[NTG-context] Why must I move frame to center it?

[Hans Hagen]

John R. Culleton wrote:
> I am using some old code that Hans provided to create a pretty frame around 
> some text, for purposes of a "Part"  header page. Here is my current test 
> file:
> --------------------------------------
> \setupoutput[pdftex]
> \definepapersize[bok][width=6.0in,height=9.0in]
> \setuppapersize[bok][letter]
> \setuplayout[marking=on,margin=.5in,textwidth=4.5in,width=middle,location=middle]
> \setupcolors[state=start]
> \starttext
> %\startstandardmakeup
> \noheaderandfooterlines
> \startuseMPgraphic{bg}
>     StartPage ;
>       path p ; p := Page enlarged  -10mm ;
>       path q ; q := Page enlarged -15mm ;
>       drawoptions(withcolor .85white) ;
>       fill reverse topboundary    q -- topboundary    p -- cycle ;
>       fill reverse bottomboundary q -- bottomboundary p -- cycle ;
>       drawoptions(withcolor .65white) ;
>       fill reverse leftboundary   q -- leftboundary   p -- cycle ;
>       fill reverse rightboundary  q -- rightboundary  p -- cycle ;
>   
add setbounds currentpicture to Page ;
>     StopPage ;
> \stopuseMPgraphic
>   
hm, inches ...
> \vglue -1.55in  %???
> \hglue -.75in    %???     
> \useMPgraphic{bg}
> \vglue -6.5in
> \hglue .2in
> \centerline{\bf Here is some text}
> %\stopstandardmakeup
> \stoptext
> -------------------------------------------------
>
> Now here is my question: why must I use the negative \vglue and \hglue lines 
> (arrived at by experimentation) to place the frame properly on the page? Did 
> I mess something up?
>   


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                                          Hans Hagen | PRAGMA ADE
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