QPainter drawArc question



  • Hi. I'm trying to use QPainter drawArc(const QRectF & rectangle, int startAngle, int spanAngle) for arc drawing.
    And have a problem with it.
    I have a coordinates of 3 arc points A, B, C.
    A - arc start point
    B - arc end point
    C - arc center with known radius (10).

    Question. How to calculate rectangle, startAngle and spanAngle for this arc?

    Look Arc Image



  • NOT TESTED, JUST MATH (and i hope it is correct ^^)
    eg.: Ax - x coordinate of A point, atan === ArcTan, abs === AbsoluteValue
    Rect Top Left Point: (Ax,By)
    Rect Bottom Right Point: (Cx+R,Cy+R)
    ...or minus im not sure about the coordinate system inside drawing but i think youll get the point i hope
    Remember 0 Degree is at 3 o'clock so:
    Angle to B point:
    atan( abs(By-Cy) / abs(Bx-Cx) ) * 16 (it should be in 1/16th Degree thats why *16)
    Angle to A Point:
    (-90 - atan( abs(Cx-Ax) / abs(Ay-Cy))) * 16 (90 couse 0 is at 3 o'clock, "-" because clockwise)

    http://i.imgur.com/cCMNFrc.png



  • Arc can be located in any of the quarters. I'm drawing using coordinates from CNC file (G-Code).


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