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Rui Ni, Myron L. Braunstein, George J. Andersen; Interaction of optical contact, shadows and motion in determining perceived scene layout. Journal of Vision 2004;4(8):615. doi: 10.1167/4.8.615.
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Ni et al. (VSS 2003) reported that the perceived distance of a floating object in a scene was influenced by the position of an object on the ground, especially if the two objects were undergoing a common horizontal translation. Earlier, Kersten et al. (1997) found that the perceived position of an object was determined by the motion of its shadow. In the present study we compared the effectiveness of a shadow to that of a second object on the ground in determining the perceived position of a floating object. The displays were motion pictures of a real scene into which we inserted two cylinders, one on the ground and one above the ground. Observers adjusted the location of a marker on the ground to match the perceived distance of the top cylinder. In the first experiment the lower cylinder was either textured or black and had one of four levels of thickness: 0, 1/3, 2/3 or 1 times the thickness of the top cylinder. The black cylinder with 0 thickness had the appearance of a shadow. Judged position of the top cylinder did not vary with thickness for the textured cylinders, but moved closer to the position of the lower cylinder when it was black and had zero thickness. In the second experiment we varied the height of the top cylinder. The lower object was either a textured cylinder of the same thickness as the top cylinder or a shadow. The height of the top cylinder affected judged distance with two textured cylinders but not with one cylinder and a shadow. In the third experiment we presented three alternative shadows, which could match the top cylinder in motion, in size, or in both. Distance judgments for the top cylinder indicated that observers relied mainly on common motion to match the shadow with the object, but that size also affected the judgments. These results show some similarities between the effect of a second object and that of a shadow in determining the perceived position of a floating object in a 3D scene but also show important differences.
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