如何在透明矩形上绘制阴影？

When I add a dropshadow bitmap effect to a rectangle, the dropshadow takes into account the transparency of the rectangle (makes sense). Is there a way to render a dropshadow on a transparent rectangle 'as if' the rectangle were opaque? ie what would appear is a rectangle-shaped 'hole', with a dropshadow.When I add a dropshadow bitmap effect to a rect