Note that the 'S' stands for 'solid' :)

  I think that it would also be useful if you explained the difference
between intersection and clipping and when it's better to use clipping
instead of intersection.

-- 
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 20 Sep 1999 06:32:51 -0400, Nieminen Juha wrote:
>Ron Parker <par### [at] fwicom> wrote:
>: It's also
>: possible that POV automatically ignores objects that are entirely
>: further away than the closest intersection so far.  If it doesn't,
>: it should.
>
>  How can it know if the object is further or not without testing the
>ray-object intersection?

By taking the dot product of each of the six bounding box coordinates 
with the ray direction.  If they're all further away, then the object
must be.

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Nieminen Juha wrote:
> 
> Ken <tyl### [at] pacbellnet> wrote:
> : intersection = cutting
> : difference   = scooping
> 
>   I have always thought about the difference as cutting parts out from the
> main object.

In english the word "scooping" implies that but perhaps not as definitavely as
you have stated it.
 
> : clipping     = cutting and hollowing
> 
>   Is clipping considered CSG? Note that the 'S' stands for 'solid' :)
> 
>   I think that it would also be useful if you explained the difference
> between intersection and clipping and when it's better to use clipping
> instead of intersection.

You might be right here.

Intersections will slice or cut an object and then apply a face where the
slice has been made.

Clipping slices an object but will not leave a face where the slice has
occured. Instead the object that has been sliced will now appear to be
hollowed out since all solid objects in POV-Ray are not truly solid.

-- 
Ken Tyler

See my 1000+ Povray and 3D Rendering and Raytracing Links at:
http://home.pacbell.net/tylereng/index.html

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Ron Parker <par### [at] fwicom> wrote:
: By taking the dot product of each of the six bounding box coordinates 
: with the ray direction.  If they're all further away, then the object
: must be.

  You forget that the bounding box is not necesarily around the object...
nor it is a box... :)

-- 
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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clipped_by should be used instead of difference when the new surface is
not seen (for example, if you clip a torus and attach two cylinders to the
ends to form a bended tube, the clipped surfaces are inside the tube and thus
are not seen).
  Why it should be used? Because it's faster to render.

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

Post a reply to this message

On 20 Sep 1999 11:15:00 -0400, Nieminen Juha wrote:
>Ron Parker <par### [at] fwicom> wrote:
>: By taking the dot product of each of the six bounding box coordinates 
>: with the ray direction.  If they're all further away, then the object
>: must be.
>
>  You forget that the bounding box is not necesarily around the object...
>nor it is a box... :)

You forget that if it's not around the object, it shouldn't matter to the
ray-intersection routine (obviously in practice it works in such a way that
vampire objects are possible, but that's a side-effect that could easily 
disappear with better optimization.)  And rest assured that finite objects 
have bounding boxes whether or not such boxes are specified in the script; 
that's how the min_extent and max_extent keywords in the superpatch work.

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On Sun, 19 Sep 1999 20:10:33 -0700, Ken <tyl### [at] pacbellnet> wrote:

>intersection = cutting

  Ken, I've always thought of intersection as an area of common overlap
among objects.

-- 
Alan ---------------------------------------------------------------
http://www.povray.org - Home of the Persistence of Vision Ray Tracer
news.povray.org - where POV-Ray enthusiasts around the world can get
together to exchange ideas, information, and experiences with others

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Same here...

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