template< class T, class M >
  template< class S >
  bool a3d::Vector3<T,M>::fromScreenPosition( S sx, S sy,
                                              a3d::Vector3<T,M> &x,
                                              a3d::Vector3<T,M> &v ){
    a3d::Matrix4f Mm;
    glGetFloatv( GL_MODELVIEW_MATRIX, (float*)Mm );
    a3d::Matrix4f Mmi = Mm.inverse();
    
    GLint size[4];
    glGetIntegerv( GL_VIEWPORT, size );
    int width = size[2]-size[0];
    int height = size[3]-size[1];
    if( width <= 0 || height <= 0 ){
      return false; }
    
    a3d::Matrix4f Mp;
    glGetFloatv( GL_PROJECTION_MATRIX, (float*)Mp );
    if( fabsf( Mp[10] -1.0f ) < std::numeric_limits<float>::epsilon() ||
        fabsf( Mp[10] +1.0f ) < std::numeric_limits<float>::epsilon() ){
      return false; }
    
    float n = Mp[14]/( Mp[10] -1.0f );
    float f = Mp[14]/( Mp[10] +1.0f );
    
    sy = sy < 0 ? sy = -sy : height-sy;
    
    a3d::Matrix4f Mpi = Mp.inverse();
    
    a3d::Vector4f v0_4( 2.0f*sx/width -1.0f,
                        2.0f*sy/height -1.0f,
                        -1.0f, 1.0f );
    v0_4 = n * v0_4;
    v0_4 = Mpi * v0_4;
    v0_4 = Mmi * v0_4;
    
    a3d::Vector4f v1_4( 2.0f * sx/width -1.0f,
                        2.0f * sy/height -1.0f,
                        +1.0f, 1.0f );
    v1_4 = f * v1_4;
    v1_4 = Mpi * v1_4;
    v1_4 = Mmi * v1_4;
    
    x = a3d::Vector3<T,M>( v0_4[0], v0_4[1], v0_4[2] );
    v = a3d::Vector3<T,M>( v1_4[0] - v0_4[0],
                           v1_4[1] - v0_4[1],
                           v1_4[2] - v0_4[2] ).normalized();