The stimuli were random dot stereograms of vertically orientated elliptical cylinders. The cylinders were 10cm in height and had a fixed width of 6cm. The surfaces of the cylinders were rendered with red Gaussian blobs and were rendered taking into account individual interocular distances. Red was used as the shutter goggles have minimum crosstalk at longer wavelengths. The standard deviation of the Gaussian blobs was 1mm and scaled in accordance with the monitor calibration. The surface of the cylinder was rendered such that regions of its surface could be occluded from an eye depending on that eye’s viewpoint of the surface, as would occur naturally when viewing a real object (Gillam & Borsting, 1988). The standard deviations of the blobs in their
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y dimension scaled with the position of the blob on the cylinder’s surface. This reflected the projective foreshortening of the blob that would naturally occur given the view of the blob from each eye. Foreshortening was applied uniformly over the blob such that the blob was rendered tangent to the cylinder’s curved surface. The stimuli therefore contained consistent textural information about 3-D shape. Typically textural information in stereograms is consistent with a flat frontoparallel surface and therefore conflicts with that of the stereoscopically
rendered shape (Zabulis & Backus, 2004), this in itself could act to introduce bias in perceived 3-D shape.
Stimuli were presented at distances of 40, 60, 80 and 100cm centred on the median plane. In each case the monitor was positioned at a distance such that the plane of the calibration loom on which the stimuli were rendered was situated at the simulated object distance. The loom itself was only present during the screen calibration. Cylinders were also presented at two viewing heights depending on the experimental condition, these were eye height and 20cm above the centre of the cylinder i.e. 15cm above the top of the vertically orientated cylinder. There were four viewing conditions which are illustrated in Figure 4.2, these were as follows, (1) Lidded: cylinders were viewed from above and a view of the cylinder’s upper surface/lid was provided, this was rendered in the same way as the rest of the cylinder’s surface (Figure 4.2a), (2) Contoured: cylinders were viewed from above but the cylinder’s upper surface/lid was not rendered (Figure 4.2a), (3) Rotated: cylinders were viewed from above but were rotated around their vertical axis so that the surface of the cylinder was frontoparallel to the observer (Figure 4.2b) and (4) ACC: cylinders were presented at eye height i.e. in a standard apparently circular cylinder (ACC) configuration (Figure 4.2c).
In the ACC condition vergence provides the predominant source of distance information for scaling disparity in the estimation of 3-D shape, this is the standard stimulus configuration in which biases in perceived disparity-defined shape have been demonstrated (e.g. Johnston, 1991). In the Rotated condition vertical gaze angle provides additional information about object distance that could be used to scale disparity, but the region of the cylinder’s surface in view is the same as that of the ACC condition. In the contoured condition, in addition to vertical gaze angle, a view of the elliptical contour of the top edge of the cylinder provides information about 3- D shape. Finally, in the lidded condition the upper surface or ‘lid’ of the cylinder is also rendered. In addition to the information provided in the rotated condition, this condition provides a salient disparity discontinuity (between the cylinders body and lid) and disparity information about the slant of the cylinder’s upper surface. both could be used to better estimate 3-D shape.
Figure 4.2: Diagram showing the experimental setup (left column) and a schematic representation of the stimuli (right column) for each of the four viewing conditions. (a) For the ‘lidded’ and ‘contoured’ conditions the stimuli were viewed from above. In the ‘lidded’ condition the cylinders top/lid was rendered whereas in the ‘contoured’ condition the top/lid was absent. (b) In the ‘rotated’ condition, stimuli were again viewed from above, but the stimuli were rotated around the vertical axis such that they appeared frontoparallel to the observer at each viewing distance. (c) In the ACC condition stimuli were viewed at eye level, this is the standard configuration for this type of task. In all conditions stimuli were viewed along the median plane.
During the ACC condition the response keyboard was positioned on the table surface to which the monitor platform was attached, this is the standard experimental viewing situation. Throughout all viewing conditions the table surface and monitor platform
were covered with matt black cloth. In the lidded, contoured and rotated conditions the keyboard was positioned on a matt black raised table surface in front of the observer (Figure 4.2). The height of the raised surface was matched to that of the bottom of the rendered 3-D stimuli. The raised surface fitted around the bite bar mount and consisted of three sections in a ‘C’ shaped configuration with its open side facing the monitor. This configuration ensured that the observer’s view of the stimuli on the monitor screen was not occluded by the table surface at any viewing distance. The raised table surface served to provide haptic information consistent with the computer rendered stimuli standing on a real support surface, information about the height of an observer’s eye(s) above the support surface is required to estimate distance from vertical gaze angle (Cutting & Vishton, 1995). Observer VD completed two versions of the ‘Contoured’ condition; one with the table surface present and one when that table surface was absent. This served as a check as to the role of appropriate haptic feedback about the support surface for the estimation of distance from height in the field. The stimuli were viewed in a dark room with no other sources of illumination and periodic breaks were taken to minimise dark adaptation. Observers were aware that they were viewing computer generated stimuli and that the raised table surface did not extend all the away to the monitor.