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Kajondecha, Cheng, and Hoshino: Human perception of contour in halftoned density step image Figure 5. Perception ratio of average patterns diagonal and perpendicular pattern vs density differences 192-180 at four distances and a dot cycle of 1–4 mm when the halftone angle is 45°. The plates show the conditions where the observation distance is a 0.5 m, b 1m,c 2m,andd 3m. halftone frequencies of 1mm, 2mm, 3mm, and 4mm.In a digital system, the halftone screen angle is simulated by placement of the dots within the halftone cells. Figure 7(a) shows a comparison between a screen angle of 0° and 45° at an observation distance of 0.5 m relativetoaveragevalueof diagonal and perpendicular pattern under an illumination of 1.010 3 Lux. The result shows that at a halftone cycle 1mm, the halftone image with the screen angle 45° can be detected more easily than the halftone image with the screen angle 0°. According to the results shown in Fig. 7, the perception ratio of the halftone image with a screen angle of 45° is higher than that with a screen angle of 0° at a halftone cycle of 1mm. The sensitivity of our human visual system is reported to decrease at angles of 45° and 135°. 6 Therefore, the stimuli of the halftone dot when the halftone angle is 45° decreases, and we perceive the contour more easily when the angle is 0°. However,atcyclesof2mm, 3mm, and 4mm, the ratios do not show as clear a difference as when the dot cycle is 1mm, possibly because the dot stimulus is too strong. Therefore, in some cases, the opposite relationship can arise for all of the other cycles (2 mm, 3mm, and 4mm). Effect of Illumination One factor that affects contour perception of a halftoned image is illumination. Figure 8 shows the perception ratio dependence on density difference at a halftone angle of 45°. The results show a comparison of when the illumination conditions are 1.010, 1.010 2 , and 1.010 3 Lux and observation distance is 1mand 3m, under halftone cycles of 1mmand 4mm, respectively. Figure 8(c) shows that the perception ratio increases with illumination when the dot cycle is 1mmat an observation distance of 3m. Figures 8(b) and 8(d) show that the perception ratio decreases when the dot cycle is 4mmat observation distances 1mand 3m with increasing illumination. In summary, the perception 404 J. Imaging Sci. Technol. 515/Sep.-Oct. 2007
Kajondecha, Cheng, and Hoshino: Human perception of contour in halftoned density step image Figure 6. Relationship between discrete Fourier transform of halftone frequency and modulation transfer function of HVS: a halftone frequency 1 dot/mm and b halftone frequency 0.33 dot/mm. Figure 7. Perception ratio of average pattern diagonal and perpendicular dependence on density differences. Density values ranged between 192 and 180, the observation distance was a 0.5 m and b 1.0 m, and illumination was 1.010 3 Lux. ratio increases with decreasing illumination at a dot cycle 4mm, whereas the perception ratio increases when illumination increases at a dot cycle of 1mmat an observation distance of 3m. In the perception of contour, competition arises between Stevens effect and the decrease in sensitivity in a highfrequency area under low illumination. When the dot cycle is 1mm, the Stevens effect becomes dominant, and the perception ratio increases with illumination. However, when the dot cycle 4mm, the perception ratio increases with a decrease in illumination. The reason for the increasing in the perceptionratioatadotcycleof4mm under decreased illumination is considered to be that the MTF of the HVS shifts to the low frequency range, resulting in a decrease in the sensitivity of the halftone dot. 7 CONCLUSIONS Contour perception was investigated under various halftone and observation conditions. Perception ratio was observed to increase with density difference. At large halftone dot cycles of 3mm, the ease of contour perception increases as J. Imaging Sci. Technol. 515/Sep.-Oct. 2007 405
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