Matter.js Gravity Point - matter.js

Question

Is it possible to create a single gravity / force point in matter.js that is at the center of x/y coordinates?
I have managed to do it with d3.js but wanted to enquire about matter.js as it has the ability to use multiple polyshapes.
http://bl.ocks.org/mbostock/1021841

Answer 1

The illustrious answer has arisen:
not sure if there is any interest in this. I'm a fan of what you have created. In my latest project, I used matter-js but I needed elements to gravitate to a specific point, rather than into a general direction. That was very easily accomplished. I was wondering if you are interested in that feature as well, it would not break anything.
All one has to do is setting engine.world.gravity.isPoint = true and then the gravity vector is used as point, rather than a direction. One might set:
engine.world.gravity.x = 355;
engine.world.gravity.y = 125;
engine.world.gravity.isPoint = true;
and all objects will gravitate to that point.
If this is not within the scope of this engine, I understand. Either way, thanks for the great work.

Answer 2

You can do this with the matter-attractors plugin. Here's their basic example:
Matter.use(
'matter-attractors' // PLUGIN_NAME
);
var Engine = Matter.Engine,
Events = Matter.Events,
Runner = Matter.Runner,
Render = Matter.Render,
World = Matter.World,
Body = Matter.Body,
Mouse = Matter.Mouse,
Common = Matter.Common,
Bodies = Matter.Bodies;
// create engine
var engine = Engine.create();
// create renderer
var render = Render.create({
element: document.body,
engine: engine,
options: {
width: Math.min(document.documentElement.clientWidth, 1024),
height: Math.min(document.documentElement.clientHeight, 1024),
wireframes: false
}
});
// create runner
var runner = Runner.create();
Runner.run(runner, engine);
Render.run(render);
// create demo scene
var world = engine.world;
world.gravity.scale = 0;
// create a body with an attractor
var attractiveBody = Bodies.circle(
render.options.width / 2,
render.options.height / 2,
50,
{
isStatic: true,
// example of an attractor function that
// returns a force vector that applies to bodyB
plugin: {
attractors: [
function(bodyA, bodyB) {
return {
x: (bodyA.position.x - bodyB.position.x) * 1e-6,
y: (bodyA.position.y - bodyB.position.y) * 1e-6,
};
}
]
}
});
World.add(world, attractiveBody);
// add some bodies that to be attracted
for (var i = 0; i < 150; i += 1) {
var body = Bodies.polygon(
Common.random(0, render.options.width),
Common.random(0, render.options.height),
Common.random(1, 5),
Common.random() > 0.9 ? Common.random(15, 25) : Common.random(5, 10)
);
World.add(world, body);
}
// add mouse control
var mouse = Mouse.create(render.canvas);
Events.on(engine, 'afterUpdate', function() {
if (!mouse.position.x) {
return;
}
// smoothly move the attractor body towards the mouse
Body.translate(attractiveBody, {
x: (mouse.position.x - attractiveBody.position.x) * 0.25,
y: (mouse.position.y - attractiveBody.position.y) * 0.25
});
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/matter-js/0.12.0/matter.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/matter-attractors#0.1.6/build/matter-attractors.min.js"></script>
Historical note: the "gravity point" functionality was proposed as a feature in MJS as PR #132 but it was closed, with the author of MJS (liabru) offering the matter-attractors plugin as an alternate. At the time of writing, this answer misleadingly seems to indicate that functionality from the PR was in fact merged.
Unfortunately, the attractors library is 6 years outdated at the time of writing and raises a warning when using a newer version of MJS than 0.12.0. From discussion in issue #11, it sounds like it's OK to ignore the warning and use this plugin with, for example, 0.18.0. Here's the warning:
matter-js: Plugin.use: matter-attractors#0.1.4 is for matter-js#^0.12.0 but installed on matter-js#0.18.0.
Behavior seemed fine on cursory glance, but I'll keep 0.12.0 in the above example to silence it anyway. If you do update to a recent version, note that Matter.World is deprecated and should be replaced with Matter.Composite and engine.gravity.