Get Off My Lawn - 1 year ago 81
Javascript Question

# Rotate points around an origin

I am trying to rotated 4 points around an origin. When I don't rotate the points, they show up in the correct location. I am doing that like this:

let origin = this.transform.position;
for (let i in this._pointsOrig) {
let point = this._pointsOrig[i];
this._points[i] = new Vector2(
point.x + origin.x,
point.y + origin.y
);
}

Which gives me this result:

Note: the blue cross is the origin, and the green rectangle outlines the points

When I attempt to rotate the rectangle, like this:

// degrees is between 0 & 360, so convert to radians
let rotation = this.transform.rotation.degrees * (Math.PI / 180);
let cos = Math.cos(rotation);
let sin = Math.sin(rotation);
let origin = this.transform.position;

for (let i in this._pointsOrig) {
let point = this._pointsOrig[i];
this._points[i] = new Vector2(
(cos * (point.x - origin.x)) + (sin * (point.y - origin.y)) + origin.x,
(cos * (point.y - origin.y)) - (sin * (point.x - origin.x)) + origin.y
);
}

I get this result where the rectangle is now on the bottom left.

I am not sure what I need to do so the points rotate around the origin.

I would try using

(cos * point.x) + (sin * point.y) + origin.x,
(cos * point.y) - (sin * point.x) + origin.y

It looks like the point.x and point.y are the values before shifting by (origin.x,origin.y). So shifting coordinates before rotation has an undesired effect.

You might find the rotation is in the wrong direction

(cos * point.x) - (sin * point.y) + origin.x,
(cos * point.y) + (sin * point.x) + origin.y

so changing signs could work.

If the rotation is alway 90° we know than cos(90°)=0, sin(90°)=1 so it simplifies to

this.points[i] = new Vector2(
- point.y + origin.x,
point.x + origin.y );

If problems persist it might help to include some actual values for origin, _pointsOrig[i] and the result.

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