Fig 1.
Illustration of the geometrical rules of optics that express the height and width of an object as a proportion of the VEH.
A) Y is the object’s total height, E the distance between the bottom of the object and the horizon, and y and e are the respective heights expressed as visual angles. The height of the object (Y) can be expressed as a proportion of the observer’s VEH (E) by means of the VEH ratio defined as: Y/E = 1—tan(e-y) / tan(e) [5,11,13]. B) X is the object’s total width, E the distance between the bottom of the object and the horizon, and x and e are the same quantities expressed as visual angles. The width of the object (X) can be expressed as a proportion of the observer’s VEH (E), by means of a VEH ratio defined as: X/E = 2 tan(x/2) / tan(e) ≈ x / e [13].
Fig 2.
The measurement device consisted of a laptop mated with a tunnel and a diving mask. Participants looked at the visual scene displayed onto the screen. The visual scene was a picture of a room in which a doorway-like aperture was formed between two movable partitions. Perceptual adjustments were performed using a right-handed trackball.
Fig 3.
The visual scene was made from a photograph of a real room (220 cm horizontal X 260 cm vertical) in which a doorway-like aperture was formed between two movable panels. In the 130cm-VEH condition, the floor was raised about 25 cm. In this illustration, the superimposed solid white lines indicate the convergence lines from the lower middle of the photography toward the center of the screen. Note that the vanishing point remained at the center of the screen whatever the visual condition. The dotted line indicates the level of the eyes at the center of the screen. Neither the solid nor the dashed white lines were actually present in the visual scenes.
Fig 4.
Passability and subjective eye-height tasks.
In the passability task, participants were asked to adjust the opening of the doorway until it was perceived to be just wide enough to pass through without turning or scrunching the shoulders. The door could be adjusted by means of a hand-held trackball that controlled the lateral translation of the right and left black-painted movable panels to produce a doorway like aperture varying in width in an expanding or retracting manner. The measure obtained was the critical aperture (A, in cm) which is the minimum width of the aperture perceived by the subject to be “passable”. In the subjective eye-height task, participants adjusted a horizontal line to be at his or her perceived level of the eyes in the visual scene. The measure obtained was the so-called subjective eye-level, expressed as a deviation angle in degrees relative to the center of the screen, or reported as the subjective visual eye-height when computed in degrees relative to the visual floor.
Fig 5.
Subject posture and visual stimuli for the control experiments.
In the first control experiment subjects performed the critical aperture and subjective eye-height tasks either in an upright standing posture or while lying prone on a cushioned table. In the second control experiment, performed only in the prone position, the viewpoint into the virtual room was either held static or displaced dynamically to evoke the visual sensation of floating freely within the 3D visual environment.
Fig 6.
Mean of shoulder-width ratios as a function of gravity session and visual condition (plotted with ± SEM; N = 16). SEM = Standard Error of the Mean, VEH = Visual Eye-Height. (see S1 File for raw data).
Table 1.
Critical aperture and shoulder width ratio.
Table 2.
Subjective eye-level and subjective visual eye-height (sVEH).
Fig 7.
Subjective eye-level and subjective visual eye-height.
A) Mean of subjective eye-level computed in degrees relative to the center of the screen as function of visual condition and gravity session (plotted with ± SEM; N = 13). B) Mean of subjective visual eye-height computed in degrees relative to the floor as function of gravity session and visual condition (plotted with ± SEM; N = 13). SEM = Standard Error of the Mean, VEH = Visual eye-height, sVEH = Subjective visual eye-height relative to the floor and in degrees. (see S1 File for raw data).
Table 3.
Predicted and actual diminution of the critical aperture.
Fig 8.
Ratio of A versus sVEH for each condition.
Mean of A°/sVEH° ratio as function of gravity session and visual condition (plotted with ± SEM; N = 13). SEM = Standard Error of the Mean, A° = Critical aperture and sVEH° = Subjective visual eye-height relative to the floor, each expressed as a visual angle in degrees. (see S1 File for raw data).
Table 4.
Critical aperture and subjective visual eye-height of the control experiments.