Four simulated samples limited in color index as indicated. Stars with i > 80° are plotted in red. The gray envelope function, calculated as described in Sect. 3.3, is shifted from the right hand side of the distribution to the left considering a slope so that it scans the high-inclination region (see text). At each step the efficiency (Eq. (4), with α = 80°) is calculated for the subsample below the envelope function. The red line in the bottom diagrams shows the evolution of ϵ as the region limited by the function is expanded to the left, and the black line indicates the total fraction of stars in the region (N_Stars ~ 1). As expected, ϵ tends to  ~0.17 for N_Stars ~ 1. The dotted gray line in the upper diagrams indicates the region where ϵ = 0.26, therefore the efficiency is increased by 50% with respect to a non-preselected sample.