The planetary direct spectrum extraction steps. Top left: the original stellar spectral matrix. The red dashed lines indicate T₁ and T₄, while the white dashed lines mark T₂ and T₃. Bottom left: the master stellar spectrum, derived by averaging all spectra within the eclipse phase (T₂₃). Top right: the planetary residual flux matrix, obtained by dividing the original flux matrix by the master stellar spectrum. Bottom right: the final WASP-18 b direct-extracted spectrum (DES; black) and the Gaussian-filtered spectrum (light blue).