title: Building an "inner stroke" effect from a Polygon
body: <p>I'm drawing a whole bunch of <a href="http://msdn.microsoft.com/en-us/library/system.windows.shapes.polygon.aspx" rel="nofollow noreferrer">Polygons</a> onto a canvas, most of which share an edge with at least one other Polygon.  I'd like to indicate a Polygon is "special" by outlining it, however due to the overlapping edges the Stroke on a Polygon tends to be partially drawn over by another Polygon causing the Stroke to look thinner than it should in places.  Additionally, depending on draw order, a Polygon may have its Stroke almost entirely covered by those around it.  Spacing the Polygons out is not a very attractive option, as visible gaps between Polygons are much less preferable to this "thin stroke" problem.</p>
  
  <p>I reason that the effect I'm looking for can be achieved by generating each Polygon as a pair of Polygons instead, such that one Polygon is completely contained within the other and this smaller Polygon's Stroke runs up to (but does not overlap) the outer Polygon's Stroke.  The inner Polygon would have a transparent Fill at all times, and a non-transparent Stroke only when I wish to indicate that the now pair of Polygons are "special".</p>
  
  <p>My question boils down to; how can I derive such an inner Polygon from the outer one, or failing that how else might I achieve this effect?</p>
  
  <p>The Polygon's in question can be constrained to the <a href="http://en.wikipedia.org/wiki/Regular_polygon" rel="nofollow noreferrer">regular polygons</a> if need be, and the Strokes are simple 1 thickness solid lines with no fancy miters or the like.  I'd like to be able to adjust the thickness of the Stroke at some point, but nothing fancier than that.</p>