Anaglyph images are used to provide a stereoscopic 3D effect, when viewed with 2 color glasses (each lens a chromatically opposite color, usually red and cyan). Images are made up of two color layers, superimposed, but offset with respect to each other to produce a depth effect. Usually the main subject is in the center, while the foreground and background are shifted laterally in opposite directions. The picture contains two differently filtered colored images, one for each eye. When viewed through the "color coded" "anaglyph glasses", they reveal an integrated stereoscopic image. The visual cortex of the brain fuses this into perception of a three dimensional scene or composition.
Anaglyph images have seen a recent resurgence due to the presentation of images and video on the Internet, Blu-ray HD disks, CDs, and even in print. Low cost paper frames or plastic-framed glasses hold accurate color filters that typically, after 2002, make use of all 3 primary colors. The current norm is red and cyan, with red being used for the left channel. The cheaper filter material used in the monochromatic past, dictated red and blue for convenience and cost. There is a material improvement of full color images, with the cyan filter, especially for accurate skin tones.
Video games, theatrical films, and DVDs can be shown in the anaglyph 3D process. Practical images, for science or design, where depth perception is useful, include the presentation of full scale and microscopic stereographic images. Examples from NASA include Mars Rover imaging, and the solar investigation, called STEREO, which uses two orbital vehicles to obtain the 3D images of the sun. Other applications include geological illustrations by the USGS, and various online museum objects. A recent application is for stereo imaging of the heart using 3D ultra-sound with plastic red/cyan glasses.
Anaglyph images are much easier to view than either parallel (diverging) or crossed-view pairs stereograms. However, these side-by-side types offer bright and accurate color rendering, not easily achieved with anaglyphs. Recently, cross-view prismatic glasses with adjustable masking have appeared, that offer a wider image on the new HD video and computer monitors.
History
The first method to produce anaglyph images was developed 1853 by Wilhelm Rollmann, a German, in Leipzig.
Producing anaglyph images
In historical methods using camera filters, on film, two images from the perspective of the left and right eyes were projected or printed together as a single image, one side through a red filter and the other side through a contrasting color such as blue or green or mixed cyan. As outlined below, one may now, typically, use an image processing computer program to simulate the effect of using color filters, using as a source image a pair of either color or monochrome images. Since the 1970s Stephen Gibson offers his patented "Deep Vision" system using cyan instead of green filters. These colors cover the entire visible spectrum, providing more natural color. "Deep Vision"-spectacles have the red filter over the right (!) eye and the cyan over the left. (To bypass Gibson's patent some companies produce red/cyan-glasses with the colors on the wrong sides.)
Popular professional programs such as Adobe Photoshop provide the basic digital tools for processing of anaglyphs. They do not provide instruction for anaglyph in their basic documentation. Various websites offer free instruction related to 3D for Photoshop. Simple, low cost programs, dedicated to anaglyph creation, are also available. In recent simple practice, the left eye image is filtered to remove blue & green. The right eye image is filtered to remove red. The two images are usually positioned in the compositing phase in close overlay registration (of the main subject). In Photoshop, for example, a function called "screen" in the "layers" (F7) option allows the two filtered layers to be combined transparently on top of each other, although the compositing can also be done by pasting the selected channels. The filtration itself can easily be done in the "curves" function, which allows removal of any red, green, or blue layer with a simple slider. Experienced Photoshop users can sometime process a good cross-view 3D pair into a color anaglyph in 2 to 5 minutes if little fine-tuning is required. Various steps can also be used to maximize the quality of the result. These may include size matching of the frames to within a few pixels, rotation (if needed), gamma leveling, and digital sharpening of the softer red image. It should be noted that both the red layer in the image and the red filter in the glasses contain no visible blue or green color information, but masked colors do exist that the eyes cannot see through a red filter that can interact with the colors contained in the cyan image after compositing and blending.
How anaglyph works
Viewing anaglyphs through appropriately colored glasses results in each eye seeing a slightly different picture. In a red-blue anaglyph, for instance, the eye covered by the red filter sees the red parts of the image as "white", and the blue parts as "black" (with the brain providing some adaption for color); the eye covered by the blue filter perceives the opposite effect. True white or true black areas are perceived the same by each eye. The brain blends together the image it receives from each eye, and interprets the differences as being the result of different distances. This creates a normal stereograph image without requiring the viewer to cross his or her eyes.
Viewing
A pair of eyeglasses with two filters of the same colors, once used on the cameras (or now simulated by image processing software manipulations) is worn by the viewer. In the case above, the red lens over the left eye allows only the red part of the anaglyph image through to that eye, while the cyan (blue/green) lens over the right eye allows only the blue and green parts of the image through to that eye. Portions of the image that are red will appear dark through the cyan filter, while portions of colors composed only of green and blue will appear dark through the red filter. Each eye therefore sees only the perspective it is supposed to see.
Red sharpened anaglyph glasses
Simple paper, uncorrected gel glasses, cannot compensate for the 250 nanometer difference in the wave lengths of the red-cyan filters. With simple glasses, the red filtered image is somewhat blurry, when viewing a close computer screen or printed image, The (RED) retinal focus differs from the image through the (CYAN) filter, which dominates the eyes' focusing. Better quality, molded acrylic glasses frequently employ a compensating differential diopter power to balance the red filter focus shift relative to the cyan, which reduces the innate softness, and diffraction of red filtered light. Low power reading glasses worn along with the paper glasses, also sharpen the image noticeably.
The correction is only about 1/2 + diopter on the red lens. However, some people with corrective glasses are bothered by difference in lens diopters, as one image is a slightly larger magnification than the other. Though endorsed by many 3D websites, the diopter "fix" effect is still somewhat controversial. Some, especially the nearsighted, find it uncomfortable. There is about a 400% improvement in acuity with a molded diopter filter, and a noticeable improvement of contrast and blackness. The American Amblyopia Foundation uses this feature in their plastic glasses for school screening of children's vision, judging the greater clarity as a significant plus factor.
Anachrome filters
Plastic glasses, developed in recent years, provide both the diopter "fix" noted above, and a change in the cyan filter. The formula provides intentional "leakage" of a minimal (2%) percentage of red light with the conventional range of the filter. This assigns two-eyed "redness cues" to objects and details, such as lip color and red clothing, that are fused in the brain. Care must be taken, however, to closely overlay the red areas into near-perfect registration, or "ghosting" can occur. Anachrome formula lenses work well with black and white, but can provide excellent results when the glasses are used with conforming, "anachrome friendly" images. The US Geological Survey, has thousands of these "conforming", full-color images, which depicts the geology and scenic features of the U.S. Nat
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