[NTG-context] shading in metapost

Hi all, I have a somewhat silly question about metapost/metafun. To emphasize parts of a picture, I place a bright red arrow or circle/ellipse on top of the picture. I would like to make this arrow more visible by putting a shade beneath it (I attach a jpg that will hopefully show the desired effect). Right now, I simply draw the arrow twice with a transparent black color, once with a smaller pencircle, so the effect is like a shadow that is darker in the center than at the edges. However, I think the effect would be more dramatic if I could have a transparent area with a smooth grading. Is this possible at all in metapost? filling a path with a transparent AND shaded color? All best Thomas