MatrixXf r_uu_center = r_uu.block(2 * R, 2 * R, 4 * R + 1, 4 * R + 1);
MatrixXf r_uv = central_convolve(r_uu_center, h);
assert(r_uv.rows() == 2 * R + 1);
MatrixXf r_uu_center = r_uu.block(2 * R, 2 * R, 4 * R + 1, 4 * R + 1);
MatrixXf r_uv = central_convolve(r_uu_center, h);
assert(r_uv.rows() == 2 * R + 1);