Comparison of the generalization performances of three predictors. Top row: linear method. Middle row: linear method with a non-linear pre-processing. Bottom row: random forest. All results are computed on the Horsehead pillar, i.e., the test set. Left and middle columns: spatial distribution of the observed and predicted column density. Both images share the same color scale. Right column: ratio of the predicted column density over the observed one. The limits of the color scale correspond to a ratio interval from 1∕3 to 3.