\setupcolors [state=start]

\startuseMPgraphic{fig1} w := 4cm ; h := 2cm ; ww := 1cm ; hh :=
1.5cm ; fill
(0,0)--(0,h)--(w,h)--(w,0)--(ww,0)--(w,hh)--(ww,hh)--(ww,0)--cycle
; \stopuseMPgraphic

\starttext

\input tufte
\input tufte

\startcolumnset
\placefigure[tblr]{blabla}{\useMPgraphic{fig1}}
\placefigure[tbrl]{blabla}{\useMPgraphic{fig1}}
\stopcolumnset

\input tufte

\startcolumnset

\placefigure[tblr:1]{blabla}{\useMPgraphic{fig1}}
\placefigure[tblr:2]{blabla}{\useMPgraphic{fig1}}

\stopcolumnset

\input tufte

\stoptext
