Can I avoid bad/different rendering of FillPolygon versus DrawPolygon?

Answer 1

You want use a bitmap and do something like this. Note this is not runnable code.

    Dim image As New Bitmap(<my_Xsize>, <my_Ysize>)
    Dim gr As Graphics = Graphics.FromImage(image)
    <Draw your FillPolygon>
    <Erase the leftside of the polygon>
    <Draw your Polygon>

     gr.DrawImage(image, <myXcoor>, <myYcoor>)

By using a bitmap you can fiddle down to the pixel level to get the exact thing you want.

Answer 2

Hack: Draw the whole arrow as opaque in an intermediate buffer, then draw the whole buffer onto your image using the appropriate transparency.