\starttext

\startMPpage[offset=10pt]
    draw(btex this is a test etex) scaled 10 ;
    fill boundingbox currentpicture withcolor red withtransparency (1,.5) ;
\stopMPpage

\startMPpage[offset=10pt]
    draw(btex this is a test etex) scaled 10 ;
    fill bbox(currentpicture) withcolor green withtransparency (1,.5) ;
\stopMPpage

\startMPpage[offset=10pt]
    draw(btex this is a test etex) scaled 10 ;
    fill boundingbox currentpicture withcolor black withtransparency (1,.5) ;
    fill bbox(currentpicture) withcolor blue withtransparency (1,.5) ;
\stopMPpage

\stoptext
