dvec2 RBFVectorFieldGenerator2D::randomVector() {
    dvec2 v(1, 0);

    auto d = theta_(mt_);
    auto c = std::cos(d);
    auto s = std::sin(d);
    return (mat2(c, s, -s, c) * v) * static_cast<float>((x_(mt_) * 2 - 1));
}
 Real theta() const {
     return theta_(0.0);
 }