点到直线距离

// (x0,y0)到(x1,y1)和(x2,y2)确定的直线的距离

double disBetweenPointAndLine(double x0,double y0,double x1,double y1,double x2,double y2)
{
    //化为ax+by+c=0的形式
    double a = y1-y2;
    double b = x2-x1;
    double c = x1*y2-x2*y1;
    double d = (a*x0+b*y0+c)/sqrt(a*a+b*b);
    /*
    如果是线段判断垂足

    double xp = (b*b*x0-a*b*y0-a*c)/(a*a+b*b);
    double yp = (-a*b*x0+a*a*y0-b*c)/(a*a+b*b);
    double xb = (x1>x2)?x1:x2;
    double yb = (y1>y2)?y1:y2;
    double xs = x1+x2-xb;
    double ys = y1+y2-yb;
    if(xp > xb || xp < xs || yp > yb || yp < ys)
    {
        d = sqrt((x0 - x1) * (x0 - x1) + (y0 - y1) * (y0 - y1));
        if(d > sqrt((x0 - x2) * (x0 - x2) + (y0 - y2) * (y0 - y2)))
            d = sqrt((x0 - x2) * (x0 - x2) + (y0 - y2) * (y0 - y2));
    }
    */
    return fabs(d);
}

线段间最短距离

//{(x1,y1),(x2,y2)}{(x3,y3),(x4,y4)}
//导入点到直线或线段的距离模板

double GetLineDistance(double x1,double y1,double x2,double y2,double x3,double y3,double x4,double y4)
{
    double tmpDouble = disBetweenPointAndLine(x1,y1,x3,y3,x4,y4);
    if(tmpDouble > disBetweenPointAndLine(x2,y2,x3,y3,x4,y4))
        tmpDouble = disBetweenPointAndLine(x2,y2,x3,y3,x4,y4)；
    if(tmpDouble > disBetweenPointAndLine(x3,y3,x1,y1,x2,y2))
        tmpDouble = disBetweenPointAndLine(x3,y3,x1,y1,x2,y2)；
    if(tmpDouble > disBetweenPointAndLine(x4,y4,x1,y1,x2,y2))
        tmpDouble = disBetweenPointAndLine(x4,y4,x1,y1,x2,y2)；

    return tmpDouble;
}

double disBetweenPointAndLine(double x0,double y0,double x1,double y1,double x2,double y2)
{
    //化为ax+by+c=0的形式
    double a = y1-y2;
    double b = x2-x1;
    double c = x1*y2-x2*y1;
    double d = (a*x0+b*y0+c)/sqrt(a*a+b*b);
    /*
    如果是线段判断垂足
    */
    double xp = (b*b*x0-a*b*y0-a*c)/(a*a+b*b);
    double yp = (-a*b*x0+a*a*y0-b*c)/(a*a+b*b);
    double xb = (x1>x2)?x1:x2;
    double yb = (y1>y2)?y1:y2;
    double xs = x1+x2-xb;
    double ys = y1+y2-yb;
    if(xp > xb || xp < xs || yp > yb || yp < ys)
    {
        d = sqrt((x0 - x1) * (x0 - x1) + (y0 - y1) * (y0 - y1));
        if(d > sqrt((x0 - x2) * (x0 - x2) + (y0 - y2) * (y0 - y2)))
            d = sqrt((x0 - x2) * (x0 - x2) + (y0 - y2) * (y0 - y2));
    }

    return fabs(d);
}