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@bobbydavid
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Last active August 29, 2015 14:17
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Example illustrating zoom and pan with a "rolling" Mercator projection. Drag left-right to rotate projection cylinder, and up-down to translate, clamped by max absolute latitude. Ensures projection always fits properly in viewbox.

<!DOCTYPE html>
<meta charset="utf-8">
<style>
svg {
background-color: lavender;
border: 1px solid black;
}
path {
fill: oldlace;
stroke: #666;
stroke-width: .5px;
}
</style>
<body>
<script src="http://d3js.org/d3.v3.min.js"></script>
<script src="http://d3js.org/topojson.v1.min.js"></script>
<script>
var width = 600,
height = 400,
rotate = 60, // so that [-60, 0] becomes initial center of projection
maxlat = 83; // clip northern and southern poles (infinite in mercator)
var projection = d3.geo.mercator()
.rotate([rotate,0])
.scale(1) // we'll scale up to match viewport shortly.
.translate([width/2, height/2]);
// find the top left and bottom right of current projection
function mercatorBounds(projection, maxlat) {
var yaw = projection.rotate()[0],
xymax = projection([-yaw+180-1e-6,-maxlat]),
xymin = projection([-yaw-180+1e-6, maxlat]);
return [xymin,xymax];
}
// set up the scale extent and initial scale for the projection
var b = mercatorBounds(projection, maxlat),
s = width/(b[1][0]-b[0][0]),
scaleExtent = [s, 10*s];
projection
.scale(scaleExtent[0]);
var zoom = d3.behavior.zoom()
.scaleExtent(scaleExtent)
.scale(projection.scale())
.translate([0,0]) // not linked directly to projection
.on("zoom", redraw);
var path = d3.geo.path()
.projection(projection);
var svg = d3.selectAll('body')
.append('svg')
.attr('width',width)
.attr('height',height)
.call(zoom);
d3.json("world-50m.json", function ready(error, world) {
svg.selectAll('path')
.data(topojson.feature(world, world.objects.countries).features)
.enter().append('path')
redraw(); // update path data
});
// track last translation and scale event we processed
var tlast = [0,0],
slast = null;
function redraw() {
if (d3.event) {
var scale = d3.event.scale,
t = d3.event.translate;
// if scaling changes, ignore translation (otherwise touch zooms are weird)
if (scale != slast) {
projection.scale(scale);
} else {
var dx = t[0]-tlast[0],
dy = t[1]-tlast[1],
yaw = projection.rotate()[0],
tp = projection.translate();
// use x translation to rotate based on current scale
projection.rotate([yaw+360.*dx/width*scaleExtent[0]/scale, 0, 0]);
// use y translation to translate projection, clamped by min/max
var b = mercatorBounds(projection, maxlat);
if (b[0][1] + dy > 0) dy = -b[0][1];
else if (b[1][1] + dy < height) dy = height-b[1][1];
projection.translate([tp[0],tp[1]+dy]);
}
// save last values. resetting zoom.translate() and scale() would
// seem equivalent but doesn't seem to work reliably?
slast = scale;
tlast = t;
}
svg.selectAll('path') // re-project path data
.attr('d', path);
}
</script>
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