On 2013–03–04 Roland Thiers wrote:

> I'm learning a little bit of metapost language, using metafun manual.
> I made some background for a title (surely the code is awful, thanks 
> for suggestions of improvement) but it's static, i'd like to adapt it to 
> the widthof the text. I don't know if it is possible from my code 
> and if it is i don't know how to do !

You can use the variable OverlayWidth to obtain the width of the
overlay, which is determined by the provided text. You can use it as
follows:

\setuplayout
  [backspace=6cm]

\startuniqueMPgraphic{essai}
  path p,p',q,q';

  w:=2OverlayWidth;

  p := (0,0.1cm)     -- (w,0.3cm);
  q := (11cm,-0.7cm) -- (w,2.2cm);
  p':= (w,1.3cm)     -- (-0.4cm,1.4cm);
  q':= (0.3cm,2.4cm) -- (0.2cm,-0.3cm);

  pickup pencircle scaled 1mm;
  drawoptions(withcolor 0.3white);

  draw p; draw p';
  draw q; draw q';

  pair A; A:=q'intersectionpoint p';
  pair B; B:=q intersectionpoint p';
  pair C; C:=p intersectionpoint q;
  pair D; D:=q'intersectionpoint p;
  path s; s:= A--B--C--D--cycle;

  fill s withcolor .8white;
\stopuniqueMPgraphic

\defineoverlay
  [encadrement]
  [\uniqueMPgraphic{essai}]

\defineframed
  [boxframed]
  [frame=off,
   foregroundstyle=bolditalic,
   foregroundcolor=darkred,
   background=encadrement]

\starttext
  \boxframed{Devoir de mathématiques}
  \blank [2cm]
  \boxframed{Devoir de mathématiques and some more text}
\stoptext


Marco