qt使用弧度角绘制两点之间的弧

我想画两点之间的弧线 . 我知道两点的位置和弧度的角度 . 我成功地编写了一个小程序来计算圆的中心，然后才能有效地绘制圆弧 . 但是当我画一个圆来验证时，当我使用小值来表示弧度时，圆线不会越过给定的两个点 .

#include <QApplication>
#include <QGraphicsEllipseItem>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QDebug>
#include <cmath>
#include <QPainter>

void cross(QPainterPath* path, double x, double y);

int main( int argc, char **argv )
{
    QApplication app(argc, argv);
    QGraphicsScene scene;
    scene.setSceneRect( 0.0, 0.0, 500.0, 500.0 );
    QPainterPath* path = new QPainterPath();

    double x1, x2, y1, y2, l, rad, r;
    double x3, y3, xx, yy;
    const double PI = 3.14159265358979323846;

    //first point
    x1=250;
    y1=250;
    //second point
    x2=350;
    y2=300;
    //radians - play with it. This is low value - this is buggy
    rad=0.002;

    l=sqrt (pow((x1-x2),2) + pow((y1-y2),2)); //distance between (x1,y) and (x2,y2)
    u=180.0 * rad / PI; //transform radians in angle
    r=(l/2.0)/sin(rad/2.0); //this is radius

    //point in the middle of (x1,y) and (x2,y2)... half of l
    x3 = (x1+x2)/2;
    y3 = (y1+y2)/2;

    //find center of circle
    if(rad>0){
        xx = x3 + sqrt(pow(r,2)-pow((l/2),2))*(y1-y2)/l;
        yy = y3 + sqrt(pow(r,2)-pow((l/2),2))*(x2-x1)/l;
    }else{
        xx = x3 - sqrt(pow(r,2)-pow((l/2),2))*(y1-y2)/l;
        yy = y3 - sqrt(pow(r,2)-pow((l/2),2))*(x2-x1)/l;
    }


    //draw circle to verify
    path->moveTo(xx, yy);
    path->addEllipse(QRectF(xx-r,yy-r,r*2,r*2));

    cross(path, x3,y3);
    cross(path, xx,yy);
    cross(path, x1,y1);
    cross(path, x2,y2);


    qDebug() << "r =" << r << " xx =" << xx << " yy =" << yy ;
    qDebug() << "Verify r - distance from (x1,y1) to center of circle" << sqrt (pow((x1-xx),2) + pow((y1-yy),2));
    qDebug() << "Verify r - distance from (x2,y2) to center of circle" << sqrt (pow((x2-xx),2) + pow((y2-yy),2));

    scene.addPath(*path);

    QGraphicsView view( &scene );
    view.show();
    return app.exec();
}

void cross(QPainterPath* path, double x, double y){
    path->moveTo(x, y-5);
    path->lineTo(x, y+5);
    path->moveTo(x-5, y);
    path->lineTo(x+5, y);
}

但是，从两个点到圆心的距离等于计算的半径 . 我哪里错了？