Geek Answers Handbook

Get a positive or negative angle of 3 points

I rotate points around a center point in 2D space. The points are the center point, the old mouse position and the new mouse position. My rotation function works fine, and I can calculate the angle perfectly. But I want to calculate a negative angle if the user moves his mouse in a direction that should be interpreted as counterclockwise.

For example, moving the mouse to the right (positive x axis) should rotate clockwise if you are above (less than) the y value of the center point, but it should rotate counterclockwise if you are actually lower (more) the y value of the center point.

Here is what I have:

PointF centerPoint;
PointF oldPoint;
PointF newPoint;

double Xc = centerPoint.X;
double Yc = centerPoint.Y;
double Xb = oldPoint.X;
double Yb = oldPoint.Y;
double Xa = newPoint.X;
double Ya = newPoint.Y;

double c2 = (Math.Pow(Xb - Xa, 2) + Math.Pow(Yb - Ya, 2));
double a2 = (Math.Pow(Xb - Xc, 2) + Math.Pow(Yb - Yc, 2));
double b2 = (Math.Pow(Xa - Xc, 2) + Math.Pow(Ya - Yc, 2));

double a = Math.Sqrt(a2);
double b = Math.Sqrt(b2);

double val = (a2 + b2 - c2) / (2 * a * b);
double angle = Math.Acos(val);

, , , , , .

+5
4

, :

double v1x = Xb - Xc;
double v1y = Yb - Yc;
double v2x = Xa - Xc;
double v2y = Ya - Yc;

double angle = Math.Atan2(v1x, v1y) - Math.Atan2(v2x, v2y);
+7
private double AngleFrom3PointsInDegrees(double x1, double y1, double x2, double y2, double x3, double y3)
{
    double a = x2 - x1;
    double b = y2 - y1;
    double c = x3 - x2;
    double d = y3 - y2;

    double atanA = Math.Atan2(a, b);
    double atanB = Math.Atan2(c, d);

    return (atanA - atanB) * (-180 / Math.PI); 
    // if Second line is counterclockwise from 1st line angle is 
    // positive, else negative
}
+3

, , ,

angle = angle > Math.PI ? angle - 2*Math.PI : angle;

. , , , .

0

, (x1, y1) (x2, y2), , Atan2(). , , "".

0

More articles:


All Articles