通过传入的两个点计算他们之间的角度

float getAngle(Point fromPoint, Point toPoint)
{
    double len_y = toPoint.y - fromPoint.y;
    double len_x = toPoint.x - fromPoint.x;
    if( 0 == len_x && fromPoint.y <= toPoint.y )
    {
        return 0;
    }
    else if( 0 == len_x && fromPoint.y > toPoint.y )
    {
        return 180;
    }
    
    float angle = 0.f;
    double tan_yx = tan_yx = std::abs(len_y)/std::abs(len_x);
    if(len_y > 0 && len_x < 0)
    {
        angle = atan(tan_yx)*180/M_PI - 90;
    }
    else if (len_y > 0 && len_x > 0)
    {
        angle = 90 - atan(tan_yx)*180/M_PI;
    }
    else if(len_y < 0 && len_x < 0)
    {
        angle = -atan(tan_yx)*180/M_PI - 90;
    }
    else if(len_y < 0 && len_x > 0)
    {
        angle = atan(tan_yx)*180/M_PI + 90;
    }
    
    return angle;
}