Simulated spectrum of a RG star from a PHOENIX spectrum of a star with an effective temperature of 4800 K, surface gravity of 2.5, and [Fe/H] of 0.0 (gray line). Resolution and sampling were downgraded to that of LAMOST. The red line corresponds with the triangular weighting function, w_ℓ, of the line, ℓ, and the blue line to the flat weighting function, w_c, employed for computing the continuum level. N_ℓ (red dots) and N_c (blue dots) indicate the points that belong to the triangular and rectangular weighting functions, respectively.