Circle Dragging IV by mbostock

This is a variant of dragging SVG circles that uses better hit-testing. Rather than choose the top circle that contains the active pointer, this chooses the circle closest to the active pointer using a Voronoi overlay. This is similar to Tovi Grossman’s bubble cursor. (See also earlier D3 3.x examples by Andrew Couch, Christopher Manning and Nate Vack.)

The Voronoi diagram is shown with a light-gray stroke for demonstration purposes (and because it looks cool!); in real usage, you probably wouldn’t show it. You can refine this technique by clipping the Voronoi cells to a larger circle that surrounds the smaller draggable circles, or by performing a distance check in side drag.subject.

Compare this technique to searching for closest circle using Canvas.

index.html

<!DOCTYPE html>
<meta charset="utf-8">
<style>

path {
  pointer-events: all;
  fill: none;
  stroke: #666;
  stroke-opacity: 0.2;
}

.active circle {
  stroke: #000;
  stroke-width: 2px;
}

</style>
<svg width="960" height="500"></svg>
<script src="//d3js.org/d3.v4.min.js"></script>
<script>

var svg = d3.select("svg"),
    width = +svg.attr("width"),
    height = +svg.attr("height"),
    radius = 32;

var circles = d3.range(20).map(function() {
  return {
    x: Math.round(Math.random() * (width - radius * 2) + radius),
    y: Math.round(Math.random() * (height - radius * 2) + radius)
  };
});

var color = d3.scaleOrdinal()
    .range(d3.schemeCategory20);

var voronoi = d3.voronoi()
    .x(function(d) { return d.x; })
    .y(function(d) { return d.y; })
    .extent([[-1, -1], [width + 1, height + 1]]);

var circle = svg.selectAll("g")
  .data(circles)
  .enter().append("g")
    .call(d3.drag()
        .on("start", dragstarted)
        .on("drag", dragged)
        .on("end", dragended));

var cell = circle.append("path")
  .data(voronoi.polygons(circles))
    .attr("d", renderCell)
    .attr("id", function(d, i) { return "cell-" + i; });

circle.append("clipPath")
    .attr("id", function(d, i) { return "clip-" + i; })
  .append("use")
    .attr("xlink:href", function(d, i) { return "#cell-" + i; });

circle.append("circle")
    .attr("clip-path", function(d, i) { return "url(#clip-" + i + ")"; })
    .attr("cx", function(d) { return d.x; })
    .attr("cy", function(d) { return d.y; })
    .attr("r", radius)
    .style("fill", function(d, i) { return color(i); });

function dragstarted(d) {
  d3.select(this).raise().classed("active", true);
}

function dragged(d) {
  d3.select(this).select("circle").attr("cx", d.x = d3.event.x).attr("cy", d.y = d3.event.y);
  cell = cell.data(voronoi.polygons(circles)).attr("d", renderCell);
}

function dragended(d, i) {
  d3.select(this).classed("active", false);
}

function renderCell(d) {
  return d == null ? null : "M" + d.join("L") + "Z";
}

</script>

Circle Dragging IV

Updated Example →

index.html