Constant angular motion about matter-js

[liabru]

You're right, if you always apply the force directly upwards then I don't think this should happen.
Please could you provide a jsfiddle so I can debug it?

[Rodeoclash]

http://jsfiddle.net/Rodeoclash/rq2qzkL0/1/

Ok, the process of building the fiddle helped me isolate the problem. After I create the triangle, I'm setting the centre of the body after creating it using:

var vertCenter = Vertices.centre(this.body.vertices);
this.body.positionPrev.x = vertCenter.x;
this.body.positionPrev.y = vertCenter.y;
this.body.position.x = vertCenter.x;
this.body.position.y = vertCenter.y;

This appears to introduce the problem. However, if I don't set it, then the spaceship does not rotate around its centre point (but I don't get the bug!)

You'll need to focus the JSFiddle demo window and you can use spacebar to apply thrust to the ship.

[liabru]

Ah, the reason is because the position argument of Body.applyForce should be supplied as world-relative.

So just change your code to be:

Body.applyForce(ship, ship.position, rotatedForcePoint);

I guess this might be a little unintuitive, so maybe I should change it to be body-relative!

Also it's safer (and easier) to pass everything via the Body.create options argument as you know the constructor will do everything you need (as changing vertices requires a number of additional operations)

var ship = Body.create({
 position: {
   x: 400,
   y: 300
 },
 vertices: [
   {x: 0, y: shipHeight},
   {x: shipWidth / 2, y: 0},
   {x: shipWidth, y: shipHeight}
 ]
});

[Rodeoclash]

That fixed it, thanks!