[NTG-context] clipping a MetaPost graphic

hi all I made a MetaPost graphic and I want to use this graphic for a background. The graphic is too large and I want to clip it to the size of 84mm width and 54mm height. With "clip currentpicture to unitsquare xyscaled (86mm,54mm)" I get the size I wanted but the clipped field should be placed in the center of the currenpicture. I tried: "clip currentpicture to unitsquare xyscaled (86mm,54mm) shifted center boundingbox currentpicture" but that became not the effect I wanted.How can I shift the clipping area to the center of the currenpicture? thx for help severin