Top left: dictionary learnt from training data set with line features. Atoms typically contain a mixture of line emissions, with different sign and resolution. Top right: dictionary learnt from training data set with continuum features. Atoms are correlated, with breaks such as the 4000 Å break. Bottom left: component of the final dictionary corresponding to the original line dictionary. Some fluctuations from the continuum are added. Bottom right: component of the final dictionary corresponding to the original continuum dictionary. Extra line emissions are added to the continuum.