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Xiaoli He, Jacob Feldman, Manish Singh; The influence of contour geometry on structure-from-motion: from symmetry to parallelism. Journal of Vision 2017;17(10):414. doi: https://doi.org/10.1167/17.10.414.
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Structure-from-motion (SFM) studies have shown that observers have vivid 3D percepts in dynamic dot displays that are projectively consistent with 3D rotation. Empirical and computational studies have focused on the motion profile of the dots, with only a few reports that people can still perceive 3D structures when the occluding contours are projectively inconsistent with 3D rotation (Ramachandran et al. 1988; Froyen et al., JOV2013; Tanrikulu et al., VSS2014, VSS2015). These studies suggest that occluding contour geometry may influence SFM percepts. We previously showed that SFM displays with fixed, asymmetric contours (inconsistent with 3D rotation) still generate vivid 3D percepts (He et al., VSS2016). Given the importance of skeletal models in shape representation, we focus here on the spectrum from symmetry to parallelism. Stimulus shapes consisted of two vertically oriented sine curves with different relative phases (0, pi/3, 2pi/3, pi). Contour geometry remained fixed within each dynamic display; hence only the symmetric shapes are projectively consistent with rigid rotation, and the displays get progressively less consistent with increasing deviation from symmetry. Dots moved horizontally with different speed profiles, varying from constant speed to a cosine speed profile (projectively consistent with 3D rotation in depth). In a method of constant stimuli, observers judged whether each display looked like a solid 3D object. Observers perceived vivid 3D structure in parallel displays, which are grossly inconsistent with 3D rotation, even with as little as 60% cosine motion. Parallel display did require a higher proportion of cosine motion to perceive 3D structure than symmetric displays, but the difference was surprisingly small. The results demonstrate that projective consistency is less important in SFM than conventionally assumed. Contour geometry and dot motion both play roles as well, in a way that sometimes overrides projective consistency.
Meeting abstract presented at VSS 2017
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