I have an application where rectangles (or I can make it thick lines.  It doesn't matter to me - I'll do whatever I can make work) that are colored in grayscale overlap.  However, rather than putting one shape on top of another I want to have their overlap combine that shadings of them both.  So, the area where two light gray shapes overlap should be darker gray, and the overlap of two dark gray shapes should be black.  Ideally I would like to have an arbitrary number of shapes overlapping in a single region if possible.  Is there a way to do this?  Thanks!!