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Math Help - Update direction of arc from three points

  1. #1
    Newbie
    Joined
    May 2009
    Posts
    13

    Update direction of arc from three points

    Hi,
    I want to find the direction of the arc drawn from three points for programming purpose.
    Depending on Point2 I have to decide whether to draw clock wise or counterclock wise arc from Point1 to Point3 through point2.

    My Logic is working in most cases except one case, as shown in figure.

    Point1= P1 =(P1X,P1Y)
    Point2= P2 =(P2X,P2Y)
    Point3= P3 =(P3X,P3Y)
    CenterPoint of Arc C(CX,CY) - I am calculating center from three points

    P1P3angle = Angle from Point1 to Point3
    P1P2angle = Angle from Point1 to Point2

    diff = p1p3angle - p1p2angle;

    by Default: Direction = CounterClockwise
    if (diff < 0 || diff > 180)
    {
    Direction = Clockwise
    }
    if (p1p3angle == 0)
    {
    if (AngleDifference < -180)
    Direction = CounterClockwise
    else
    Direction = Clockwise
    }
    if (p1p3angle == 90)
    {
    if (diff< -180)
    Direction = CounterClockwise
    }
    Attached Thumbnails Attached Thumbnails Update direction of arc from three points-sample.jpg  
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