The <a href="https://en.wikipedia.org/wiki/Convex_hull_of_a_simple_polygon" class="extiw" title="en:Convex hull of a simple polygon">Convex hull of a simple polygon</a>. The polygon is shown in blue, its four pockets are yellow, and the whole region shaded in either color is the convex hull.
<a href="//commons.wikimedia.org/wiki/User:David_Eppstein" class="mw-redirect" title="User:David Eppstein">David Eppstein</a>
<span class="int-own-work" lang="en">Own work</span>