Inertia not updated when calling Body.setMass()

[touho]

I started using Matter.js in my game editor where you can create bodies and edit them later on. I noticed that these two code snippets give different results:

var body = Matter.Bodies.rectangle(x, y, w, h, { density: 0.01 });

var body = Matter.Bodies.rectangle(x, y, w, h, { density: 0.001 });
Matter.Body.setDensity(body, 0.01);

Both of the snippets will set density to 0.01, but since inertial is only calculated on creation of the body, these snippets will result in different body.inertial value. Therefore bodies act really weird.

When density changes, mass is recalculated. Shouldn't inertial be also recalculated when mass changes? If you think so, I would be happy to contribute.

[touho]

Not recalculating inertia is probably good optimization if plenty of bodies change their mass a little. But atleast a mention in the comment or taking third parameter Body.setMass = function(body, mass, recalculateInertia) would be helpful.

[liabru]

Good spot. Recalculating inertia in this case is easy, we can just do:

var moment = body.inertia / (body.mass / 6);
body.inertia = moment * (mass / 6);

I'll tag this as a bug, thanks.

[touho]

// allow options to override the automatically calculated properties
Body.set(body, {
	axes: options.axes || body.axes,
	area: options.area || body.area,
	mass: options.mass || body.mass,
	inertia: options.inertia || body.inertia
});

This code might break if inertia is set in setMass function. Might have to separately call setInertia afterwards.

I really like how you have structured the library. Well done!

[Technostalgic]

Ran into this bug too, had me real confused for a hot minute.
For me, I was using Matter.Body.setDensity to set the density of a body, but when I tested it, it seemed that the body's rotational velocity was being computed correctly, but the inertia didn't account for its linear velocity.

I'm still a bit confused as to why the inertia not being updated only seemed to be effecting the linear velocity and not the rotational velocity.

on an unrelated note:

liabru
var moment = body.inertia / (body.mass / 6);
body.inertia = moment * (mass / 6);

I assume is supposed to be:
var moment = body.inertia / (body.mass / 6);
body.inertia = moment * (body.mass / 6);