Question

Hi I’m using this C# code to rotate polygons in my app – they do rotate but also get skewed along the way which is not what i want to happen. All the polygons are rectangles with four corners defined as 2D Vectors,

    public Polygon GetRotated(float radians)
    {

        Vector origin = this.Center;
        Polygon ret = new Polygon();
        for (int i = 0; i < points.Count; i++)
        {
            ret.Points.Add(RotatePoint(points[i], origin, radians));
        }
        return ret;
    }

    public Vector RotatePoint(Vector point, Vector origin, float angle)
    {
        Vector ret = new Vector();
        ret.X = (float)(origin.X + ((point.X - origin.X) * Math.Cos((float)angle)) - ((point.Y - origin.Y) * Math.Sin((float)angle)));
        ret.Y = (float)(origin.Y + ((point.X - origin.X) * Math.Sin((float)angle)) - ((point.Y - origin.Y) * Math.Cos((float)angle)));
        return ret;
    }

Answer 1

Looks like your rotation transformation is incorrect. You should use:

x' = x*Cos(angle) - y*Sin(angle)
y' = x*Sin(angle) + y*Cos(angle)

For more information, check various sources on the internet. 🙂