Browsing by Author "Wang, Chengyu"
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Item Open Access Sampling in Computational Cameras(2022) Wang, ChengyuThis dissertation contributes to computational imaging by studying the intersection of sampling and artificial intelligence (AI). It has been demonstrated that AI shows superior performance in various image processing problems, ranging from super- resolution to classification. In this work we demonstrate that combining AI with intelligent data sampling enables new camera capabilities.We start with traditional image signal processing (ISP) in digital cameras and show that AI has significantly improved the performance of ISP functions, such as demosaicing, denoising, and white balance. We than demonstrate a deep-learning (DL)-based image signal processor that regroups the ISP functions and achieves fast image processing with an end-to-end network. We further study the image compression strategies and show that AI is also a helpful tool for imaging system design. Following the study on image processing, we turn to the camera autofocus con- trol. With the demonstration of a DL-based autofocus pipeline and saliency de- tection network, we show that AI achieves 5 - 10x faster autofocus compared to traditional contrast maximization and allows content-based autofocus control. We also demonstrate an all-in-focus imaging pipeline to produce all-in-focus images or videos. This shows that AI extends the concept of camera control from optimizing an instantaneous image to producing the control trajectory that optimizes the sampling eciency and long-term image or video quality. Next we consider coherent phase retrieval. We first study the Fisher information and the Cram ́er-Rao lower bound on the mean squared error of coherent signal estimation from the squared modulus of its linear transform. Then we demonstrate two coding strategies to achieve optimal phase retrieval, and we use simulations to show practical implementations of these strategies. These simulation take the advantage of well-developed deep learning libraries. Finally we focus on Fourier ptychography, a technique combining aperture synthe- sis and phase retrieval. We build a snapshot ptychography system using a camera array and deep neural estimation, which achieves 6.7ˆ improvement in resolution compared to a single camera. We also present simulations considering various aper- ture distributions and multiple snapshots to show design considerations of such as system.
Item Embargo Why “Winners” Accommodate and “Losers” Repress: Heterogenous Career Prospects and Choices of Leaders when Facing Protests(2024) Wang, ChengyuTo repress or accommodate protests is a fundamental problem that political leaders face, especially in authoritarian regimes. The literature on state repression mostly focuses on how factors such as protest characteristics and government resources influence protest outcomes. Departing from this literature, my thesis understands repression and accommodation as choices that political leaders make with their own ends in mind. Building on the literature on political selection, which takes career advancement as the main motivation of leaders, the thesis analyzes choices at the subnational level in China and investigates how heterogeneity in self-evaluated career prospects of local communist party leaders shapes their choices. The thesis employs distance to the age ceiling for promotion, age at taking office, and years in office to measure the promotion stress local party leaders experience. The thesis proposes that promotion stress encourages leaders to embrace career risks that the repression choice entails. Estimating logit models with fixed effects, the thesis finds that: (1) at the deputy provincial level, party leaders close to the age ceiling choose repression more than do young and promising party leaders; and (2) at the city level, party leaders who take up office at an age young enough that promotion is possible but still old enough to handicap them for ultimate promotion to the provincial level choose repression more compared with older party leaders with effectively no chance to reach the provincial level. The findings confirm the theorized pattern: stressed “losers” repress more at both levels and secure “winners” repress less at the deputy provincial level. The thesis contributes to our understanding of the repression choice and brings new nuance to our understanding of the impact of political selection institutions on governance choices.