Isolating Forces

This example demonstrates how to patch force.initialize such that a force applies only to a subset of nodes in a simulation.

function isolate(force, filter) {
  var initialize = force.initialize;
  force.initialize = function() { initialize.call(force, nodes.filter(filter)); };
  return force;
}

Another way of having forces only apply to some nodes is to set the force strength to zero for those nodes by using a custom strength accessor (e.g., x.x). But if you have lots of isolated forces and lots of nodes, it is faster to isolate the force to a subset of nodes than to set the strength to zero for unaffected nodes.

<!DOCTYPE html>
<meta charset="utf-8">
<canvas width="960" height="500"></canvas>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

var n = 20;

var nodes = d3.range(n * n).map(function(i) {
  return {
    index: i,
    color: i < 200 ? "brown" : "steelblue"
  };
});

var canvas = document.querySelector("canvas"),
    context = canvas.getContext("2d"),
    width = canvas.width,
    height = canvas.height;

var simulation = d3.forceSimulation(nodes)
    .force("y", d3.forceY())
    .force("brown", isolate(d3.forceX(-width / 6), function(d) { return d.color === "brown"; }))
    .force("steelblue", isolate(d3.forceX(width / 6), function(d) { return d.color === "steelblue"; }))
    .force("charge", d3.forceManyBody().strength(-10))
    .on("tick", ticked);

function ticked() {
  context.clearRect(0, 0, width, height);
  context.save();
  context.translate(width / 2, height / 2);
  nodes.forEach(drawNode);
  context.restore();
}

function drawNode(d) {
  context.beginPath();
  context.moveTo(d.x + 3, d.y);
  context.arc(d.x, d.y, 3, 0, 2 * Math.PI);
  context.fillStyle = d.color;
  context.fill();
}

function isolate(force, filter) {
  var initialize = force.initialize;
  force.initialize = function() { initialize.call(force, nodes.filter(filter)); };
  return force;
}

</script>

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