3-D Film

The Three Dimensional Cinema the term 3-D (or 3D) is used to describe any visual presentation system that attempts to maintain or recreate moving images of the third dimension, the illusion of depth as seen by the viewer. The technique usually involves filming two images simultaneously, with two cameras positioned side by side, generally facing each other and filming at a 90 degree angle via mirrors, in perfect synchronization and with identical technical characteristics. When viewed in such a way that each eye sees its photographed counterpart, the viewer's visual cortex will interpret the pair of images as a single three-dimensional image. Modern computer technology also allows for the production of 3D films without dual cameras.

Anaglyph images
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, which typically, after 2002 make use of all 3 primary colors. The current norm is red for one channel (usually the left) and a combination of both blue and green in the other filter. That equal combination is called cyan in technical circles, or blue-green. 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 costly 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. With the techniques outlined below it is possible to convert stereo pairs from any source into anaglyph images.

Polarized 3D glasses
Polarized 3D glasses create the illusion of three-dimensional images by restricting the light that reaches each eye, an example of stereoscopy. To present a stereoscopic motion picture, two images are projected superimposed onto the same screen through orthogonal polarizing filters. The viewer wears low-cost eyeglasses which also contain a pair of orthogonal polarizing filters. As each filter only passes light which is similarly polarized and blocks the orthogonally polarized light, each eye only sees one of the images, and the effect is achieved.

The difficulty arises because light reflected from a motion picture screen tends to lose a bit of its polarization. However, this problem is eliminated if a "silver" or "aluminized" screen is used. This means that a pair of aligned DLP projectors, some polarizing filters, a silver screen, and a computer with a dual-head graphics card can be used to form a relatively low-cost system for displaying stereoscopic 3d data simultaneously to tens of people wearing polarized glasses. Such a system, called a GeoWall, has been used for several years now in the Earth Sciences thanks to the GeoWall Consortium, with several open source and commercial packages available.

When stereo images are to be presented to a single user, it is practical to construct an image combiner, using partially silvered mirrors and two image screens at right angles to one another. One image is seen directly through the angled mirror whilst the other is seen as a reflection. Polarized filters are attached to the image screens and appropriately angled filters are worn as glasses. A similar technique uses a single screen with an inverted upper image, viewed in a horizontal partial reflector, with an upright image presented below the reflector, again with appropriate polarizers. Polarizing techniques are most simply used with cathode ray technology, as polarizers are used within ordinary LCD screens for control of pixel presentation - this can interfere with these techniques. In 2003 Keigo Iizuka discovered an inexpensive implementation of this principle on laptop computer displays using cellophane sheets.

Polarized stereoscopic pictures have been around since 1936, when Edwin H. Land first applied it to motion pictures. The so called "3-D movie craze" in the years 1952 through 1955 was almost entirely offered in theaters using polarizing projection and glasses. Only a minute amount of the total 3D films shown in the period used the anaglyph color filter method. What is new is the use of digital projection, and also the use of sophisticated IMAX 70mm film projectors, with very reliable mechanisms. Whole new generations of 3D animation films are beginning to show up in the theaters, all using some form of polarization. Polarization is not easily applied to home 3-D broadcast or DVD presentation. At this point only anaglyph glasses may be used to view the new HD shows and are beginning to be aired occasionally by NBC and the Discovery Channel.

Alternate-frame sequencing
Alternate-frame squencing (sometimes called Alternate Image, or AI) is a method of showing 3-D film that is used in some venues. It is also used on PC systems to render 3-D games into true 3-D.

Applications in film
The movie is filmed with two cameras like most other 3-D films. Then the images are placed into a single strip of film in alternating order. In other words, there is the first left-eye image, then the corresponding right-eye image, then the next left-eye image, followed by the corresponding right-eye image and so on.

The film is then run at 48 frames-per-second instead of the traditional 24 frames-per-second. The audience wears very specialized LCD shutter glasses that have lenses that can open and close in rapid succession. The glasses also contain special radio receivers. The projection system has a transmitter that tells the glasses which eye to have open. The glasses switch eyes as the different frames come on the screen.

This system is not generally used anymore in venues in favor of polarization. It is used, however, in home 3-D movie systems.

Applications in Gaming
The same method of alternating frames can be used to render modern 3-D games into true 3-D, although it has been used to give a 3D illusion on consoles as old as the Sega Master System and Nintendo Famicom. Here, special software/hardware to used generate two channels of images, ofset from each other to create the stereoscopic effect. High frame rates (typically ~100fps) are required to produce seamless graphics, as the perceived frame rate will be half the actual rate (each eye sees only half the frames). Again, LCD shutter glasses synchronised with the graphics card complete the effect.

Autostereoscopy
Autostereoscopy is a method of displaying three-dimensional images that can be viewed without the use of special headgear or glasses on the part of the user. These methods produce depth perception in the viewer even though the image is produced by a flat device.

Several technologies exist for autostereoscopic 3D displays. Currently most of such flat-panel solutions are using lenticular lenses or parallax barrier. If the viewer positions their head in certain viewing positions, they will perceive a different image with each eye, giving a stereo image. Consequently, eye strain and headaches are usual side effects of long viewing exposure to of autostereoscopic displays that use lenticular lens or parallax barriers. These displays can have multiple viewing zones allowing multiple users to view the image at the same time. Other displays use eye tracking systems to automatically adjust the two displayed images to follow the viewer's eyes as they move their head.

A wide range of organisations have developed autostereoscopic 3D displays, ranging from experimental displays in university departments to commercially available displays. Examples include: Alioscopy, Apple, Dimension Technologies, Fraunhofer HHI, Holografika, i-Art, NewSight, Philips, SeeFront, SeeReal Technologies, Spatial View, and Tridelity. Sharp also claim to have the technology, although not for commercial sale at the moment.

Stereogram
A stereogram is an optical illusion of depth created from flat two-dimensional image. Originally, stereogram referred to a pair of stereo images which could be viewed using stereoscope. Other types of stereograms include anaglyphs and autostereograms.

Stereogram was discovered by Charles Wheatstone in 1838. He found an explanation of binocular vision which led him to construct a stereoscope based on a combination of prisms and mirrors to allow a person to see 3D images from two 2D pictures. Stereograms were re-popularized by the creation of autostereogram on computers, where a 3D image is hidden in a single 2D image, until the viewer focuses the eyes correctly. The Magic Eye series is a popular example of this. Magic Eye books refer to autostereograms as stereograms, leading most people to believe that the word stereogram is synonymous to autostereogram. Salvador Dalí created some impressive stereograms in his exploration in a variety of optical illusions.

Pulfrich effect
The Pulfrich effect is a psycho-optical phenomenon wherein lateral motion by an object in the field of view is interpreted by the brain as having a depth component, due to differences in processing speed between images from the two eyes. The effect is generally induced by placing a dark filter over one eye. The phenomenon is named for German physicist Carl Pulfrich who first described it in 1922.

In the classic Pulfrich effect experiment a subject views a pendulum swinging in a plane perpendicular to the observer’s line of sight. When a neutral density filter (a darkened lens – typically grey) is placed in front of, say, the right eye the pendulum seems to take on an elliptical orbit, appearing closer as it swings toward the right and farther as it swings toward the left.

The widely accepted explanation of the apparent depth is that a reduction in retinal illumination (relative to the fellow eye) yields a corresponding delay in signal transmission, imparting instantaneous spatial disparity in moving objects. This seems to occur because visual system latencies are generally shorter for (the visual system responds more quickly to) bright targets compared to dim targets. This motion with depth is the visual system’s solution to a moving target when a difference in retinal illuminance, and hence a difference in signal latencies, exists between the two eyes.

The Pulfrich effect has typically been measured under full field conditions with dark targets on a bright background, and yields about a 15ms delay for a factor of ten difference in average retinal illuminance. These delays increase monotonically with decreased luminance over a wide (> 6 log-units) range of luminance. The effect is also seen with bright targets on a black background and exhibits the same luminance-to-latency relationship. The effect can occur spontaneously in several eye diseases such as cataract, optic neuritis, or multiple sclerosis. In such cases, symptoms such as difficulties judging the paths of oncoming cars have been reported.

The Pulfrich effect has been utilized to enable a type of stereoscopy, or 3-D visual effect, in visual media such as film and TV. As in other kinds of stereoscopy, glasses are used to create the illusion of a three-dimensional image. By placing a neutral filter (eg., the darkened lens from a pair of sunglasses) over one eye, an image, as it moves right to left (or left to right, but NOT up and down) will appear to move in depth, either toward or away from the viewer.

Because the Pulfrich effect depends on motion in a particular direction to instigate the illusion of depth, it is not useful as a general stereoscopic technique; for example it cannot be used to show a stationary object apparently extending into or out of the screen; similarly, objects moving vertically will not be seen as moving in depth. It can, however, be effective as a novelty effect in contrived visual scenarios. One advantage of material produced to take advantage of the Pulfrich effect is that it is fully compatible with "regular" viewing without the need for "special" glasses.

The effect achieved a small degree of popularity in television in the 1990s. For example, it was used in a "3D" motion television commercial in the 1990s, where objects moving in one direction appeared to be nearer to the viewer (actually in front of the television screen) and when they moved in the other direction, appeared to be farther from the viewer (behind the television screen). To allow viewers to see the effect, the advertiser provided a large number of viewers with a pair of filters in a paper frame. One eye's filter was a rather dark neutral gray while the other was transparent. The commercial was in this case restricted to objects (such as refrigerators and skateboarders) moving down a steep hill from left to right across the screen, a directional dependency determined by which eye was covered by the darker filter.

The effect was also used in the 1993 Doctor Who charity special Dimensions in Time and a 1997 special TV episode of 3rd Rock from the Sun. In many countries in Europe, a series of short 3D films, produced in the Netherlands, were shown on television. Glasses were sold at a chain of gas stations. These short films were mainly travelogues of Dutch localities. A Power Rangers episode sold through McDonalds used "Circlescan 4D" technology which is based on the Pulfrich effect. Animated programs that employed the Pulfrich effect in specific segments of its programs include The Bots Master and Space Strikers; they typically achieved the effect through the use of constantly-moving background and foreground layers. The videogame Orb-3D for the Nintendo Entertainment System used the effect (by having the player's ship always moving) and came packed with a pair of glasses. So did Jim Power: The Lost Dimension in 3-D for the Super Nintendo, using constantly-scrolling backgrounds to cause the effect. In the United States and Canada, six million 3D Pulfrich glasses were distributed to viewers for an episode of Discovery Channel's Shark Week in 2000.

ChromaDepth
Chromadepth is a patented system from the company Chromatek that produces a stereoscopic effect based upon differences in the diffraction of color through a special prism-like holographic film fitted into glasses. Chromadepth glasses give the illusion of colors taking up different positions in space, with red being in front, and blue being in back. Any media piece can be given a 3D effect as long as the color spectrum is put into use with the foreground being in red, and the background in blue. From front to back the scheme follows the visible light spectrum, from red to orange, yellow, green and blue.

The system was used variously for comic-books, educational books for children, light show displays at such planetariums as the Hayden Planetarium in New York City, and other printed and projected applications. This method was also used for I Love the '80s 3-D and glasses could be bought at Best Buy. Chromadepth has recently been put into use in Crayola's 3D Chalk. This technique was also used by the production company Quirky Motion for the music video promoting New Cassettes 2007 single "recover/retreat" as part of the BBC Electric Proms.

By Srivenkat Bulemoni, Filmmaking Techniques