The incorporation of haptic interfaces into collaborative virtual environments is challenging when the users are geographically distributed. Reduction of latency is essential for maintaining realism, causality and the sense of co-presence in collaborative virtual environments during closely-coupled haptic tasks. In this study we consider the effects of varying amounts of simulated constant delay on the performance of a simple collaborative haptic task. The task was performed with haptic feedback alone or with visual feedback alone. Subjects were required to make a coordinated movement of their haptic displays as rapidly as possible, while maintaining a target simulated spring force between their end effector and that of their collaborator. Increasing simulated delay resulted in a decrease in performance, either in deviation from target spring force and in increased time to complete the task. At large latencies, there was evidence of dissociation between the states of the system that was observed by each of the collaborating users. This confirms earlier anecdotal evidence that users can be essentially seeing qualitatively different simulations with typical long distance network delays.
Unacceptable moire distortion may result when images that include periodic structures such as halftone dots are scanned. In the frequency domain, moire patterns correspond to visible aliased frequencies. In the spatial domain, moire patterns are evident as cyclic changes in the size of halftone dots, producing visible periodic "beat" patterns.Moire pattern formation depends on the following factors: (1) the halftone screen frequency, (2) the scan frequency, (3) the angle between the scan direction and the halftone screen, (4) the scanner aperture size and shape, (5) quantization errors from the thresholding operation, (6) scanner and printer noise, and (7) the ink flow in the paper during printing. This paper analyzes the visibility of moire patterns in terms of these factors. In addition, the paper describes an approach to reducing the visibility of moire patterns by directly manipulating the moire formation factors. With an appropriate selection of the scan frequency and screen angle for a given screened image, moire beat frequencies in the scanned -Optical Instrumentation Engineers.2). The formation of moire patterns depends on the following factors: (1) the halftone screen frequency, (2) the scan frequency, (3) the angle between the scan direction and the halftone screen, (4) the scanner aperture size and shape, (5) quantization errors from the thresholding operation, (6) scanner and printer noise, and (7) the ink flow on the paper during printing. In the spatial domain, moire patterns can be described as visible "beat" patterns resulting from the incorrect reproduction of halftone dots. As shown in Fig. 3, halftone dots 1 and 2 are the same size before digitization. After digitization, halftone dot 1 consists of one black pixel and halftone dot 2 consists of two black pixels. A band of two -pixel dots appears darker than a band of one -pixel dots, producing "beat" patterns. In the frequency domain, moire patterns are seen as aliased frequency components resulting from the scanning of screened art that possesses infinite bandwidth. Figure 4(a) illustrates the relation between reflectance intensity and position for one horizontal scan line in Fig. 1. Figure 4(b) illustrates the frequency components of Fig. 4(a) in the Fourier domain. Figure 4(c) shows the aliased frequency components after scanning. These aliased frequency components correspond to visible moire patterns in the scanned image.Much research has been done on the analysis and reduction of moire patterns by postscan processing.'-' Rosenfeld and Kak showed that moire patterns are caused by aliased frequencies produced when sampling images containing OPTICAL ENGINEERING / July 1989 / Vol. 28 No. 7 / 805Abstract. Unacceptable moire distortion may result when images that include periodic structures such as halftone dots are scanned. In the frequency domain, moire patterns correspond to visible aliased frequencies. In the spatial domain, moire patterns are evident as cyclic changes in the size of halftone dots, producing visible periodic "beat" patterns....
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