smoothing property
The amount by which the arc is smoothed by extending the curve from the inner circular arc to the edge between vertices.
A value of 0 indicates that the corner is rounded by only a circular arc, with no flanking curves. A value of 1 indicates that there is no circular arc in the center, and the flanking curves on either side meet at the middle.
Must be in the range 0.0 to 1.0, inclusive.