Problems with masks

Is there a way that I could use a semi-transparent bitmap as a mask for a display object… That is; the solid shape of the bitmap acts as the ‘window’ for the display object.

Or… Is there a way to use lines drawn using the lineTo() commands as a mask for another display object?
I’ve tryed drawing lines on a display object and then using that display object as a mask, but it doesn’t seem to recognize these lines as a part of that display object.