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Sophie Kenny, Nikolaus Troje; Changes in camera elevation dictate perception of point-light walkers' facing direction.. Journal of Vision 2014;14(10):1017. doi: 10.1167/14.10.1017.
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The degree of perspective distortion of an object depends on the ratio of its size to its distance from the rendering camera (the field-of-view, FOV). Previously, researchers have reported that sufficient amounts of linear perspective can disambiguate the direction of an otherwise depth-ambiguous point-light display (e.g., Schouten & Verfaillie, 2010). Based on their finding that the effect of FOV on the FTV bias is modulated by the height of the camera above ground Troje, Kenny, and Weech ( 2013) hypothesised that this observation is not based on linear perspective per se, but rather the result of a bias to see the walkers feet from above rather than from below. Here, we test explicitly if the previously reported effects of linear perspective are caused by camera elevation changes that pit a facing the viewer bias (FTV) against a very strong viewing from above bias. We asked participants to indicate the perceived facing direction of point-light displays, and modified the camera elevation according to a staircase procedure targeting the 25%, 50% and 75% FTV thresholds. A univariate ANOVA showed that camera elevation caused large changes in perceived facing direction at the three FTV bias thresholds: 25% (M = -12.87°, SD = 10.14°), 50% (M = -6.27°, SD = 10.99°), or 75% (M = -1.00°, SD = 8.28°), F(2, 22) = 16.04, p <.001. Increasing amounts of negative elevation, below the horizontal plane, led to the perception of point-light displays as facing away from the viewer. Most importantly, the resulting psychometric function is identical to those obtained with linear perspective methods that incidentally modify camera elevation (Troje, Weech, & Kenny, 2013). We argue that camera elevation, not linear perspective, produces the previously observed modifications of the facing-the-viewer bias of depth ambiguous point-light walkers.
Meeting abstract presented at VSS 2014
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