Question

We have 3(three) xyz points that define a circle in 3D space, this circle needs to be converted into a polyline(for further rendering). I’m looking for a ready C or C++ function or free library that can do the job.

Don’t understand why this was closed. And I can’t even answer my own question there. Shame on you guys. But you will not stop the knowledge spreading!

Answer 1

There is a nice article and a code sample on how to build a circle by 3 points in 2D, XY plane.

http://paulbourke.net/geometry/circlesphere/

http://paulbourke.net/geometry/circlesphere/Circle.cpp

To build a 3D circle we’ll have to:

• rotate our 3 points into XY plane
• Calculate circle center
• build a circle in XY plane using the code in the article
• rotate it back into it’s original plane

For rotations it is best to use quaternions.

To find a correct quaternion I looked at Ogre3d source code:
void Quaternion::FromAngleAxis (const Radian& rfAngle, const Vector3& rkAxis)

There is one more useful function there:
Quaternion getRotationTo(const Vector3& dest, const Vector3& fallbackAxis = Vector3::ZERO) const
But I didn’t use it.

For quaterions and vectors I used our own classes. Here is the full source code of the function that does the job:

bool IsPerpendicular(Point3d *pt1, Point3d *pt2, Point3d *pt3);
double CalcCircleCenter(Point3d *pt1, Point3d *pt2, Point3d *pt3, Point3d *center);

void FindCircleCenter(const Point3d *V1, const Point3d *V2, const Point3d *V3, Point3d *center)
{
    Point3d *pt1=new Point3d(*V1);
    Point3d *pt2=new Point3d(*V2);
    Point3d *pt3=new Point3d(*V3);


    if (!IsPerpendicular(pt1, pt2, pt3) )       CalcCircleCenter(pt1, pt2, pt3, center);
    else if (!IsPerpendicular(pt1, pt3, pt2) )  CalcCircleCenter(pt1, pt3, pt2, center);
    else if (!IsPerpendicular(pt2, pt1, pt3) )  CalcCircleCenter(pt2, pt1, pt3, center);
    else if (!IsPerpendicular(pt2, pt3, pt1) )  CalcCircleCenter(pt2, pt3, pt1, center);
    else if (!IsPerpendicular(pt3, pt2, pt1) )  CalcCircleCenter(pt3, pt2, pt1, center);
    else if (!IsPerpendicular(pt3, pt1, pt2) )  CalcCircleCenter(pt3, pt1, pt2, center);
    else {
        delete pt1;
        delete pt2;
        delete pt3;
        return;
    }
    delete pt1;
    delete pt2;
    delete pt3;

}

bool IsPerpendicular(Point3d *pt1, Point3d *pt2, Point3d *pt3)
// Check the given point are perpendicular to x or y axis
{
    double yDelta_a= pt2->y - pt1->y;
    double xDelta_a= pt2->x - pt1->x;
    double yDelta_b= pt3->y - pt2->y;
    double xDelta_b= pt3->x - pt2->x;

    // checking whether the line of the two pts are vertical
    if (fabs(xDelta_a) <= 0.000000001 && fabs(yDelta_b) <= 0.000000001){
        return false;
    }

    if (fabs(yDelta_a) <= 0.0000001){
        return true;
    }
    else if (fabs(yDelta_b) <= 0.0000001){
        return true;
    }
    else if (fabs(xDelta_a)<= 0.000000001){
        return true;
    }
    else if (fabs(xDelta_b)<= 0.000000001){
        return true;
    }
    else
        return false ;
}

double CalcCircleCenter(Point3d *pt1, Point3d *pt2, Point3d *pt3, Point3d *center)
{
    double yDelta_a = pt2->y - pt1->y;
    double xDelta_a = pt2->x - pt1->x;
    double yDelta_b = pt3->y - pt2->y;
    double xDelta_b = pt3->x - pt2->x;

    if (fabs(xDelta_a) <= 0.000000001 && fabs(yDelta_b) <= 0.000000001){
        center->x= 0.5*(pt2->x + pt3->x);
        center->y= 0.5*(pt1->y + pt2->y);
        center->z= pt1->z;

        return 1;
    }

    // IsPerpendicular() assure that xDelta(s) are not zero
    double aSlope=yDelta_a/xDelta_a; //
    double bSlope=yDelta_b/xDelta_b;
    if (fabs(aSlope-bSlope) <= 0.000000001){    // checking whether the given points are colinear.
        return -1;
    }

    // calc center
    center->x= (aSlope*bSlope*(pt1->y - pt3->y) + bSlope*(pt1->x + pt2 ->x)
                         - aSlope*(pt2->x+pt3->x) )/(2* (bSlope-aSlope) );
    center->y = -1*(center->x - (pt1->x+pt2->x)/2)/aSlope +  (pt1->y+pt2->y)/2;

    return 1;
}

//! Builds a circle in 3D space by 3 points on it and an optional center
void buildCircleBy3Pt(const float *pt1,
                      const float *pt2,
                      const float *pt3,
                      const float *c,       // center, can be NULL
                      std::vector<float> *circle)
{
    /*  Get the normal vector to the triangle formed by 3 points
        Calc a rotation quaternion from that normal to the 0,0,1 axis
        Rotate 3 points using quaternion. Points will be in XY plane 
        Build a circle by 3 points on XY plane 
        Rotate a circle back into original plane using quaternion
     */
    Point3d p1(pt1[0], pt1[1], pt1[2]);
    Point3d p2(pt2[0], pt2[1], pt2[2]);
    Point3d p3(pt3[0], pt3[1], pt3[2]);
    Point3d center;
    if (c)
    {
        center.set(c[0], c[1], c[2]);
    }

    const Vector3d p2top1 = p1 - p2;
    const Vector3d p2top3 = p3 - p2;

    const Vector3d circle_normal = p2top1.crossProduct(p2top3).normalize();
    const Vector3d xy_normal(0, 0, 1);


    Quaternion rot_quat;
    // building rotation quaternion
    {
        // Rotation axis around which we will rotate our circle into XY plane
        Vector3d rot_axis = xy_normal.crossProduct(circle_normal).normalize();
        const double rot_angle = xy_normal.angleTo(circle_normal); // radians

        const double w = cos(rot_angle * 0.5);
        rot_axis *= sin(rot_angle * 0.5);

        rot_quat.set(w, rot_axis.x, rot_axis.y, rot_axis.z);
    }

    Quaternion rot_back_quat;
    // building backward rotation quaternion, same as prev. but -angle
    {
        const double rot_angle = -(xy_normal.angleTo(circle_normal)); // radians
        const double w_back = cos(rot_angle * 0.5);
        Vector3d rot_back_axis = xy_normal.crossProduct(circle_normal).normalize();
        rot_back_axis *= sin(rot_angle * 0.5);
        rot_back_quat.set(w_back, rot_back_axis.x, rot_back_axis.y, rot_back_axis.z);
    }

    rot_quat.rotate(p1);
    rot_quat.rotate(p2);
    rot_quat.rotate(p3);
    rot_quat.rotate(center);

    if (!c)
    {
        // calculate 2D center
        FindCircleCenter(&p1, &p2, &p3, &center);
    }

    // calc radius
    const double radius = center.distanceTo(p1);

    const float DEG2RAD = 3.14159f / 180.0f;
    // build circle
    for (int i = 0; i < 360; ++i)
    {
        float degInRad = i * DEG2RAD;
        Point3d pt(cos(degInRad) * radius + center.x, sin(degInRad) * radius + center.y, 0);

        // rotate the point back into original plane 
        rot_back_quat.rotate(pt);

        circle->push_back(pt.x);
        circle->push_back(pt.y);
        circle->push_back(pt.z);
    }
}