Hi,

I occasionally (usually to feed parameters to CSG macros where) need an
empty object.
(This means that the spatial extension is empty.) Consider the following
approaches:

    #declare No_object1 = box {0 0}
    #declare No_object2 = intersection {box {0 0} box {1 1}}
    #declare No_object3 = quadric {0 0 0 1}

The first is quite fast but actually not correct: The object contains
one point.
It might happen that a stray ray accidentally hits this point, causing
artefacts.
The second remedies the problem, but causes a

    Warning: Degenerate CSG bounding box (not used!).

which I dislike for aesthetical reasons. The third, which I currently
use,
is fine. But it has no bounding box either and hence uses as much time
as the second. I might add a bounding box to the third approach, but I
still hope there might be a more elegant solution.

Any suggestions?

  Mark Weyer

Post a reply to this message

Mark Weyer <wey### [at] fregemathematikuni-freiburgde> wrote:
>     #declare No_object1 = box {0 0}

> The first is quite fast but actually not correct: The object contains
> one point.
> It might happen that a stray ray accidentally hits this point, causing
> artefacts.

  Add "no_image no_reflection no_shadow" to it, so it should be completely
ignored (at least in theory).

-- 
#macro N(D)#if(D>99)cylinder{M()#local D=div(D,104);M().5,2pigment{rgb M()}}
N(D)#end#end#macro M()<mod(D,13)-6mod(div(D,13)8)-3,10>#end blob{
N(11117333955)N(4254934330)N(3900569407)N(7382340)N(3358)N(970)}//  - Warp -

Post a reply to this message

> >     #declare No_object1 = box {0 0}
>
>   Add "no_image no_reflection no_shadow" to it, so it should be completely
> ignored (at least in theory).

Not if used in difference. Or do I get something wrong?

To be more precise:

difference {
  box {<-1,-1,0> <1,1,1}
  box {0 0 no_image no_reflection no_shadow}
}

I did not try this, but if the stray ray actually hits the 0-box, I think it
would decide not to have hit the surface of the defference yet. Hence
it would continue INSIDE the object.

  Mark

Post a reply to this message

On Thu, 21 Nov 2002 16:09:02 +0100, Mark Weyer
<wey### [at] fregemathematikuni-freiburgde> wrote:
> Not if used in difference. Or do I get something wrong?
> To be more precise:
> difference {
>   box {<-1,-1,0> <1,1,1}
>   box {0 0 no_image no_reflection no_shadow}
> }

Do you really need an empty object ? Can't you directly use first box object
then instead of whole difference object ?

ABX

Post a reply to this message

> Do you really need an empty object ? Can't you directly use first box object
> then instead of whole difference object ?

It seems I forgot to repeat the following information in my
second post: It need the empty object to feed it as a parameter
to macros using CSG.

Example: I might have a macro for doors. One of its parameters
is the key shape, so I can carve in (using difference) an appropriate
keyhole. Sometimes I might need a door without a keyhole. The I
would feed the empty object for the key shape. Following your idea
I would have to make a second copy of the door macro where I omit
the carving out of the keyhole.

  Mark Weyer

Post a reply to this message

Mark Weyer wrote:
> 
> It seems I forgot to repeat the following information in my
> second post: It need the empty object to feed it as a parameter
> to macros using CSG.

If you are not up to maximum speed just use a poly or isosurface with no
solutions...

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 15 Nov. 2002 _____./\/^>_*_<^\/\.______

Post a reply to this message

On Thu, 21 Nov 2002 16:26:12 +0100, Mark Weyer
<wey### [at] fregemathematikuni-freiburgde> wrote:
> > Do you really need an empty object ? Can't you directly use first box object
> > then instead of whole difference object ?
>
> It seems I forgot to repeat the following information in my
> second post: It need the empty object to feed it as a parameter
> to macros using CSG.

I already saw this but I intentionaly provoked you to more precise answer :-)

> Example: I might have a macro for doors. One of its parameters
> is the key shape, so I can carve in (using difference) an appropriate
> keyhole. Sometimes I might need a door without a keyhole. The I
> would feed the empty object for the key shape. Following your idea
> I would have to make a second copy of the door macro where I omit
> the carving out of the keyhole.

There is a lot of ways how you can do this without empty object.

For example:

// indexes to array
#declare DoorsId=0;
#declare KeyholeId=1;
#declare Key=2;


#macro Room(Elements)
  #local Door=Elements[DoorsId];
  #ifdef(Elements[KeyholeId])
    #local Door=intersection{
                  object{Door}
                  object{Elements[KeyholeId]}
                };
    #ifdef(Elements[Key])
      #local Door=union{
                    object{Door}
                    object{Elements[Key]}
                  };
    #end
  #end
#end

ABX

Post a reply to this message

> There is a lot of ways how you can do this without empty object.

You are right. But I still prefer the empty-object-method for
some applications, because it is more elegant to my taste.

  Mark

Post a reply to this message

> If you are not up to maximum speed just use a poly or isosurface with no
> solutions...

In fact, my third approach (quadric {0 0 0 1}) is quite similar.
What is your suggestion for maximum speed?

  Mark

Post a reply to this message

On Thu, 21 Nov 2002 16:44:51 +0100, Mark Weyer
<wey### [at] fregemathematikuni-freiburgde> wrote:
> > There is a lot of ways how you can do this without empty object.
>
> You are right. But I still prefer the empty-object-method for
> some applications, because it is more elegant to my taste.

Wasting memory for not necessary CSG objects is elegant? I doubt.

ABX

Post a reply to this message