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Ryne Choi, Jacob Feldman, Manish Singh; Perceptual biases in the interpretation of non-rigid structure from motion. Journal of Vision 2019;19(10):198. doi: https://doi.org/10.1167/19.10.198.
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Structure-from-motion research has largely focused on the perception of rigidly moving 3D objects. However, many of the objects we see in the environment are animate and move non-rigidly. Models of SFM based on rigidity cannot interpret articulated non-rigid motion. Recently it has been shown that subjects are just as good at perceiving the 3D structure of objects undergoing certain forms of non-rigid transformations (Jain & Zaidi, 2011). Our overarching goal is to investigate the kinds of non-rigid object transformations that are perceivable through SFM. In this study, we focus on two types of non-rigid motion that we believe to be categorically different: length change and orientation change of a protruding part. Orientation change is a part articulation that is more biologically plausible than length change. Our stimuli consisted of an ellipsoid with a protruding narrower part. The object was shown solely by a random dot texture. The whole object rotated back and forth about its vertical axis. At the same time, the protruding part continuously changed its length or orientation with respect to the base part. We found that when the part changed in length, subjects often perceived the transformation as an orientation change in 3D. We manipulated the magnitude of length change and used the method of constant stimuli and adjustment to measure subjects’ perception of the length transformation. We found that even when the length of the part changed dramatically, subjects perceived an orientation change rather than a length change. The results demonstrate that certain non-rigid transformations are easier to perceive correctly through SFM than others. The visual representation of shape may include certain “non-rigid” transformations that are more “natural,” such as the articulation of limbs (part-wise rigidity) on biological shapes. The visual system may be biased towards these more biomechanically plausible interpretations of motion.
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