The Basics of Stereoscopic Imaging
3D is based on the principle of stereoscopy, which creates the illusion of depth in an image. The easiest way to enhance depth perception in the brain is to provide the eyes of the viewer with two different images, representing two perspectives of the same object, with a minor deviation exactly equal to the perspectives that both eyes naturally receive in binocular vision.
Put in simpler terms, 3D is stereo for your eyes. To achieve this, you need two images that are offset approximately the same distance as the average spacing between human eyes – about four inches. This is often achieved by shooting video with two cameras, or a special camera with two lenses. There are also post-production techniques that can add depth-mapping to 2D imagery to create a stereoscopic effect.
Three Types of 3D
Today’s 3D technology – the stuff that is getting all of the current media and retail attention – generally falls into one of two categories: 3D that you watch with Active Shutter Glasses and 3D that you watch with Polarized, or Passive glasses. The third technique, called Anaglyph, uses two color layers superimposed but offset with respect to each other to produce a depth effect. Anaglyph 3D has been around since the 1950s (yes the old red/blue paper glasses) but results in poor image quality and heavily tinted imagery. For the purposes of a Blu-ray 3D discussion, let’s examine the two modern types of 3D technology:
Polarized, or Passive Technology
Most of us have seen Avatar or other movies in IMAX 3D with polarized 3D glasses. These glasses create the illusion of three-dimensional images by restricting the light that reaches each eye, creating a stereoscopic, or 3D, effect.
To create that 3D effect, two images are projected onto the same screen through different polarizing filters. The viewer wears low-cost eyeglasses which also contain a pair of different polarizing filters. Through the filters, each eye sees a different picture (each filter allows the light which is similarly polarized and blocks the light polarized in the opposite direction). This is used to produce a three-dimensional effect by projecting the same scene into both eyes, but depicted from slightly different perspectives.
The advantage to this technology is that the passive glasses are really inexpensive (between $1 and $10 per pair); however, most believe that the 3D effects generated through this technology are not as sharp as the ones generated through a display with active technology. This is because the high definition image contains both the left eye and the right eye image in the same frame (usually squeezed side-by-side or top/bottom) and the screen decodes the single frame into two – one for the left one for the right. This means that each eye’s image is actually half the resolution of the full frame HD image.
Active Technology:
Active technology has been adopted by the Blu-ray Disc Association and is supported by most consumer electronics firms, including LG, Samsung, Panasonic, Sony and more. With this technology, an HDTV will display one image to your left eye and one image to your right eye. Since the effective frame rate is halved, these HDTVs need to have double the refresh rate of HDTVs (60 Hz). This is why you will find that all 3D HDTVs have a minimum frame rate of 120 Hz (most have a frame rate around 240 Hz or even 480 Hz).
Active liquid crystal shutter glasses are then worn by the viewer and quickly block each eye in sequence to ensure that each eye only sees the corresponding image being displayed on the 3D TV set. The active shutter glasses are kept in sync with the HDTV using Bluetooth, infrared, or radio technology. These special glasses usually contain liquid crystals that can be made opaque, thus acting as a shutter. These glasses are battery-operated (battery life estimated at around 80 hours or so).