Or maybe a method in MP to set the bounding box of the resulting overlay?
setbounds in combination with leftenlarged, topenlarged etc.
Thanks Wolfgang, this is the kind of solution I was looking for: \startuseMPgraphic{cell:triangle} path p ; p := unittriangle rotated 90 xscaled 2.5 OverlayWidth yscaled 2.5 OverlayHeight ; draw p withcolor red ; setbounds currentpicture to p topenlarged 6mm ; \stopuseMPgraphic \defineoverlay[triangle][{\uniqueMPgraphic{cell:triangle}}] \starttext \bTABLE[frame=off] \bTR \bTD[nx=2,align=middle,background=triangle] \dontleavehmode a\crlf text \eTD \eTR \bTR[toffset=1mm] \bTD left \eTD \bTD right \eTD \eTR \bTR \bTD another \eTD \bTD row \eTD \eTR \eTABLE \stoptext Thanks also for the explanation of backgroundoffset: it's an offset relative only to the background, and it moves in the opposite direction of the content offset of \framed. Looks like \framed[...,offset=VALUE,backgroundoffset=frame,...] is equivalent to \framed[...,offset=VALUE,backgroundoffset=-VALUE,...]. I tried to add something to the WIKI to show how it works, but the previews I got were not consistent with what I'm obtaining on my PC (with LMTX and "context --luatex"), so I gave up. Massi