FillPolygon alpha not working on Windows Phone8

Oct 10, 2013 at 4:32 PM
for some reason WriteableBitmap.FillPolygon(path.Points, c); with c beeing {#00FFFFFF} draws a black figure. With c = {#BEFFFFFF} it is a slightly gray square and #FFFFFFFF is white.

I clear the writeablebitmap before the call with bitmap.Clear(Color.White) and also tried invalidating it. Still it draws a black square instead of a 100% transparent white one.

Can someone help me out with this please?