Optimal Output Level Selection for Multilevel Halftoning Based on Visual Experiments Using Spatially Modulated Gratings

Abstract

Experiments have shown that multitoning with output levels equally spaced in CIE L* does not produce uniform texture at different gray levels. In this study, psychophysical experiments are described, investigating the influence of spatial modulation on the perception of lightness difference. A variety of different patterns (square wave, checker-board, and 50% blue noise patterns) were tested at various frequencies and amplitudes (contrast). A significant frequency dependent effect was observed where the lightness difference perception is reduced for high frequency modulated gratings at low L* values. The magnitude of this effect is highly related to the frequency of the modulation. There is no evidence to indicate that either modulation type or modulation amplitude has a significant effect on the lightness difference perception. Based on the experimental results, an effective lightness metric (Le*) is derived as a function of spatial frequency. The Le* metric is intended to provide a space that is linearly related to lightness difference perception for frequency modulated patterns. Results from a further visual experiment have verified that multitoning patterns produced using levels that are equally spaced in Le* produces results with more uniform texture visibility across the entire tone scale.