Evaluating object visibility with simulated visual impairment using real and rendered scenes

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Evaluating object visibility with simulated visual impairment using real and rendered scenes

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Visual accessibility is "the effectiveness with which vision can be used to travel safely through the space and to pursue the intended activities in the space (Legge, Yu, Kallie, Bochsler, & Gage, 2010)." A major issue for visual accessibility involves the visibility of objects under visually challenging conditions, especially those faced by people with impaired vision. This research investigates visibility of objects under conditions of artificially induced blur, and concludes with simple design principles for improving object visibility when vision is compromised by reductions in acuity and contrast sensitivity. Additionally, this research bridges a gap between object visibility studies in physical environments, and studies of visibility using image processing and screen displays of the same objects and scenes. Psychophysical measurements of visibility of objects shown on the desktop display accounted for 58% of the variability in visibility measurements of the same scenes and objects in the physical environment. The technical aspects of virtual modeling—from stimulus conception to photometrically and geometrically accurate displaying of stimulus images—are described. Chapter 1 of this dissertation provides an overview of this research. In Chapter 2, experiments are described in which object visibility was tested in a physical environment. The objects were boxes and cylinders. The subjects were normally sighted young adults who wore blur goggles that simulate the effects of visually impaired acuity and contrast sensitivity loss. Interactions between illumination source (i.e., luminaire location), surface reflectance, and object shape led to the conclusion that curved objects were more visible than flat objects with blurry vision. However we wished to confirm this conclusion for a wider range of viewing conditions. Chapter 3 extended the work of the physically based study to a computer desktop displayed environment. Several models for digitally simulating physical blur goggles were examined. A high dynamic range (HDR) image of the point spread function (PSF) of the physical blur goggles, convolved with HDR images of the stimuli, plus a slight contrast attenuation, provided the best agreement between visibility performance of subjects viewing the objects on a computer display and the performance of the subjects who viewed the corresponding real objects (Chapter 2). A comparison between photographs and renderings showed no difference between the two image types. The main effects and interaction findings of the physical experiment (from Chapter 2) were largely reproduced with our desktop model. The desktop model was used to study novel conditions (i.e., new object locations and orientations), were it was found that under diffuse overhead illumination near the center of the room, the curved objects were actually less visible than flat objects. In all other cases that were tested, curved objects still provided enhanced visibility over flat objects. Chapter 4 contains a technical report on the methods used to create photometrically accurate HDR images based on physically based 3D objects, materials, and scenes. It illustrates the 3D modeling, rendering, photography, and photometry methods for reproducing photometrically and geometrically accurate scenes on a calibrated computer display. In addition to modeling and calibration techniques, Chapter 4 illustrates methods for spatial filtering (i.e. visual blur), and non-spatial image processing, including color balancing and contrast reduction, required for accurate reproduction of the physically based experiment. Example code for all major steps, including spatial and non-spatial filtering, is provided. Collectively, this series of experiments demonstrated a simple heuristic for designing visually accessible spaces: namely that adding curvature to objects can enhance object visibility under reduced acuity and reduced contrast sensitivity situations. This enhancement, however, is lost under diffuse illumination, where other methods for increasing contrast within or around the object would be more reliable. Moreover, this research demonstrates a complete workflow from conception of a physical environment to complete virtual modeling of the physical space, as well as digitization of models for visual impairment. This research provides a foundation for further study of visibility in more complex and realistic environments (including motion, as demonstrated in this dissertation's supplemental video archive), linking virtual desktop models to physical environments.


University of Minnesota Ph.D. dissertation. December 2012. Major: Psychology. Advisors: Gordon E. Legge, Paul R. Schrater. 1 computer file (PDF), xvi, 138 pages.

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Kallie, Christopher Scott. (2012). Evaluating object visibility with simulated visual impairment using real and rendered scenes. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/144216.

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