POV-Ray : Newsgroups : povray.advanced-users : passing variable to macro for use in #for statement : Re: passing variable to macro for use in #for statement Server Time: 24 May 2019 01:38:42 GMT
  Re: passing variable to macro for use in #for statement  
From: Kenneth
Date: 20 Sep 2018 19:00:00
clipka <ano### [at] anonymousorg> wrote:
> Am 19.09.2018 um 17:58 schrieb Bald Eagle:

> > Along the same lines, is it possible to assign an image_map file type
> > to a variable-- like png, jpeg, etc. ?

>
> ...reasonably clean would be the following construct:
>
>     #macro FormatKeyword(Format)
>       #if (Format = "png")
>         png
>       #elif (Format = "jpeg")
>         jpeg
>       ....
>       #end
>     #end
>
>     #declare Format = "png";
>     image_map { FormatKeyword(Format) "foo.png" }

Hey, that's GREAT! I struggled with this idea years ago, and had to use some
ugly kludges to get this result. Using a macro didn't occur to me (I probably
thought it wouldn't work.) Thanks, it's a really nice construct.

This would be the (longer) way that I would write such a macro (just for
visual/mental clarity):

#macro FormatKeyword(Format)
#if (Format = "png")
png
#else
     #if(Format = "jpeg")
     jpeg
     #else
          #if(Format = "bmp")
          bmp
          #else
          #end
     #end
#end
#end // of macro

#declare Format = "png";
box{0,1
pigment{
image_map{FormatKeyword(Format) "crater demo as PNG.png" interpolate 2}

.... which works very nicely.

A couple of questions/observations, though: The docs say two things at
"3.3.2.6.1 The if...else...end Directives" (in v3.7.1 beta 9's help file--
although I haven't yet checked the online wiki):

"...an optional #elseif clause is now supported."

and

"NOTE: Nesting directives in the following manner has been known to cause
problems during the parse phase.
#if( #if(yes) yes #end ) #end
"

The even shorter keyword  #elif  (for #elseif) is not mentioned there (even
though it works); and, its keyword 'color' in scsne code looks like a
'non'-keyword. Should both of those little errors be fixed?

More importantly, I've never had a problem using nested #if's (when including
all of the #else keywords, anyway.) I *think* my macro is a good demonstration
of what the docs mean by nested #ifs as explained there(?) So are the new
keywords #elseif and #elif designed to eliminate any *possible* nested-#ifs
problems (along with being 'shorthand' versions of such constructs)? Just
curious.


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