Rotating point by angle

Question

I know that the theory of rotating a point by an angle is on the internet a million times, but I don't get my code to work properly.

I have a line with 2 points, when you click on 1 of the 2 points, you will rotate the point relative to the other point. In my testcase I have a upper left point and a bottom right point, so a diagonal line.

I want to make sure the line snaps to a 90 degrees rotation so It will always be a straight line (either vertically or horizontally). So I first get the current angle, then get the angle that It should be, and calculate the difference.

Point startPoint = obj2.Location;
Point currentEndPoint = new Point(obj2.Location.X + obj2.Size.Width, obj2.Location.Y + obj2.Size.Height);
Point newEndPoint = e.Location;
double angle = MathHelper.GetAngleOfVerticalLine(startPoint, newEndPoint);
double angleToBe = MathHelper.GetClosestNumber(angle, new double[] { 0, 90, 180, 270, 360 });
double angleToDo = 0.0; // -5
if (angle < angleToBe)
{
    angleToDo = Math.Abs(angle - angleToBe);
}
else
{
    angleToDo = angleToBe - angle;
}
angleToDo %= 360;
Point newSize = MathHelper.RotatePoint(newEndPoint, startPoint, angleToDo);

obj.Size = (Size)newSize;

public static Point RotatePoint(Point pointToRotate, Point centerPoint, double angleInDegrees)
{
    double angleInRadians = angleInDegrees * (Math.PI / 180);
    double cosTheta = Math.Cos(angleInRadians);
    double sinTheta = Math.Sin(angleInRadians);
    return new Point
    {
        X =
            (int)
            (cosTheta * (pointToRotate.X - centerPoint.X) -
            sinTheta * (pointToRotate.Y - centerPoint.Y) + centerPoint.X),
        Y =
            (int)
            (sinTheta * (pointToRotate.X - centerPoint.X) +
            cosTheta * (pointToRotate.Y - centerPoint.Y) + centerPoint.Y)
    };
}

But the results that I get, are not straight lines but they are kind of random. The angle, angleToBe and angleToDo work properly. The RotatePoint method should be the problem then, but I'm not 100% sure about that.

What doesn't work properly? Getting the angle, or getting the point, or what? — davidsbro, Sep 16 '13 at 23:51
I forgot the last paragraph, I´m sorry. As I stated, I get random lines and the problem most likely occurs in the RotatePoint method or a combination of my total code / usage. — Erwin Okken, Sep 17 '13 at 09:41

score 0 · Answer 1 · answered Sep 19 '13 at 14:38

0

Can't you use the Matrix.Rotate class to do the heavy lifting? Source: http://msdn.microsoft.com/en-us/library/s0s56wcf.aspx (Of course the math is half the fun).

answered Sep 19 '13 at 14:38

riezebosch

1,796
15
28

That is exactly what I'm doing now (because the math doesn't seem to work out well for me). Translating it to the origin, then rotate and then translate back does the job. – Erwin Okken Oct 08 '13 at 14:37

Rotating point by angle

1 Answers1