In six experiments we demonstrate that the vertical-horizontal illusion that is evoked when viewing photographs and line drawings is relatively small, whereas the magnitude of this illusion when large objects are viewed is at least twice as great. Furthermore, we show that the illusion is due more to vertical overestimation than horizontal underestimation. The lack of a difference in vertical overestimation between pictures and line drawings suggests that vertical overestimation in pictures depends solely on the perceived physical size of the projection on the picture surface, rather than on what is apparent about an object's represented size. The vertical-horizontal illusion is influenced by perceived physical size. It is greater when viewing large objects than small pictures of these same objects, even when visual angles are equated.

Viewed from the center of projection, a perspective picture presents the pictorial depth information of a scene. Knowing the center of projection, one can reconstruct the depicted scene. Assuming another viewpoint is the center of projection will cause one to reconstruct a transformed scene. Despite these transformations, we appreciate pictures from other viewpoints. The compensation hypothesis states that the visible picture surface allows observers to compensate for transformations by locating the center of projection and experiencing pictorial space from there. Weshow that observers neither completely compensate for nor experience transformations of space as geometry would predict. We propose a modified compensation hypothesis according to which different degrees of visibility of the picture surface invoke different degrees of compensation.Pictures are flat surfaces that show scenes in depth. For the visual system, pictures present the problem ofintegrating conflicting flatness and depth information. Practically, understanding how picture perception resembles or differs from perceiving real space will allow us to create and use spatial displays more effectively. In this paper, we address two problems: (1) how we perceive pictures from different viewpoints even though pictures are geometrically correct for only one viewpoint and (2) how depth information and flatness information interact in perceiving depth in pictures.A picture mimics the light from a scene to one viewpoint, called the center ofprojection. To all other viewpoints, the picture presents a geometrically transformed pictorial space. However, experience suggests that we can appreciate pictures from many viewpoints. Kubovy (1986) has called this phenomenon the robustness ofperspective. Some researchers (e.g., Goldstein, 1987;Kubovy, 1986;Pirenne, 1970; have proposed that robustness results because seeing the picture surface allows observers to compensate for these transformations ofpictorial space. That is, observers perceive the layout of pictorial space as if they were viewing the This research was supported by Grant MH 47317 to M.K., principal investigator. The authors thank Jake Mazulewicz, Anu Kesari, and Albert Downs for their able help in conducting the experiment, and Ron Simmons for building the apparatus. The authors also thank Marco Bertamini for discussions on the nature of explanations and descriptions, Hal Sedgwick for his thoughts on cross-talk, the visual world, and the visual field, Denny Proffitt for his thoughts on "compellingness" in pictures, movies at SIGGRAPH, and registered variables, and Marco Bertamini and Mukul Bhalla for their suggestions, which greatly improved this paper. Correspondence should be addressed to T. Yang or M. Kubovy, Department of Psychology, Gilmer Hall, University ofVirginia, Charlottesville, VA 22903-2477 (e-mail: yang@virginia.edu or kubovy@virginia.edu).picture from the center of projection. We call this explanation the compensation theory ofperspective robustness.In this paper, we present two versio...

Relative size judgments were collected for two objects at 30.5 m and 23.8 m from the observer in order to assess how performance depends on the relationship between the size of the objects and the eye level of the observer. In three experiments in an indoor hallway and in one experiment outdoors, accuracy was higher for objects in the neighborhood of eye level. Weconsider these results in the light of two hypotheses. One proposes that observers localize the horizon as a reference for judging relative size, and the other proposes that observers perceive the general neighborhood of the horizon and then employ a height-in-visual-fieldheuristic. The finding that relative size judgments are best around the horizon implies that information that is independent of distance perception is used in perceiving size.Size constancy and size perception at a distance are old problems with theoretical implications for the nature of the perceptual process. Is perception ofsize derived inferentially from information about distance? Does perception of size rely on the general characteristics of the terrestrial environment in which we evolved? The current studies focus on the use ofone source ofinformation about the relative size of objects at a distance: the horizon line.In this paper, we first consider the geometry ofhorizon information about size and discuss the various ways horizon information could be used for size perception. We then present a set ofstudies that examine how horizon information influences relative size perception. Our methodology avoids the complications that come with involving changes in perceived distance. We propose that, if the horizon is used as a reference for perceiving relative size, observers should better discriminate small height differences near this reference.Before we can assess the role of horizon information about size, we need to consider the different ways in which an observer can use the horizon. For a standing observer viewing objects on the same ground plane, the explicit or implicit horizon line intersects objects at the observer's eye height, thereby specifying the absolute size ofthe object as a multiple of the observer's eye height (Sedgwick, 1973). This size information is independent of the distance of the object from the observer. Sedgwick (1973) was the first to observe that absolute size information is This research was supported by NIMH Grant MH52640-02 and NASA Grant NCC 2-925. We thank Eric Pranzarone and Stein Kristensen for assistance in data collection and Geoffrey Bingham, Ulric Neisser, and H. A. Sedgwick for comments on an earlier version of this article. Address correspondence to M. Bertamini, Staffordshire University, Psychology Division, College Road, Stoke-on-Trent ST4 2DE, England (e-mail: m.bertamini@staffs.ac.uk) or to T. L. Yang, Gilmer Hall, Department of Psychology, University of Virginia, Charlottesville, VA 22903 (e-mail: yang@virginia.edu). available in units ofthe standing observer's eye height, as is shown in the following formula:where H is object height as...

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