Anaglyph

History

The principle is described in 1853 by Rollman. Charles d' Almeida makes known this mode of restitution of the relief to the Academy of Science of Paris. Louis Ducos Of Hauron improves it, finishes developing it in 1891 (it gives him also this name). The anaglyphs are then the object of an important production intended for tourism, teaching, the leisures, publicity… Louis Lumière adapts the process to the cinema in 1936, but the process remains marginal there.

Constitution of an anaglyph

An anaglyph consists of two superimposed images (called counterparts) of complementary colors representing the same scene but seen slightly shifted points: generally left sight in red and right sight in cyan. These homologous images are thus not identical: the shift (called Parallax or disparity) is not the same one for all the elements of the image; it all the more large, in a direction or the other, that the elements are located far from the plan of the physical image, for example printed, projected on a screen or is posted on the screen of a computer or a television set.

Principle of restitution of the relief

The restitution of the Relief is given while placing a filter of the one of these two complementary colors on an eye and a filter of the other color on the other eye: thus, each eye perceives only the visible elements of the image through the corresponding filter.

If two elements of left and right image are seen sufficiently close, with an only horizontal shift, the brain will interpret them like representative the same object. This object will be seen more or less far in front of or behind the plan on which the image is physically formed, according to the value and the direction of the shift, called parallax, between the elements left and right.

Thus, if the shift is different for various elements of the image, that which looks at the image through filters will have the impression to see an image whose points are located in all space. One sees this stereoscopic image then.

Colors of the homologous images

Several combinations of complementary colors were tested: formerly especially magenta and green, more recently red and cyan or yellow and blue, currently most widespread.

The filters red and cyan are well adapted to the representation of the colors yellow, ochers, green if it is rather pale (the green grass is rather well returned), mauves, crimsons, blue, all the nuances of gray and brown. An underwood in autumn is returned perfectly. It happens that the image is overall clearer through one of the filters, in this case he is better to correct the image, which is easy with the principal software of images.

Anaglyphs made up of drawings on white or clear zone give opposite results of the same drawings on dark bottom: it is then necessary to turn over the glasses for good to see them.

A remarkable improvement of the technique of the anaglyphs is the Infitec process, based on interferential filters: each filter lets pass three bands wavelengths, the left filter and the right filter letting pass from the different colors all. One can then show “anaglyphs” comprising all the colors, without phantom images.

Principle of manufacture

An anaglyph is carried out starting from two sights respectively right and left. The basic principle is to extract - in the case from anaglyph red-cyan the red component from the image left and the components green and blue (cyan = green + blue) from the right image then to recombine these three components in a single image.

The majority of the software of image processing make it possible to separate, then to recombine the components of color of the images: by combining the components red of the sight of left, and green and blue from the sight of right-hand side, one obtains a suitable anaglyph, in which the colors yellow, ochers, brown, mauve, blue, green pale are well restored by filters red and cyan. It is here about the basic process, there exist many other more elaborate processes and which make it possible to correct the problems inherent in the anaglyphs (see the Limites paragraph) and thus to obtain better anaglyphs.

Manufacture of the glasses to see the anaglyphs: paperboard, scissors, adhesive and especially filters:

Kodak Wratten N° 26 or 29 (45 €) for the red and Kodak Wratten N° 44 (75 €) for the cyan.

One finds easily in the trade by Internet of the glasses for anaglyphs at low prices, especially in great quantities, and of very suitable quality.

To obtain only one eyeglass anaglyphic, sufficient for seeing well in relief the galleries of images diffused on Internet, one can address to the French Stereophony-Club.

Limits

The points of colors which must be visualized as only one point with final should not be too distant (if possible not more than one tenth of the width of the figure), to prevent that the brain does not separate the images, this thus limits the effect of relief which one can give to an image.

However, an image anaglyph can be projected on a white wall: starting from a width of image of 1,20 m, one can ad infinitum restore a space image of two meters, which is not possible on the screen of a computer.

Moreover, the distance between the observer and the plan of the image anaglyph are a parameter which exploits in an important way the impression of relief.

Retinal competition

A problem can be noted if an element in the original scene has a color which approaches that of one of the filters: this element will be seen like very clearly by the bearing this filter and very dark eye by the other eye. The most frequent cases are for example: a quite blue sky, a car bright red. That will cause an imbalance between the two eyes which will return the observation of the unpleasant anaglyph (the element in question seems to flicker). This problem is often called “retinal competition” and can be solved or attenuated by a filtering of the colors (the die-saturation of the colors is the most common operation and one will speak then about anaglyphs “half-colors”). The solution can be also to use other colors for glasses of the glasses: the glasses yellow-blue will make it possible to reproduce more accurately the colors red and flesh but these glasses are less widespread than the traditional glasses red-cyan.

Phantoms

Another problem which rises him from “the universality” of the system: the anaglyphs can be projected, printed or visualized on a screen of computer or television. However all these devices do not restore the colors in the same way and, on their side, the glasses all are not identical - some will be more or less dark and are not “perfect”. The consequence is that filtering is not perfect, the separation of the two images is not total and each eye will see a little the image intended for the other eye. One then sees appearing parasitic images known as “phantom images” which will decrease the comfort of vision and thus the impact of the image. If it is difficult to obtain that everyone has a screen well regulated and perfect glasses it is better to avoid in the anaglyphs of too important contrasts which will cause phantoms. An image processing on computer (reduction in contrast) can there too attenuate the problem.

Compression of the images

The algorithm of compression of the digital images most widespread as the JPEG were designed to compress standard photographs and always does not give satisfaction to treat images anaglyphs.

To increase the compression ratio, these algorithms separately process the data “luminosity” and information “color” (or chrominance) of the image to be compressed. To simplify, one can say that the compression of information “color” is not carried out pixel by pixel but per block of 2 or 4 pixels which are then regarded as having the same color. One speaks about “under-sampling of the chrominance”. However the anaglyphs rest on a precise coding of the colors for restoring the relief of the scene well represented. The under-sampling of the chrominance will make that the two images left and right-hand side will dribble one on the other causing of the unpleasant phantom images having the shape of silver plated nets.

Fortunately of many software allow to record images JPEG by decontaminating the under-sampling of the chrominance, the quality of the stereo image is some largely improved (at the price of a slightly higher size of file).

Applications

The anaglyphs were used, as of the XIXe century, to project stereoscopic images, while allotting to each spectator of the coloured filter glasses and while placing in front of each projector the corresponding filter. These projections related at the beginning on still images, then to cinema at the beginning of the XXe century.

Later, the anaglyphs were used in television, with little success because of the interferences between the signals of luminosity and those of color, which had with the standards of transmission of television.

The anaglyphs are often used in the press, in particular in scientific publications to present figures, for example of anatomy, in relief. Two-tone glasses are then united with the publication.

The anaglyphs are currently sails very about it on the photograph galleries in relief on Internet, because it is the only practical means to show in full screen of the stereoscopic images on computer, without special installation.

See too

Stereogram
Stéréoscopie

External bonds

http://marsrovers.nasa.gov/gallery/3d/ Galerie of anaglyphs of the Planet Mars makes during the mission Mars Exploration Rover organized by NASA.
Anaglyphs on Flickr
Article in connection with the integration of an effect of relief anaglyph in panoramic sequences QuickTime VR

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