Skip to content

Instantly share code, notes, and snippets.

@mbostock
Forked from mbostock/.block
Last active February 9, 2016 02:02
  • Star 2 You must be signed in to star a gist
  • Fork 1 You must be signed in to fork a gist
Star You must be signed in to star a gist
Save mbostock/6252418 to your computer and use it in GitHub Desktop.
Dynamic Simplification II
license: gpl-3.0

A combination of the map zooming and dynamic simplification demonstrations: as the map zooms in and out, the simplification area threshold is adjusted so that it is always appropriate to the current scale. Thus, the map looks good and renders quickly at all points during the animation.

An additional rendering optimization is that points outside the current viewport are much more heavily simplified that points inside the viewport. This is, in effect, a cheap approximation of viewport clipping.

<!DOCTYPE html>
<meta charset="utf-8">
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>
var width = 960,
height = 500;
var chesapeake = [-75.959, 38.250];
var scale,
translate,
visibleArea, // minimum area threshold for points inside viewport
invisibleArea; // minimum area threshold for points outside viewport
var simplify = d3.geo.transform({
point: function(x, y, z) {
if (z < visibleArea) return;
x = x * scale + translate[0];
y = y * scale + translate[1];
if (x >= 0 && x <= width && y >= 0 && y <= height || z >= invisibleArea) this.stream.point(x, y);
}
});
var zoom = d3.behavior.zoom()
.size([width, height])
.on("zoom", zoomed);
// This projection is baked into the TopoJSON file,
// but is used here to compute the desired zoom translate.
var projection = d3.geo.mercator()
.translate([0, 0])
.scale(4000);
var canvas = d3.select("body").append("canvas")
.attr("width", width)
.attr("height", height);
var context = canvas.node().getContext("2d");
var path = d3.geo.path()
.projection(simplify)
.context(context);
d3.json("us-states.json", function(error, json) {
if (error) throw error;
canvas
.datum(topojson.mesh(topojson.presimplify(json)))
.call(zoomTo(chesapeake, 0.05).event)
.transition()
.duration(5000)
.each(jump);
});
function zoomTo(location, scale) {
var point = projection(location);
return zoom
.translate([width / 2 - point[0] * scale, height / 2 - point[1] * scale])
.scale(scale);
}
function zoomed(d) {
translate = zoom.translate();
scale = zoom.scale();
visibleArea = 1 / scale / scale;
invisibleArea = 200 * visibleArea;
context.clearRect(0, 0, width, height);
context.beginPath();
path(d);
context.stroke();
}
function jump() {
var t = d3.select(this);
(function repeat() {
t = t.transition()
.call(zoomTo(chesapeake, 1.25).event)
.transition()
.call(zoomTo(chesapeake, 0.05).event)
.each("end", repeat);
})();
}
</script>
Display the source blob
Display the rendered blob
Raw
Sorry, something went wrong. Reload?
Sorry, we cannot display this file.
Sorry, this file is invalid so it cannot be displayed.
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment