Converting color images into grayscale ones suffer from information loss. In the meantime, it is one fundamental tool indispensable for single channel image processing, digital printing, and monotone e-ink display. In this paper, we propose an optimization framework aiming at maximally preserving color contrast. Our main contribution is threefold. First, we employ a bimodal objective function to
... [Show full abstract] alleviate the restrictive order constraint for color mapping. Second, we develop an efficient solver that allows for automatic selection of suitable grayscales based on global contrast constraints. Third, we advocate a perceptual-based metric to measure contrast loss, as well as content preservation, in the produced grayscale images. It is among the first attempts in this field to quantitatively evaluate decolorization results.