This example demonstrates dynamic topology-preserving shape simplification using topojson.presimplify, a new feature in TopoJSON 1.3. The presimplify method computes the importance of each point using a configurable importance function; the default implementation uses Cartesian triangular area for Visvalingam simplification. Then, a custom geometry stream (d3.geo.transform) uses the previously-computed point importance to filter the geometry during rendering.