Dynamic Simplification IV

This map dynamically simplifies the geometry in response to zooming, so that the smallest displayed detail is approximately one square pixel. Use the mousewheel, or pinch on touch devices, to zoom.

<!DOCTYPE html>
<canvas width="960" height="500"></canvas>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://unpkg.com/topojson-client@2"></script>
<script src="https://unpkg.com/topojson-simplify@2"></script>
<script>

var canvas = d3.select("canvas"),
    context = canvas.node().getContext("2d"),
    width = canvas.property("width"),
    height = canvas.property("height");

var land,
    borders;

var minZ, // minimum area threshold for simplification
    transform = d3.geoIdentity().clipExtent([[0, 0], [width, height]]),
    simplify = d3.geoTransform({point: function(x, y, z) { if (z >= minZ) this.stream.point(x, y); }});

var zoom = d3.zoom()
    .scaleExtent([1 / (1 << 5), 1 << 2])
    .on("zoom", zoomed);

context.fillStyle = "#bbb";
context.strokeStyle = "#fff";
context.lineJoin = "round";
context.lineCap = "round";

var path = d3.geoPath()
    .projection({stream: function(s) { return simplify.stream(transform.stream(s)); }})
    .context(context);

d3.json("us-states.json", function(error, us) {
  if (error) throw error;

topojson.presimplify(us);
  land = topojson.feature(us, us.objects.states);
  borders = topojson.mesh(us, us.objects.states, function(a, b) { return a !== b; });

// An arbitrary scale and center point to set the initial view.
  // This projection is baked into the TopoJSON file,
  // but is used here to compute the desired zoom translate.
  var scale = 0.05, point = d3.geoMercator()
      .translate([0, 0])
      .scale(4000)
      ([-75.959, 38.250]);

canvas
      .call(zoom)
      .call(zoom.transform, d3.zoomIdentity
          .translate(width / 2, height / 2)
          .scale(scale)
          .translate(-point[0], -point[1]));
});

function zoomed(d) {
  var t = d3.event.transform;
  minZ = 1 / (t.k * t.k);
  transform.translate([t.x, t.y]).scale(t.k);
  context.clearRect(0, 0, width, height);
  context.beginPath();
  path(land);
  context.fill();
  context.beginPath();
  path(borders);
  context.stroke();
}

</script>

https://d3js.org/d3.v4.min.js

https://unpkg.com/topojson-client@2

https://unpkg.com/topojson-simplify@2