Surface brightness in J band, I_J, as a function of the K band, I_K, (upper frame) and I_H as a function of I_K (lower frame) for the main filament area shown in Fig. 2. The y = x function is denoted by a black line. The colour scale of the 2D histogram indicates the logarithmic density of the points, both here and in other figures. The data are convolved to 40 resolution.