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

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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 -

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> >     #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

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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

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> 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

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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 _____./\/^>_*_<^\/\.______

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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

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> 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

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> 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

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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

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In article <3DDCFF73.C06ACF8C@frege.mathematik.uni-freiburg.de>,
 Mark Weyer <wey### [at] fregemathematikuni-freiburgde> wrote:

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

It is in no way "elegant". It needlessly slows things down, takes 
memory, and is just an ugly hack. The array method looks like the best 
way to do it: simple, flexible, doesn't waste processing power or do 
other odd things that could cause trouble. You don't need a "nothing" 
object.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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Mark Weyer <wey### [at] fregemathematikuni-freiburgde> wrote:
> What is your suggestion for maximum speed?

  Don't use an object at all. That achieves maximum speed.

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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Mark Weyer <wey### [at] fregemathematikuni-freiburgde> wrote in 
news:3DDCF201.8E1826C9@frege.mathematik.uni-freiburg.de

> Any suggestions?

maybe :

isosurface { function { 1 } gradient 0.01 accuracy 100000 
contained_by { box { -1e5,+1e5 } }


-- 
#macro g(U,V)(.4*abs(sin(9*sqrt(pow(x-U,2)+pow(y-V,2))))*pow(1-min(1,(sqrt(
pow(x-U,2)+pow(y-V,2))*.3)),2)+.9)#end#macro p(c)#if(c>1)#local l=mod(c,100
);g(2*div(l,10)-8,2*mod(l,10)-8)*p(div(c,100))#else 1#end#end light_source{
y 2}sphere{z*20 9pigment{function{p(26252423)*p(36455644)*p(66656463)}}}//M

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Mark Weyer wrote:

> I occasionally (usually to feed parameters to CSG macros where) need an
> empty object.
>
> [...]
>
> Any suggestions?

sphere { 0, 0 clipped_by { sphere { 1, 0 } }

-- 
Ignacio Fernández Galván
Linux User #289967 (counter.li.org)
PGP Pub Key ID: 0x01A95F99 (pgp.escomplinux.org)

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