Electronic imaging system improvements via novel hardware architecture and halftoning algorithms
Electronic imaging systems such as image rendering and printing devices are providing higher quality with improved technologies. Physical hardware limitation has been pushed to a newer level. By utilizing some of these new imaging technologies, three improvements are presented in this document, including banding reduction using pulse width modulation, multitoning using DBS screens, and hybrid screen design. In Ch. 1, we propose a system to reduce electrophotographic laser printer banding artifacts due to optical photoconductor drum velocity fluctuations. The drum velocity fluctuations are sensed with an optical encoder mounted on the drum axis. Based on the line-to-line differential encoder count, we modulate the laser pulse width to compensate fluctuations in development that would otherwise occur. We present an analysis of the system, including the compensation algorithm that determines the desired pulse-width as a function of differential encoder count. Characterization of the system is based on printing, scanning, and processing a special test page that yields information about line-spacing and absorptance fluctuations. This data is synchronized with the encoder count signal that was recorded during the printing of the test page. The experimental results show the efficacy of the system. In Ch. 2, we propose a methodology for multilevel screen design using Direct Binary Search. We define a multitone schedule, which for each absorptance level specifies the fraction of each native tone used in the multitone cell. Based on the multitone schedule, multitone patterns are designed level-by-level by adding native tones under the stacking constraint. At each level, the spatial arrangement of the native tones is determined by a modified DBS search. We explore several different multitone schedules that illustrate the image quality tradeoffs in multitone screen design. In Ch. 4, we propose a screen design method that generates a new hybrid class of halftones---stochastic dispersed-dot in highlights and periodic clustered-dot in midtones. The traditional solution for generating hybrid halftones is the so-called supercell technique. While maintaining the spatial resolution, supercell increases the number of the output levels of a periodic micro-clustered-dot screen by adding dots asynchronously based on the order described in a dispersed-dot macroscreen. The traditional supercell screen employs a Bayer macroscreen and consequently results in visible false textures in highlight area. Simply replacing the Bayer macroscreen with a stochastic macroscreen yields the maze-like artifact due to the embedded upsampling process. In this work, we propose a screen design method which optimizes the employed multiple macroscreens and microscreens simultaneously using DBS. The resulting final screen is decomposable for a memory-efficient implementation. We also propose an adaptive microcell-based edge enhancement algorithm for further improvement. Several results are demonstrated using the proposed screens.
Allebach, Purdue University.
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