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Last active Feb 9, 2016
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Dynamic Simplification
license: gpl-3.0

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.

<!DOCTYPE html>
<meta charset="utf-8">
<script src="//"></script>
<script src="//"></script>
var width = 960,
height = 500;
var area = 1, simplify = d3.geo.transform({
point: function(x, y, z) {
if (z >= area), y);
var canvas ="body").append("canvas")
.attr("width", width)
.attr("height", height);
var context = canvas.node().getContext("2d");
var path = d3.geo.path()
d3.json("us.json", function(error, topo) {
if (error) throw error;
function render() {
context.clearRect(0, 0, width, height);
function resimplify() {
var t =;
(function repeat() {
t = t.transition()
.tween("area", areaTween(20))
.tween("area", areaTween(.5))
.each("end", repeat);
function areaTween(area1) {
return function(d) {
var i = d3.interpolate(area, area1);
return function(t) {
area = i(t);
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