3D glasses and other 3D display devices
How 3D effect work
Our ability to see stereo-vision comes from each of our eyes seeing a slightly different view of the world. Our brain integrates these two images into one three-dimensional picture. The key element in producing the stereoscopic depth effect is parallax. Parallax is the horizontal distance between corresponding left and right image points. The stereoscopic image is composed of two images generated from two related perspective viewpoints, and the viewpoints are responsible for the parallax content of a view.
How 3D displays work
Electro-stereoscopic displays provide parallax information to the eye by using a method related to that employed in the stereoscope. The 3D display systems normally in use use on of the following methods:
* Separate display for each eye (used in HMDs)
* Shutter glasses (most common method)
* Color filter glasses (used in some old 3D movies)
* Polarizing glasses (used in some modern 3D movies)
Color filter glasses
Color filter glasses were one of the oldest 3D glasses. The system works so that both eyes have a different color filter in front of them. This causes that left eye can only see few colors and right eye some other colors. When the led eye’s colors are used to draw the emage which it should see and same is used for right eye, the combined image can be viewed with suitable glasses in 3D. The most common color combinations are red+green and blue+green. The color filtering limits that there are only few possible colors in use in th epicture so the images made using this method are not very nice to look.
Color filter glasses have been used in 3D movies and some early computer games. The advantage of this method is that the 3D material can be stored to any standard color video media and viewed with normal display devices as long as you wear the right color filter glasses. The galsses are very inexpensive beacuse you only need very cheap plastic filters for them. You can even make your own glasses from piece of cardboard and suitable filters (standard lighting GEL numbers R26 and R83 should be quite suitable for red+green glasses).
This technique causes colors in the image to be compromised because you have too many different colros in different eyes. Practically you loose almost all your color, so you can see objects coming out of the screen but they are gray. The colors also create some eyestrain and distortion.
Polarizing glasses
This method is usually used with projection displays when 3D material needs to be displayed. Every viewer has to wear special glasses which have two polarizing lenses which have their polarization directions adjusted to be 90 degrees different. This makes is possible that left eye sees it’s picture without problems but everything ment to right eye (sent out at different polarization) seems to be black. Same applies also to right eye.
The material which has to be shown is typically projected using two projectors (film projector, slide projector or video projector) which each have polarizing lenses in front of them (adjusted to meet the polarization directions of the glasses). The projection surface must be specially made so that it does not do any harm to the polarization (many traditional projection surface materials are not suitable, silver stripe screen is recommended). The advantage of this method is that the pictures can be in full color and the viewing glasses are still quite inexpensive.
LCD shutter glass method
In the LCD shutter glass 3D display, the left and right images are alternated rapidly on the monitor screen. When the viewer looks at the screen through shuttering eyewear, each shutter is synchronized to occlude the unwanted image and transmit the wanted image. Thus each eye sees only its appropriate perspective view. The left eye sees only the left view, and the right eye only the right view.
A field-sequential 3D (stereoscopic) video signal is a normal video signal (PAL, NTSC or SECAM) which has been specially recorded with left and right images stored on the even and odd fields of the video signal. The 3D video signal is usually viewed while wearing a pair of LCD shutter glasses which only allow the left eye to see left images and the right eye to see right images.
If the images (the term “fields” is often used for video and computer graphics) are refreshed (changed or written) fast enough (often at twice the rate of the planar display), the result is a flickerless stereoscopic image. This kind of a display is called a field-sequential stereoscopic display.
Lenny Lipton has develloped this technology very much: he holds many patents and has commercially used this technology.
Format Attributes for LC shutter glass 3D formats
Format Fields/Sec Medium Video viewing hardware Interlace, NTSC 60 NTSC Normal TV, odd/even field sync box Interlace, PAL 50 PAL Normal TV, odd/even field sync box Side-by-side, NTSC 120 NTSC View/Record box, computer monitor Side-by-side, PAL 100 PAL View/Record box, computer monitor Above-and-Below 120 PC Sync-Doubling Emitter, VGA monitor Stereo-Ready 120 Workstation Normal workstation monitor White-Line-Code 70-90 PC White line code decoder box for glasses
Here is a short summary of 3D glass controlling schemes for computer displays (taken from 3D PC Systems page):
+-------------+---------------+----------+-----------------+----------+ | 3D Method |Vertical Output| Fields |Effective Refresh| Vertical | | | Frequency |per Second|Rate for a Stereo|Resolution| | | of video card | | Field Pair | | +-------------+---------------+----------+-----------------+----------+ |Interlaced | 60Hz | 60Hz | 30Hz | half | +-------------+---------------+----------+-----------------+----------+ |Page Flipping| 60Hz | 60Hz | 30Hz | full | +-------------+---------------+----------+-----------------+----------+ |Sync-Doubled | 60Hz | 120Hz | 60Hz | half | +-------------+---------------+----------+-----------------+----------+
The biggest drawback of LC-Shutterglasses besides the compatibility and ergonomy issue is Crosstalk. Due to the persistance of the monitor tube, the inability of the LC-panels to block the light entirely, sync errors and other factors one see “Ghostimages” sometimes.� The right eye sees some residue of the image dedicated to the left eye and vice versa.
Flicker
Another common scapegoat for inadequate hardware, software, and lack of stereo training is flicker, which is most noticeable in standard frequency (e.g. 60 Hz) field sequential systems. It varies with many factors, especially screen brightness, screen size and room illumination. The image may still flicker even at 120 Hz screen refresh if the image is not updated in the proper way. Decreasing the level of ambient illumination in the room can reduce the room flicker to imperceptible levels. Reducing screen luminosity with brightness and contrast controls will reduce image flicker to low or imperceptible levels.
How glasses are controlled
There have been many methods for controlling the LC shutter glasses. The liquid crystal shutter elements (size usually 3/4″x1″) itself have to be driven using AC voltage, because DC would destroy the liquid crystals. The driving signals are typically around 3-8V and frequency is usually three hundred Hz. The shutter elements are usually designed so that when no voltage is connected to them you can see through them and when you apply the AC control voltage those elements become black.
The most common way to connect the glasses is that the LC shutter glasses are wired to a controller which is connected to video source what you want to watch (usually computer or VCR). Sometimes IR or other wireless links are used between the glasses and controller. Using IR or radio link enables easily controlling multiple glasses from one controller.
Controller itself is connected to video source through some suitable connector. If you are watching stereo program from VCR, the controller get the LC glass controlling info from the video signal. This same method is used also in many computer interfaces. Some system do not just take the info from the sync signals, but they actually modify the video signal.
In PC virtual reality applications serial port and parallel port interfaces become quite common. In this implementation the software takes care of changing the data in display memory and sends the signal to which eye the picture is to the PC glass controller through serial or parallel port. The most widely used implementations of this technique have been very simple circuit which have used one or two output pins for telling what eye should be visible and what should not be. Sega 3D glass serial port interface is quite classical this kind of interface. Serial port interface have also been used with Commodore Amiga and Apple Macintosh.
Commodore Amiga joystick port is bidirectional. It has power output and it can be used for controlling small peripherals like LC shutter glass interface. AMI VR system has used this method. Using joystick port has also the same limitations like serial and parallel port interfaces: software has to handle the controlling of LC shutter glasses.
When 3D graphics has become more and more used, some workstation manufacturers have put special ports to their 3D graphics adapter for easy interfacing of 3D LC shutter glasses. Nowadays Silicon Graphics workstations have a LC shutter glass interface as a standard feature.
3D devices and virtual reality
3D display devices a a necessity for generating virtual reality environment. The only way to make the filling that you are really inside the virtual reality word is to show the virtual world around you in 3D. There have been many methods for generating 3D displays, but the most common of them used with computers are LC shutter glasses and head mount displays (HMD).
Sega 3D glasses
Sega 3D glasses are simple and inexpensive LCD shutter glasses used in many home brew virtual reality systems. Those glasses can be used in combination with normal computer monitor to show realistic 3D pictures.
Original use
Sega designed special glasses for it’s video game console to be able to produce 3D video games. Those glasses used LCD shutter method for producing 3D images. In this method the picture is snown to different eyes after each other. The LCD panels in front of both eyes are controlled so that one eye sees one screen image and the other eye sees the second image etc. With normal TV it was possible to show 25 or 30 images per second (depending on TV standard used) to both eyes. The image flickers quite much, but gives well noticeable 3D effect. 3D games for sega game consoles did not sell very well and Sega dropped those glasses from it’s product lines.
Using Sega 3D glasses in PC virtual reality
Ever since the Sega 3D glasses are used in many home virtual reality projects. There is free software support for those glasses in some nice VR programs (for example rend386). One of the problem for home experiments have been to figure out how to connect those glasses to PC.
There have been two approaches for controlling the glasses form the computer: serial port and parallel port. For connecting the glasses to those PC ports, the small adapter box (made for Sega game console) is practically useless. The home VR experiments had to develop their own circuit. The program which runs in computer must them make sure that the display contents in the screen in changed for every frame and the state of the shutter glasses is changed according to that.
