Special Report: 3D Video in Pro AV
In part one of a two-part special report, our video expert details how 3D video works and which technologies work best depending on need.
There is a maximum parallax limit for 3D displays, and that is when both eyes are looking straight ahead with their axes parallel. This condition is known as positive parallax, and it’s never exceeded in real life. But positive parallax only occurs when viewing objects at a distance, not when viewing objects on a monitor or projection screen.
The opposite condition is negative parallax, where the center axes of the eyes cross over each other. To our brain, this implies that the object being viewed is in front of the screen and is similar to what happens when we look at a pencil that is held just inches from our face. Not surprisingly, viewing images with negative parallax for any length of time results in eyestrain and headaches.
Our eyes normally exhibit zero parallax when viewing electronic displays. So we need some way to induce parallax, to separate the left and right eye images, and then use special eyewear to help the brain re-converge the images as it would in real life. The easiest way is to create dual images with opposite (complementary) colors and pair them with similar glasses. This process is known as color anaglyph 3D and is the oldest and only universal 3D viewing system. The most common color combination is cyan and red, but green and magenta and blue and yellow have also been used.
While color anaglyph 3D works surprisingly well, it has an obvious limitationloss of color resolution. It is, however, the least expensive way to show 3D content because the glasses can be made from paper at a cost of a few pennies apiece when produced in large quantities. Anaglyph has been used extensively to show 3D movies over the past 60 years and there have even been a few DVD and Blu-ray titles released in the anaglyph format.
The second process for creating 3D images is to stack a pair of projectors and equip them with polarizers. The polarizers are either oriented at right angles to each other (linear polarization) or at opposite circular polarization angles (also known as Xpol). Each projector shows only the picture information for one eye and the projectors are aligned to create the correct parallax.
The viewer then wears a pair of glasses with matching polarizers for the left and right eye. In theory, the opposing polarization “twist” between each minimizes unwanted ghost images (or cross talk) from the other projector. In reality, there’s always some cross talk, but if the projectors are set up correctly and the eyewear is matched, we’ll see converged 3D images.
This method is known as passive 3D, and is widely used in movie theaters. Passive 3D can also be implemented on LCD and plasma monitors by applying a layer of circular-polarized material directly to the screen surface, aligning it with the rows of pixels. Every other pixel row has the same polarization twist, so the left eye sees the odd-numbered rows of pixels and the right eye sees the even-numbered rows. Our eyes converge these two sets of images into one using depth cues.
The third process for viewing 3D requires more horsepower, and is known as active-shutter or active-refresh 3D. In this system, individual frames of left- and right-eye information are sequenced at a very fast frame rate, typically double the rate used for normal TV. At the same time, a synchronizing signal is sent by the display or an outboard transmitter to a pair of special, battery-operated 3D glasses. These glasses contain miniature LCD panels that switch on and off at the same rate as the left and right eye sequencing of the 3D display. In this way, only one shutter is open in any given instant, revealing to that eye only the image that it should be seeing. Because of our persistence of vision, we don’t notice any flicker at this fast frame rate; the left- and right-eye images appear to be present simultaneously. As with the other methods, our brain then converges the two images into one to create the illusion of 3D.