= Control polygon
A <control polygon> joins successive <control points> by <line segments>. It gives an editable geometric skeleton, not usually the exact <curve>. An open <control polygon> has two ends; a closed <control polygon> also joins the last <control point> to the first. Store the first <control point> only once when using cyclic indices, so the closing edge is counted exactly once. Positive <partition of unity> basis functions place the <curve> inside the <convex hull> of its active <control points>; a convergent <subdivision curve> rule refines the <control polygon> toward its limit, with a certified error when an appropriate bound is available.
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