3D display

For more information on 3D television, see 3D television.

A 3D display is any display device capable of conveying a stereoscopic perception of 3-D depth to the viewer. The basic requirement is to present offset images that are displayed separately to the left and right eye. Both of these 2-D offset images are then combined in the brain to give the perception of 3-D depth. Although the term "3D" is ubiquitously used, it is important to note that the presentation of dual 2-D images is distinctly different from displaying an image in three full dimensions. The most notable difference is that the observer is lacking any freedom of head movement and freedom to increase information about the 3-dimensional objects being displayed. Holographic displays do not have this limitation, so the term "3D display" fits accurately for such technology.

Similar to how in sound reproduction it is not possible to recreate a full 3-dimensional sound field merely with two stereophonic speakers, it is likewise an overstatement of capability to refer to dual 2-D images as being "3D". The accurate term "stereoscopic" is more cumbersome than the common misnomer "3D", which has been entrenched after many decades of unquestioned misuse.

The optical principles of multiview auto-stereoscopy have been known for over 60 years. Practical displays with a high resolution have recently become available commercially.

Types of 3D displays

Stereoscopic

Main article: Stereoscopy

Based on the principles of stereopsis, described by Sir Charles Wheatstone in the 1830s, stereoscopic technology provides a different image to the viewer's left and right eyes. Examples of this technology include anaglyph images and polarized glasses. Stereoscopic technologies generally involve special spectacles.

Autostereograms

A random dot autostereogram encodes a 3D scene which can be "seen" with proper viewing technique

Main article: autostereogram

More recently, random-dot autostereograms have been created using computers to hide depth information in a field of apparently random noise, so that until viewed by diverging or converging the eyes in a manner similar to naked eye viewing of stereo pairs, the subject of the image remains a mystery. A popular example of this is the Magic Eye series, a collection of stereograms based on distorted colorful and interesting patterns instead of random noise.

Pulfrich effects

Main article: Pulfrich effect

In the classic Pulfrich effect paradigm a subject views, binocularly, a pendulum swinging perpendicular to his line of sight. When a neutral density filter (e.g., a darkened lens -like from a pair of sunglasses) is placed in front of, say, the right eye the pendulum appears to take on an elliptical orbit, being closer as it swings toward the right and farther as it swings toward the left.

The widely accepted explanation of the apparent motion with depth is that a reduction in retinal illumination (relative to the fellow eye) yields a corresponding delay in signal transmission, imparting instantaneous spatial disparity to moving objects. This occurs because the eye, and hence the brain, respond more quickly to brighter objects than to dimmer ones.

So if the brightness of the pendulum is greater in the left eye than in the right, the retinal signals from the left eye will reach the brain slightly ahead of those from the right eye. This makes it seem as if the pendulum seen by the right eye is lagging behind its counterpart in the left eye. This difference in position over time is interpreted by the brain as motion with depth: no motion, no depth.

The ultimate effect of this, with appropriate scene composition, is the illusion of motion with depth. Object motion must be maintained for most conditions and is effective only for very limited "real-world" scenes.

Prismatic & self-masking crossview glasses

"Naked-eye" cross viewing is a skill that must be learned to be used. New prismatic glasses now make cross-viewing as well as over/under-viewing easier, and also mask off the secondary non-3D images, that otherwise show up on either side of the 3D image. The most recent low-cost glasses mask the images down to one per eye using integrated baffles. Images or video frames can be displayed on a new widescreen HD or computer monitor with all available area used for display. HDTV wide format permits excellent color and sharpness. Cross viewing provides true "ghost-free 3D" with maximum clarity, brightness and color range, as does the stereoscope viewer with the parallel approach and the KMQ viewer with the over/under approach. The potential depth and brightness is maximized. A recent cross converged development is a new variant wide format that uses a conjoining of visual information outside of the regular binocular stereo window. This allows an efficient seamless visual presentation in true wide-screen, more closely matching the focal range of the human eyes.

Lenticular prints

Main article: Lenticular printing

Lenticular printing is a technique by which one places an array of lenses, with a texture much like corduroy, over a specially made and carefully aligned print such that different viewing angles will reveal different image slices to each eye, producing the illusion of three dimensions, over a certain limited viewing angle. This can be done cheaply enough that it is sometimes used on stickers, album covers, etc. It is the classic technique for 3D postcards.

A variant of this for portable electronic devices, the parallax barrier, has begun deployment.

Displays with filter arrays

The LCD is covered with an array of prisms that divert the light from in their notebook and desktop computers. These displays usually cost upwards of 1000 dollars and are mainly targeted at science or medical professionals.

Another technique, for example used by the X3D company,^{[citation needed]} is simply to cover the LCD with two layers, the first being closer to the LCD than the second, by some millimeters. The two layers are transparent with black strips, each strip about one millimeter wide. One layer has its strips about ten degrees to the left, the other to the right. This allows seeing different pixels depending on the viewer's position.

Wiggle stereoscopy

Wiggle stereoscopy

This method, possibly the simplest stereogram viewing technique, is to simply alternate between the left and right images of a stereogram. In a web browser, this can easily be accomplished with an animated .gif image, Flash object, or JavaScript script. Most people can get a crude sense of dimensionality from such images, due to parallax^{[citation needed]}.

Closing one eye and moving the head from side-to-side when viewing a selection of objects helps one understand how this works. Objects that are closer appear to move more than those further away. This effect may also be observed by a passenger in a vehicle or low-flying aircraft, where distant hills or tall buildings appear in three-dimensional relief, a view not seen by a static observer as the distance is beyond the range of effective binocular vision.

Advantages of the wiggle viewing method include:

• No glasses or special hardware required
• Most people can "get" the effect much quicker than cross-eyed and parallel viewing techniques
• It is the only method of stereoscopic visualization for people with limited or no vision in one eye

Disadvantages of the "wiggle" method:

• Does not provide true binocular stereoscopic depth perception
• Not suitable for print, limited to displays that can "wiggle" between the two images
• Difficult to appreciate details in images that are constantly "wiggling"
• Lack of 3D illusion to those who can detect the wiggling too easily.

A wiggle stereogram of the Sun alternating between left and right eye images taken by the NASA's STEREO solar observation mission. Try to look at it with just one eye.

Most wiggle images use only two images, leading to an annoyingly jerky image. A smoother image, more akin to a motion picture image where the camera is moved back and forth, can be composed by using several intermediate images (perhaps with synthetic motion blur) and longer image residency at the end images to allow inspection of details. Another option is a shorter time between the frames of a wiggle image.

Although the "wiggle" method is an excellent way of previewing stereoscopic images, it cannot actually be considered a true three-dimensional stereoscopic format. To experience binocular depth perception as made possible with true stereoscopic formats, each eyeball must be presented with a different image at the same time – this is not the case with "wiggling" stereo. The apparent effect comes from syncing the timing of the wiggle and the amount of parallax to the processing done by the visual cortex. Three or five images with good parallax may produce a better effect than simple left and right images.

Wiggling works for the same reason that a translational pan (or tracking shot) in a movie provides good depth information: the visual cortex is able to infer distance information from motion parallax, the relative speed of the perceived motion of different objects on the screen. Many small animals bob their heads to create motion parallax (wiggling) so they can better estimate distance prior to jumping.

Problems

Each of these display technologies can be seen to have limitations, whether the location of the viewer, cumbersome or unsightly equipment or great cost. The acquisition of artifact-free 3D images remains difficult. There are currently no guidelines or standards for multi-camera parameters, placement, and post- production processing, as there are for conventional 2D television.