ngraph.forcelayout

ngraph.forcelayout

This is a force directed graph layouter in 2d. It is using quad tree as an n-body solver. This repository is part of ngraph family, and operates on ngraph.graph data structure. If you want to go the 3D space, please check out ngraph.forcelayout3d

API

First of all it's worth to mention all force directed algorithms are iterative. We need to perform multiple iterations of an algorithm, before graph starts looking aesthetically pleasing.

With that in mind, the easiest way to make graph look nice is:

// graph is an instance of `ngraph.graph` object.
var layout = require('ngraph.forcelayout')(graph);
for (var i = 0; i < ITERATIONS_COUNT; ++i) {
  layout.step();
}

// now we can ask layout where each node/link is best positioned:
graph.forEachNode(function(node) {
  console.log(layout.getNodePosition(node.id));
  // Node position is pair of x,y coordinates:
  // {x: ... , y: ... }
});

graph.forEachLink(function(link) {
  console.log(layout.getLinkPosition(link.id));
  // link position is a pair of two positions:
  // {
  //   from: {x: ..., y: ...},
  //   to: {x: ..., y: ...}
  // }
});

Result of getNodePosition()/getLinkPosition() will be always the same for the same node. This is true:

layout.getNodePosition(1) === layout.getNodePosition(1);

Reason for this is performance. If you are interested in storing positions somewhere else, you can do it and they still will be updated after each force directed layout iteration.

"Pin" node and initial position

Sometimes it's desirable to tell layout algorithm not to move certain nodes. This can be done with pinNode() method:

var nodeToPin = graph.getNode(nodeId);
layout.pinNode(nodeToPin, true); // now layout will not move this node

If you want to check whether node is pinned or not you can use isNodePinned() method. Here is an example how to toggle node pinning, without knowing it's original state:

var node = graph.getNode(nodeId);
layout.pinNode(node, !layout.isNodePinned(node)); // toggle it

What if you still want to move your node according to some external factor (e.g. you have initial positions, or user drags pinned node)? To do this, call setNodePosition() method:

layout.setNodePosition(nodeId, x, y);

Monitoring changes

Like many other algorithms in ngraph family, force layout monitors graph changes via graph events. It keeps layout up to date whenever graph changes:

var graph = require('ngraph.graph')(); // empty graph
var layout = require('ngraph.layout')(graph); // layout of empty graph

graph.addLink(1, 2); // create node 1 and 2, and make link between them
layout.getNodePosition(1); // returns position.

If you want to stop monitoring graph events, call dispose() method:

layout.dispose();

Physics Simulator

Simulator calculates forces acting on each body and then deduces their position via Newton's law. There are three major forces in the system:

1. Spring force keeps connected nodes together via Hooke's law
2. Each body repels each other via Coulomb's law
3. The drag force slows the entire simulation down, helping with convergence.

Body forces are calculated in n*lg(n) time with help of Barnes-Hut algorithm implemented in quadtree module. Euler method is then used to solve ordinary differential equation of Newton's law and get position of bodies.

Please refer to physicsSimulator to see the source code and simulator parameters:

// Configure
var physicsSettings = {
  springLength: 30,
  springCoeff: 0.0008,
  gravity: -1.2,
  theta: 0.8,
  dragCoeff: 0.02,
  timeStep: 20
};

// pass it as second argument to layout:
var layout = require('ngraph.forcelayout')(graph, physicsSettings);

You can get current physics simulator from layout by checking layout.simulator property. This is a read only property. The default physics simulator can be changed by supplying a createSimulator function in the physicsSettings object.

Space occupied by graph

Finally, it's often desirable to know how much space does our graph occupy. To quickly get bounding box use getGraphRect() method:

var rect = layout.getGraphRect();
// rect.x1, rect.y1 - top left coordinates of bounding box
// rect.x2, rect.y2 - bottom right coordinates of bounding box

Manipulating bodies

This is advanced technique to get to internal state of the simulator. If you need to get a node position use regular layout.getNodePosition(nodeId) described above.

In some cases you really need to manipulate physic attributes on a body level. To get to a single body by node id:

var graph = createGraph();
graph.addLink(1, 2);

// Get body that represents node 1:
var body = layout.getBody(1);
assert(
  typeof body.pos.x === 'number' &&
  typeof body.pos.y === 'number', 'Body has position');
assert(body.mass, 'Body has mass');

To iterate over all bodies at once:

layout.forEachBody(function(body, nodeId) {
  assert(
    typeof body.pos.x === 'number' &&
    typeof body.pos.y === 'number', 'Body has position');
  assert(graph.getNode(nodeId), 'NodeId is coming from the graph');
});

install

With npm do:

npm install ngraph.forcelayout

license

MIT

Feedback?

I'd totally love it! Please email me, open issue here, tweet to me, or join discussion on gitter.