Fig 1. Stimuli and visual filters.

a) Examples of RGB natural color images from the database. b) Set of N = 50 filters obtained after digitizing to 1 bit the luminance (L+M) coordinate. The actual bandwidth occupancy of the set ($\frac{1}{N_{tot}}{\displaystyle \underset{f(p)>c}{\overset{}{\int }}p\delta (p)dp}$) turns out to be slightly higher (0.06) than the imposed limit W (0.05). c) Set of N = 50 filters obtained after digitizing to one bit the l (L/(L+M)) coordinate. The actual bandwidth occupancy of the set out to be slightly higher (0.06) than the imposed limit W (0.05), but is still the same as the actual bandwidth of the 1 bit luminance set. d) Two gray-levels luminance sketches obtained with the filters in (b). e) Color-only sketches obtained with the filters in (c). f) Set of N = 50 filters obtained after digitizing to 2 bit the luminance coordinate. The actual bandwidth occupancy of the set turns out to be slightly higher (0.07) than the imposed limit W (0.05). g) Set of N = 50 filters obtained after digitizing to 1 bit both l (L/(L+M)) and luminance (L+M) coordinates. The actual bandwidth occupancy of the set turns out to be slightly higher (0.07) than the imposed limit W (0.05), but is still the same as the actual bandwidth of the 2 bits luminance set. h) Four gray-levels luminance sketches obtained the set of filters in (f). i) Color sketches obtained with the set of filters in (g).