"Amazon_Warrior" <heu### [at] yahoocom> wrote in message 
news:43c1ab14@news.povray.org...
>
> Just wondering if you can help.  I'm a relative newbie to POV, and  I've
> been trying to follow the help file included to solve this, but to no 
> avail.
> Basically, I want to create a nested while loop containing two different
> instructions- scale and translate- and apply it to a blob object.  The 
> help
> file suggests that I put the starting loop instructions just after the
> threshold statement of the blob object, like so:
>
> blob {
> threshold 0.5
> #declare Length = 0;
> #while ( Length >= -30 )
>  #declare Scale = 2;
>  #while ( Scale >= 0.25 )
>   sphere { <0,0,0>, 20, 1 scale <1,1,0.9> }
>   translate <Length,0,0>
>   scale Scale
>  #declare Length = Length - 0.25;
>  #declare Scale = Scale - 0.1;
> #end
> #end
> }
>
> However, when I run this, I get an error message:  No matching } in blob,
> sphere found instead.  (I'm using version 3.6, if that makes any
> difference.)  I've tried putting the loops in assorted other places as 
> well,
> but... nada.

You're applying transformations (the translate and scale), multiple times, 
only to the blob object itself but not to the sphere(s). Maybe I'm 
misunderstanding this myself here, don't know for sure, it just looks like 
this error is about that particular aspect even though the blob should still 
be transformable like other objects.

> I've also noticed that the help file indicates that the format of nested
> while loops should be:
>
> #declare Variable1 = 0;
> #while ( Variable1 <= X )
>  #declare Variable2 = 0;
>  #while ( Variable2 >= Y )
>   {{{Stuff here}}}
>  #declare Variable1 = Variable1 + A;
>  #end
>  #declare Variable2 = Variable2 - B;
> #end
>
> I've not found this works.  Instead, I put both #end statements after the
> last variable (as in the previous example), and it seems to work, which is
> confusing.  I'd be really grateful if someone could tell me what I'm doing
> wrong.

For the loops to retain their own identities they require the #end to be 
placed where only the corresponding #while contains the parts you want to 
change during the looping. If you have the #end's together you aren't 
applying anything unique to the first (or outer wrapping) loop, since both 
variables change during the second (inner wrapped) loop but nothing is done 
once it leaves it.

Back to your blob looping... you seem to be creating a series of shells. The 
placing of the spheres would repeat (0*x to -30*x), with smaller sizes (2 to 
0.25 radii). This won't matter much unless you animated it during the 
process, so I'm guessing you have some idea of what is being done there 
(like maybe only a while loop test?).

The actual while loops should be okay if it is written:

blob {
 threshold 0.5
// setup for outer (first) loop
 #declare Length = 0;
 #while ( Length >= -30 )
// set up for inner (second) loop [also gets reset during outer loop]
  #declare Scale = 2;
// run the following loop
  #while ( Scale >= 0.25 )
   sphere {
   <0,0,0>, 20, 1 scale <1,1,0.9>
   translate <Length,0,0>
   scale Scale
   } // enclose sphere statement here
// inner loop variable change
  #declare Scale = Scale - 0.1;
 #end
// outer loop variable change
  #declare Length = Length - 0.25;
 #end
}

If that's still not making sense enough please go ahead and ask more about 
it. I think you're going to want to shift the position around more than 
this, anyway, to keep the parts separate and visible. For example, by adding 
movement in another direction during the scaling part.

> Also, can someone fully explain trace for me?  I can't seem to get it to
> work properly, and I would like to be able to 'trace' an object 
> repeatedly,
> so that several instances of the same object are applied.  Don't know if 
> I'm
> making much sense here?

I like to point out the sample scene files when someone asks about such 
things as 'trace'!

[main POV-Ray files location]\scenes\language

There are three or four examples using trace there.

I can only manage a loose description of what takes place and how it is 
doing it, mimicking the Help wording, so I'd rather not try. ;)  Someone 
else might have insights into using it in #while loops so I'll leave that up 
to another person, if anyone else replies.

Bob

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