In all cases the perceived stereoscopic form is that expected from the disparity and it is vivid and stable, while the residual monocular form is vague and fuzzy. Similarly figures Id and le give the concave mould, figures le and If the pseudoscopic mould. Fusion of figures la and lb presents the face with normal depth, while figures lb and lc present the face pseudoscopically (left and right images interchanged). A concave plaster mould of this head is portrayed in figures Id-If. The object in figures l a - l c was the head of a tailor's dummy, painted white and mounted flush in a white surround. Examples of stereograms made in the above way are shown in figure 1. Cyclopean presentation of familiar objects provides a test of this issue. It is frequently argued that binocular stereopsis fails to reverse apparent depth in pseudoscopic vision of this kind because of the familiarity of the object depicted.Īlternatively it may be that irrespective of familiarity the so-called pictorial depth cues (perspective, shading, interposition, etc) can dominate stereopsis. in the mould made from a solid model of a face) present a visual conflict between different sources of depth information. 2 A test case: perceptual reversibility of faces and moulds of faces Reversal of disparity by interchanging the left and right halves of a stereogram, and reversal of actual depth (e.g. Stereograms can obviously be made by taking photographs at two places near the projection point, a few inches apart. The requirements for cyclopean stimulation are thus fairly well satisfied by this arrangement. However, because the dots really are at different distances from the observer, they afford the normal pattern of binocular disparities and the object is perceived stereoscopically. The strong masking effect of the high-contrast texture helps in this respect. Ideally, then, there are no variations in texture density, and, apart from effects of light scatter, surface orientation, etc, very little monocular form is visible. Hence all dots in the texture subtend the same visual angle, independent of projection distance. Dots on the further surface are projected larger in size, but they are also further from the eye, by the same proportion. To see why this is so, consider two surfaces at different distances from the projector. The result is essentially a binocular, cyclopean stimulus. The essence of the idea is this: project a random texture on to a white object which stands on a white background, and view the scene from close to the projection point. The technique is not perfect, but it is quick and easy and may be useful to the student or researcher in many applications. The purpose of this note is to describe a very simple method of producing randomdot stereograms of complex objects without a computer. He concludes: "Of course, stereograms of complex surfaces cannot be manufactured by this simple method they necessitate the use of computers". They are quick to make, but imperfect and limited to simple depth planes. Julesz (1971, p 288) describes possible cheaper methods, such as 'sandpaper stereograms'. Cyclopean research, however, requires access to expensive computing and graphics facilities, as well as considerable investment of time and effort. The random-dot stereogram allows an observer to perceive surfaces and forms in depth, even though each monocular view appears to be a completely random texture. 1 The principle of the technique The power and usefulness of 'cyclopean' stimulation in perceptual research are now well established (Julesz 1971). Further possible applications of the technique are outlined. It is then shown that stereograms of inside-out faces (moulds) cease to be perceptually reversible only when the presentation is truly cyclopean. A simple technique for producing random-dot stereograms of real objects is described. Mark A Georgeson Department of Psychology, 8-10 Berkeley Square, Bristol BS8 1 HH, England Received 12 April 1979Ībstract. Random-dot stereograms of real objects: observations on stereo faces and moulds
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