Upon empirical testing of #13 -- actual sorting of test covers -- it becomes apparent that the lightness maybe should be done on the primary color, rather than the vivid color, as the primary color is by definition what dominates the image.
I.e. - use the vivid color's hue to place it into a partitioned color band, then use the lightness of the dominant color to sort within that band.
Upon empirical testing of #13 -- actual sorting of test covers -- it becomes apparent that the lightness maybe should be done on the primary color, rather than the vivid color, as the primary color is by definition what dominates the image.
I.e. - use the vivid color's hue to place it into a partitioned color band, then use the lightness of the dominant color to sort within that band.