c# rotation problem

Question

I have 3 particles and one of them is the center particle. I want to rotate other two particle ( stored in particles list ) relative to the center particle with the formula q' = Θq + p where q' is the new position of the rotated particle, Θ is the orientation angle and p is the position of center particle. The initial position of other two particles is stored in initialParticlePosition list. THe problem is I think the angle I calculate is wrong because of the range. I thing I should take the range as [-pi, pi) or something like this. In some parts it calculates correct but sometimes it is wrong. Can someone help me with this code or give me another method of rotating.

{

         angle = Math.Acos(Vector2.Dot(heading,new Vector2(0,-1) ));

         for (int i = 0; i < 2; i++)
         {
             tempX = (double)initialParticlePositions[i].X * Math.Cos(angle) - (double)initialParticlePositions[i].Y * Math.Sin(angle) + centerParticle.position.x;
             tempY = (double)initialParticlePositions[i].X * Math.Sin(angle) + (double)initialParticlePositions[i].Y * Math.Cos(angle) + centerParticle.position.y;
             particles[i].position.x = tempX;
             particles[i].position.y = tempY;
         }
}

Run your code with known values for the angle and plot the output on paper. Verify the output by hand. cos and sin should cope with angles outside [-pi,pi] (it's been a while since I did this sort of thing). — ChrisF
– ChrisF, Commented Jan 11, 2011 at 20:06

Community · Accepted Answer · 2017-05-23 12:26:56Z

Some methods that might help (angles always in degrees, not rad):

    public static double GetAngle(Vector v)
    {
        return Math.Atan2(v.X, -v.Y) * 180.0 / Math.PI;
    }

    public static Vector SetAngle(Vector v, double angle)
    {
        var angleInRads = angle * (Math.PI / 180.0);
        var distance = v.Length;
        v.X = (Math.Sin(angleInRads) * distance);
        v.Y = -(Math.Cos(angleInRads) * distance);
        return v;
    }

    static public Point RotatePointAroundCenter(Point point, Point center, double rotationChange)
    {
        Vector centerToPoint = point - center;
        double angle = GetAngle(centerToPoint);
        Vector centerToNewPoint = SetAngle(centerToPoint, angle + rotationChange);
        return center + centerToNewPoint;
    }

(You should start marking answers that help as answer, click the checkmark outline below the votes on the left, e.g. you could accept this answer)

Edit: Optimized the methods a bit.

+1 ... You might provide an explanation of what you're doing. Specifically, that you're translating the point so that it's relative to the origin, doing the rotation, and then translating back.

Steve H · Accepted Answer · 2011-01-11 21:24:59Z

The particle positions that are orbiting can be set with a single line of code each:

Assume p1, p2, & p3 are Vector2s and p2 & p3 are orbiting p1.

p2 = Vector2.Transform(p2 - p1, Matrix.CreateRotationZ(rotationChangeP2)) + p1;

p3 = Vector2.Transform(p3 - p1, Matrix.CreateRotationZ(rotationChangeP3)) + p1;

The Matrix.Create...() method will call the two trig functions for you.

edit. the Matrix & Vector2 structures & methods are XNA specific but included here because that's what the OP tagged his Q with.

Empyrean · Accepted Answer · 2011-01-13 01:27:28Z

0

angle = Math.Acos(Vector2.Dot(heading,new Vector2(0,-1)));

As you suspect, your combination of dot product and Acos will only give you angles in a 180 degree range.

Instead, use Atan2 on your unit vector to get a full range of angles from -pi to pi.

angle = (float)Math.Atan2((double)heading.Y, (double)heading.X);

You may need to negate the Y term if your Y axis is positive in the down direction.

edited Jan 13, 2011 at 1:27

answered Jan 12, 2011 at 5:54

Empyrean

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