[NTG-context] drawarrow and transparency

[Hans Hagen]

On 2-5-2012 17:35, Alan BRASLAU wrote:
> I have a MetaPost question concerning drawarrow withtransparency
> (or any other compound object).
>
> Is there some clever way to achieve uniform transparency
> for a compound object? For example, in the minimal example
> below, how can one obtain a transparent arrow without getting
> an X-ray vision of the arrowhead?
>
> \starttext
> \startMPpage
> 	drawarrow origin--(1cm,0) withtransparency(1,.5) ;
> 	drawarrow origin--(0,1cm) withtransparency(1,.5) ;
> \stopMPpage
> \stoptext

This is a side effect of an arrow being a composed graphic. I 
experimented a bit with transparency grouping and after several variants 
I settled to the following approach:

\starttext

\startMPpage[offset=1mm]

     drawarrow (1cm,0cm) -- (0cm,0cm) withcolor red 
withtransparency(1,.5)  ;

     draw image (
         drawarrow (1cm,0cm) -- (0cm,0cm) withcolor red 
withtransparency(1,.5)  ;
     ) shifted (0,2mm) asgroup "" ;

     draw image (
         drawarrow (1cm,0cm) -- (0cm,0cm) withcolor red 
withtransparency(1,.5)  ;
     ) shifted (0,4mm) asgroup "isolated" ;

     draw image (
         drawarrow (1cm,0cm) -- (0cm,0cm) withcolor red 
withtransparency(1,.5)  ;
     ) shifted (0,6mm) asgroup "knockout" ;

     draw image (
         drawarrow (1cm,0cm) -- (0cm,0cm) withcolor red 
withtransparency(1,.5)  ;
     ) shifted (0,8mm) asgroup "isolated,knockout" ;

     addbackground withcolor green ;

\stopMPpage

\stoptext

As we have to use xforms (a pitty that this is needed) there is the 
usual interference and something fishy with clipping is going on.

Hans

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                                           Hans Hagen | PRAGMA ADE
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