inline void unit_from_axis_angle(const vector3& axis, const float& angle){
         vector3 v(axis);
         v.norm();
         float half_angle = angle*0.5f;
         float sin_a = (float)sin(half_angle);
         set(v.x*sin_a, v.y*sin_a, v.z*sin_a, (float)cos(half_angle));
     };
     //-----------------------------------
     /**
       convert a quaternion to axis angle representation,  
       preserve the axis direction and angle from -PI to +PI
     */
     inline void to_axis_angle(vector3& axis, float& angle)const {
         float vl = (float)sqrt( x*x + y*y + z*z );
         if( vl > TINY )
         {
             float ivl = 1.0f/vl;
             axis.set( x*ivl, y*ivl, z*ivl );
             if( w < 0 )
                 angle = 2.0f*(float)atan2(-vl, -w); //-PI,0  
             else
                 angle = 2.0f*(float)atan2( vl,  w); //0,PI  
         }else{
             axis = vector3(0,0,0);
             angle = 0;
         }
     };