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Donovan Phoenix February 2016

Always return a CCW rotation from a 3 point arc method

I'm trying to create 3 point arcs from a table into a autocad (R12/lt2) DXF file. the problem comes is that the file seems to ALWAYS save arcs in a CCW rotation. so the method below works properly if the pont data i feed it is CCW but does not work if the rotation is CW the arc's angles gets 'flipped'.I can't seem to figure out a way to detect when a CW arc's data is detected to rectify the error

The method calculates the arc with 3 points on the arc's circumference (not the start, center and end point)

Here is My method:

public static List<double> threePointArch(double startX, double startY, double midX, double midY, double endX, double endY)
    List<double> returnValues = new List<double>();
    //Calculate center point from 3 points on a circle
    //calculate Line R
    double lineR = (midY - endY) / (midX - endX);
    //Calculate Line T
    double lineT = (startY - midY) / (startX - midX);
    //calculate x Center
    double centerX = ((lineR * lineT * (startY - endY)) + (lineR * (midX + startX)) - (lineT * (endX + midX))) / (2 * (lineR - lineT));
    //calculate y Center by subsitution
    double centerY = ((endY + midY) / 2) + ((centerX - ((endX + midX) / 2)) * (-1 / lineR));
    //Calculate Raduis
    double raduis = Math.Sqrt((startX - centerX) * (startX - centerX) + (startY - centerY) * (startY - centerY));

    //Calculate Start Angle
    double startAngle = Math.Atan2(startY - centerY, startX - centerX) * (180 / Math.PI);
    //Calculate End Angle
    double endtAngle = Math.Atan2(endY - centerY, endX - centerX) * (180 / Math.PI);

    //if (endtAngle < 0)
    //    endtAngle = endtAngle + 360;

    return returnValues;

I have been trying for 3 weeks now with every fix i could think of, i tried calculatin


Spektre March 2016

I assume this is in 2D so use cross product to detect CW/CCW

let p0,p1,p2 be your arcs points as 3D vectors (x,y,z=0.0) then:


are the edge vectors. now compute the cross product


returning normal vector n of plane your arc lies on now just test the sign of its z coordinate:

if (n.z<0.0) CW else CCW

the sign depends on the configuration of your coordinate system so if it does not work use

if (n.z>0.0) CW else CCW

if (n.z==0.0) then you are in trouble and try to avoid that at all costs by dividing such entities to 2 arc's.

Also see arc via 3 points in specific direction

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Asked in February 2016
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