This example visualizes a simplified map of Japan using a Mercator projection. A slider controls the level of topological simplification, filtering out geographic features based on a triangle-area threshold computed with `topojson.presimplify`. The map is rendered as an SVG path using `d3.geoPath` with a custom `d3.geoTransform` stream that converts simplified coordinates to pixel positions via `projection.fitExtent`. The code redraws the map on slider input and window resize events.