[metapost] Redefine ..

[Laurent Méhats]

Le 04/06/2012 18:00, Troy Henderson a écrit :
> I am drawing 3D graphics, and many of my "points" are colors instead of
> pairs.  Of course, the colors are projected to pairs eventually, but I
> would like to define paths of colors directly with .. by doing something like
>
> path p;
> p:=(1,2,3)..(4,5,6)..(7,8,9);
>
> I have a vardef P that transforms colors into pairs, and thus I can always
> just do
>
> p:=P(1,2,3)..P(4,5,6)..P(7,8,9);
>
> but I would like to be able to omit this P usage.  My attempt thusfar is
>
> ---
> def pathnode primary a =
>     if color a:
>        P(a)
>     else:
>        a
>     fi;
> enddef;
>
> primarydef a .. b =
>     pathnode(a) .. pathnode(b)
> enddef;
> ---
>
> but this is not working.  I would appreciate any insight into how I might
> fix this so that I can use .. between pairs.
>
> Thanks in advance,
>
> Troy Henderson
>
>
> --
> http://tug.org/metapost/

A dirty workaround could be to scan color-path expressions looking for 
colors, then prefixing them with P (thus turning 
"(1,2,3)..(4,5,6)..(7,8,9)" into "P(1,2,3)..P(4,5,6)..P(7,8,9)"). Here is 
a naive attempt.

vardef P expr clr=
   (redpart clr, greenpart clr)
enddef;

vardef map @# expr pth=
   string fun, aux, res, chr, acc;
   numeric len;
   fun:=str @#;
   aux:=pth;
   len:=length aux;
   res:="";
   forever:
     exitif len=0;
     chr:=substring(0, 1) of aux;
     aux:=substring(1, len) of aux;
     len:=len-1;
     if chr<>"(":
       res:=res&chr;
     else: % we look for the next ")" occurrence
       acc:=chr;
       forever:
         exitif len=0;
         chr:=substring(0, 1) of aux;
         aux:=substring(1, len) of aux;
         len:=len-1;
         acc:=acc&chr;
         exitif chr=")";
       endfor
       if color scantokens acc:
         res:=res&fun&acc;
       else:
         res:=res&acc;
       fi
     fi
   endfor
   show res;
   scantokens res
enddef;

draw map P "(0, 1, 2) .. controls (3, 4, 5) .. {(6, 7, 8)} (9, 0) .. cycle";

 >> "P(0, 1, 2) .. controls P(3, 4, 5) .. {P(6, 7, 8)} (9, 0) .. cycle"

Regards,
Laurent Méhats