A line moves down the plane, and behind it a front of parabolic arcs, the beach line, traces the edges of the diagram. Eight chapters on cells, parabolas, the beach line and the two kinds of event that change it.
Every frame is computed exactly from the sweep position: the beach line is the lowest parabola of the sites already passed, and the traced edges are the parts of the Voronoi diagram at least as close to their sites as to the line. The diagram itself comes from the Delaunay triangles of the sites, whose circumcircles are also the circle events.