Stereoscopy

Physiology of the stereoscopic sight

The stereoscopic vision is due, mainly, with the binocular Neurons located in the cerebral Cortex at the level of the specific and primary education zones of the treatment of the vision (V1 Surface). These binocular neurons are the only neurons to receive the Nerve impulse of two homonymous neurons of the Rétine via the neuronal chain.

The images Left and Right of the same object, collected at the same time by both Retina S, are conveyed with the visual Cortex by the optical nerves which cross in the " Chiasma ", with the result that they are present in cells close to the visual cortex. David Hubel, Nobel Prize of medicine, as points out as the Cellule S of the parts left and right of the visual cortex also communicate by the callous Corps, which contributes to the mechanism of binocular fusion.

This double information allows by complex mechanisms utilizing other zones of the Cerveau the perception of the angle between the visual information collected by photoreceivers of an eye and its counterparts of the other eye making it possible to perceive the Relief S and the Distance.

So the people suffering from Strabism (bad adaptation of the zones of retinal photoreceivers homologous) or the people suffering from anopsy on an eye have a very faded vision of the relief and depth.

The stereoscopic Vision is normally very precise (one usually appreciates an in-depth variation of ten centimetres at a distance of ten meters), of precision inversely proportional to the distance, and limited in Amplitude: one sees with difficulty in relief a very brought closer object or a very distant object.

The stereoscopic vision is disturbed, if not prevented, by various defects of the images: vertical shifts, contradictions of the relief, excess of Parallax, " images fantômes" or attenuated vision of the right sight by the left eye and reciprocally, dissymmetry of the Luminosity…

Why look in relief?

The observation in relief is natural, it is the image “punt” which asks for an effort of interpretation, because it is only a Projection Perspective of the Espace in a Plan. The stereoscopic image restores each plan in its place in space, dimensions and the shape of each object, provided there is no effect of Maquette nor of Distorsion in-depth.

The too complex images, too tangled up, are much more readable if one observes them in relief: for example a cavity in an object seems hollow indeed; contrary to the asperities do not seem such in photo “punt”. The presentation in relief gets a feeling of presence which does not exist differently, because the spectator sees the scene as if it were there, especially in Projection in a rather obscure room so that only the photo in relief is visible.

One can also see in relief what one does not see naturally as follows: Mountain S or formations Nuage use are too distant so that one perceives the successive plans of them, but if they are photographed in relief since two drawn aside enough points, one sees them, certainly reduced size, but with all their relief and their real forms; microcrystals, Insect S or Fleur S, too small to be observed with the naked eye, taken from two brought closer enough points of view, appear very increased with their real Forme S and them Depth.

How to carry out stereoscopic images?

By images, one understands here: Photo S, Drawing S, Synthesized images or images carried out by any other means. It is a question of taking two sights of the same scene, starting from two side by side points of view of.

Three methods in photography

In two times

One can operate in two times, by moving the apparatus: in this case, it is essential that nothing moved between the two catches of sights, for example characters, animals, vehicles, sheets of the trees, waves, clouds or their shades under the effect of the wind; it is necessary also that the two catches of sights aim in the same direction, that the apparatus does not have advanced, was not moved in height nor did not rotate.

By simultaneous release of two apparatuses

One can also start two identical apparatuses at the same time: it is necessary for that apparatuses are rigidly fixed and well aligned, and that release is quite simultaneous, which imposes constraints, with apparatuses with film and especially with digital devices. Several Entreprise S Artisan ales proposes couplings of two digital devices, fixed well between them and synchronized well.

With a stereoscopic apparatus

Particular applications

There still exists of other methods, each one adapted to particular cases, in particular for the catches of sights in “macrostereoscopy”, i.e. with narrow a “base” adapted to photograph very small objects: double Diaphragm with Mirror S, dividers of images (attention to the “Deformation S in Trapezoid”), Filter S of Color S side by side in front of the Objective , mirrors semi-transparent, etc

Stereoscopic images can easily be extracted from all model kinds digital three-dimensional professionals, by choosing two isolated points of view for example of one to two tenth of the distance from the foreground, if it is a question of presenting them on a screen of computer, but not more one thirtieth if it is a question of presenting them in projection to a broad audience.

How to observe in relief of the images carried out to this end?

These images can be observed in Diapositive S, drawn on Papier, the screen of Ordinateur, etc, by various methods:

Stereoscope

with an optical instrument called Stereoscope. There exists much of varieties of stereoscopes: to two Lens S, two Mirror S, only one mirror, semi-transparent Mirror, etc

Anaglyph

by separating the sights left and right-hand side by the Color S (Anaglyph S): admittedly this process degrades the colors and excludes the presence from objects to the bright colors too close to those to the coloured Filtre S, but it is the only known manner to print images of large size easily visible in relief by not trained people with the free vision.

Images with networks

Images imbricated in vertical bands: lined” or “lenticular”, visible networks “in relief without any instrument (proceeded car-stereoscopic).

Projection in relief

Projection in polarized light, by far the easiest, effective and spectacular method, which certainly request of the means to the organizers of these projections, but which force the Spectateur S only to carry light, inexpensive and not very constraining glasses, and not deteriorating the colors at all.

Other methods

real Images projected in space, Photostéréosynthèse, Holography, etc

Free vision

In “free vision”, parallel or crossed, for those which can it, either spontaneously, or after ocular exercises. For the majority, this free Vision is not possible that at an important distance, from where the image appears somewhat stretched.

The cross vision is most accessible: the left eye looks at the straight lines figure and the right eye looks at the left figure. To help oneself, one can close his left eye initially then to place his right Main at a few centimetres of his right eye, in order to hide the right figure to him. Then the right eye is closed and one places the left hand at a few centimetres of his left eye in order to hide the left figure to him. One then opens the two eyes, each one of them seeing only one figure. To have a cast some share in the interval enters the two hands to make superimpose the two figures in a single figure.

With a little drive, the brain finishes at the end of a few seconds by adapting the vision on a clear figure in relief, as in the example below: stereoscopic

Animation in vision croisée

Shelves to be avoided, sources of eyestrain

Seen left and right-hand side too disparate (ex: colors too sharp in anaglyphs).

vertical Shifts, in particular by misalignment at the time of the catch of sights, rotation or deformations in trapezoid due for example to the use of “dividers of images” or excess of Convergence of the optical axes.

Defects of Synchronism, “temporal parallax” by which an object which moved towards the side is seen more at its correct distance.

Excess of Depth of relief: it is advisable not to exceed a typical limit of extreme variation of parallax which is in general estimated with thirtieth of the distance from observation.

Déformations of the image in growth or stretching, especially if these deformations are variable from one object to another.

Contradictions of the relief, in particular because of a bad placement of the “window”.

These shelves can be avoided, in particular by a work known as of “assembly”, as well in the cases of slides, as of pullings on paper or digital photographs.

Here an example of old stereoscopic image:

Organizations

The stereoscopy interests of the thousands of amateurs throughout the world, whose majority gather in associations, in particular the International union of Stéréoscopie and the clubs national or buildings which are affiliated for him (in France: the French Stereophony-Club). These associations also count among their members of many professionals.

The stereoscopy also interests the professionals, in varied fields (Géographie, Biologie, Chimie, Architecture, Medical imagery, “CAD”, etc). There does not exist organization which gathers the stereoscopists of all these professions. On the other hand, the essence of the knowledge accumulated by these professionals is found in the reports of annual conventions “Stereoscopic Displays and Applications”.

Notes, references

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