Alternate-Frame Sequencing
Let's start with alternate-frame sequencing. We're not going to use this method in this article, but it's important to learn about it so that you understand the strengths and weaknesses of the two main methods of producing a stereoscopic 3D display. Alternate-frame sequencing is the stereoscopic 3D system that is positioned to be the standard in the home and almost all of the new 3D-ready TVs will rely on this method to reproduce 3D stereoscopic content.
A standard TV can provide 60 frames of video per second (60 Hz), but a 3D-ready TV can provide twice that, or 120 frames per second (120 Hz). The alternate-frame sequencing system works by alternately displaying a frame of video for each eye--first a frame of video for the left eye is shown, followed by a frame for the right eye, a frame of video for the left eye again, and so on. This changes back and forth, 120 times each second.
If you were to watch TV while it delivered 3D content this way, all you'd see would be a blur of what would appear to be two perspectives overlapped on top of each other. The key to making this system work is LCD shutter glasses. These glasses alternatively block each eye, 120 times each second (60 times a second for each eye), in order to allow only the intended frame of video to be seen by the targeted eye.
With a total 120 Hz, each eye sees every second frame and thus receives 60 frames of video per second--the same rate that we're used to seeing on conventional televisions. At this speed, you shouldn't be able to perceive any strobing or flickering, but you'd probably notice that the video seems darker than you'd expect. This makes sense since, each eye is only receiving light half of the time.
The alternate frame-sequencing method of stereoscopic 3D is used not only in upcoming 3D-ready television sets, but in some new GeForce 3D Vision-ready PC monitors.
Alternate-frame sequencing is the method of 3D realization that we used in 2007 when we put together the "Wall-Sized 3D Displays: The Ultimate Gaming Room" article. Back then, 120 Hz projectors were practically unheard of, so we settled for an 80 Hz DLP projector. This projector could display 80 FPS, delivering 40 frames for each eye. While 40 frames per eye aren’t bad, at the slower shutter speed, the strobe effect is far more pronounced. This strobe effect can make it hard to watch a 3D display for long periods of time and may cause disorientation and headaches. The issue was more acceptable three years ago when intrepid early-adopters were pioneering wall-sized 3D viewing, but it wouldn't cut it in today's mass market. It's 2010 and we expect more.
Thankfully, today there are a number of 120 Hz 3D-ready projectors on the market, and that high refresh rate makes for a far smoother experience far easier on the eyes. What's the downside to the alternate-frame sequencing method? The cost of the equipment can become increasingly prohibitive. For example, a kit including a single pair of 3D glasses and an IR emitter (required to synchronize the glasses to the proper frame of video) is about $200. Each extra pair of glasses after that will typically cost $150 each. So, if you have a family of five and you want to watch a 3D film on your new 3D Blu-ray home-theater system, it'll cost you about $800 to get the glasses alone. Invite a couple to join your family for movie night, and the cost goes up to $1,100--and that's not including the cash you've already spent on your 3D-ready television or projector.
Are there any other downsides to active LCD glasses? Well, as mentioned previously, it limits the amount of light that reaches the eye, so your TV will seem darker than it does without wearing them. The glasses will also need to be powered. Modern LCD glasses are rechargeable, but this is a minor inconvenience.
However, the advantage of alternate-frame sequencing is significant--while the 3D Blu-ray specification is said to be display-agnostic, almost all of the consumer-grade TVs that have been recently announced rely on this method. That means when films like Cloudy with a Chance of Meatballs, Monsters vs. Aliens, and Avatar become available to purchase as a 3D Blu-ray disc, it might not be practical to view them in any other way. We will have to wait until the technology arrives and matures to see if this will be the case, but alternate-frame sequencing is poised to become the de-facto standard for 3D stereoscopic displays in the home.
