Halftones are intended to produce the illusion of continuous images form binary output states, so the visibility of undesired halftone textures is an essential quality factor of halftone patterns. We propose a metric to predict the visibility of color halftone textures. The metric utilizes the human visual threshold function and contrast sensitivity functions of luminance and chrominance. The threshold is related to the average background luminance level by de Vries-Rose law. An iterative approach was used to determine the distance in which the visual error just exceeds the visual threshold. This distance is the metric that predicts the critical distance that a human observer can just discriminate the textures. To verify the metric, the texture visibility was determined experimentally by a psychological experiment. The halftone stimuli were presented on an SGI monitor. Starting from an initial distance, where the halftone images appeared as continuous color patches, the subject walked toward the monitor and found the distance where he or she could just discriminate the spatial changes caused by the textures. Then the distances determined by the experiment and those predicted by the metric were compared. A good correlation was achieved. The results show that the metric is able to predict the visibility over a wide range of texture characteristics.
It has been found that the L* function defined in the CIELAB color space is not suitable to predict the human visual perception of modulated patterns at high spatial frequencies. For example, in multilevel halftoning (multitoning), when output levels are equally spaced in L*, it has been observed that the visibility of the resulting multitone patterns is not uniform across different parts of the tone scale. This leads to the hypothesis that the CIE L* function may not be a good metric to evaluate the perceived lightness differences at high-spatial frequencies as it was derived based on the perception of large area uniform patches. To investigate the relationship between suprathreshold lightness difference perception with regard to spatial frequency and amplitude modulation, we designed a psychophysical experiment, which was conducted using a lightness difference matching paradigm. The stimuli used in the experiment were horizontal square-wave gratings. The behavior of lightness difference perception under varying spatial frequencies and modulation amplitudes across the entire L* scale was studied. Consistent results were acquired that show a significant frequency-dependent effect where the effective lightness difference for high- frequency patterns is reduced for low L* values. The magnitude of this effect was found to be highly related to the spatial frequency of the modulation. Based on these results, we derived an effective lightness function that is dependent on spatial frequency. The effective lightness function can be applied to the selection of the output levels for multitoning.
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