Picture

Stereo-Vision Formats

What's a Format?

There are several distinct means for preparing time-shared multiplexed images for stereo-vision electronic displays.  Furthermore, the concept of format is distinct from the selection technique, or the means for providing each eye with its needed image (and rejecting the unnecessary image).  Format and image selection must work together, and in designing such a system the selection technique may determine the format.  For example, if the image is to be viewed in a stereoscope similar to the popular View-Master viewer (as mentioned in Chapter 1), the left and right images may be formatted so that they are side-by-side on the display screen.

In this chapter we are concerned with formats which work in conjunction with eyewear (also known as selection devices) and use the time-multiplexing approach.  Such eyewear may be active, using liquid crystal (LC) shutters like StereoGraphics' CrystalEyes or SimulEyes VR products, or they may be passive polarizing spectacles as are used in our projector ZScreen :.  In fact, even when using the passive eyewear with our projection Zscreen , the user is looking through a shutter, but one whose components are distributed between the Zscreen electro-optics and the polarizing eyewear.

The most widespread method for viewing electronic stereo-vision images uses CrystalEyes on workstations for scientific visualization applications such as molecular modeling.  In the most general terms, the format used by CrystalEyes is called the alternate field, field-sequential, or time-multiplexed technique.

Interlace Stereo

The original stereo-vision television format used interlace to encode left and right images on odd and even fields.  It's a method that's used today, and it has the virtue of using standard television sets and monitors, standard VCRs, and inexpensive demultiplexing equipment - a simple field switch that shunts half the fields to one eye and half to the other by synching the shutters' eyewear to the field rate.  The method  results in flicker - which some people find more objectionable than others.  One way to mitigate the flicker when viewing a monitor is to reduce the brightness of the image by adding neutral density filters to the eyewear.  Another problem is that each eye sees half the number of lines (or pixels) which are normally available, so the image has half the resolution.

The interlace approach or, to be more precise, the time-multiplexed low field rate approach, has turned out to have an interesting application these days when used in conjunction with an HMD using LC displays.  Because of the long persistence of LC displays, a low number of longer- lasting fields will produce a more or less flicker-free image.  If a field switch is used to alternate fields to one and then the other LC display, the result can be a stereo image with barely detectable flicker.

Above-and-Below Format

Our concern has been to create stereo-vision formats and add-on selection devices which operate within the existing infrastructure of computer graphics and video hardware, without modification to the infrastructure hardware or basic working procedures. The original method we invented has survived for computer graphics, but has disappeared for video. The method uses two subfields arranged above and below each other in a single standard field.  The images in these subfields are squeezed top to bottom by a factor of two.

At the standard 60 fields per second, it takes half the duration of an entire field, or 1/120th second, to scan a subfield.  When played back on a monitor operating at 120 fields per second, the subfields which had been juxtaposed spatially become juxtaposed temporally.  Therefore, each eye of the beholder, when wearing the proper shuttering eyewear, will see 60 fields of image per second, out of phase with the other 60 fields prepared for the other eye.  Thus, it is possible to see a flicker-free stereoscopic image because each eye is seeing a pattern of images of 1/120th second followed by 1/120th second  of darkness.  When one eye is seeing an image, the other is not, and vice versa.

Today there are many models of high-end graphics monitors which will run at field rates of 120 or higher.  Provided that a synchronization pulse  is added between the subfields in the subfield blanking area, such a monitor will have no problem in displaying such images.  The monitor will automatically "unsqueeze" the image in the vertical so the picture has the normal proportions and aspect ratio.  The term "interlace", when applied to such a display, is irrelevant because the stereo-format has nothing to do with interlace, unlike the odd-even format explained above.  The above-and-below format can work for interlace or progressive scan images.

StereoGraphics' sync doubling emitter for the above-and-below format is available for PCs.  It adds the missing synchronization pulses to the vertical blanking for a proper video signal. The shutters in the eyewear are triggered by the emitter's infrared signal.  This emitter is used to take a PC with a video board running at a standard rate, nominally 60 fields per second, and double it for the desired flicker-free rate. The emitter also provides an IR signal for our CrystalEyes eyewear whose electronics sync to the emitter's signal, which corresponds to the sync pulses (both original and added subfield pulses).

If the image has high enough resolution to begin with or, more to the point, enough raster lines, the end result is pleasing.  Below 300 to 350 lines per field the image starts to look coarse on a good-sized monitor viewed from two feet, which seems quite close.  However, that's about the distance people sit from workstation or PC monitors.  This sheds light on why this approach is obsolete for video.  NTSC has 480 active video lines.  It uses a twofold interlace so each field has 240 lines.  Using the subfield technique, the result is four 120-line fields for one complete stereoscopic image.  The raster looks coarse, and there is a better approach for stereo video multiplexing as we shall shortly explain.

At the frequently used 1280x1024 resolution, an above-and-below formatted image will wind up at 1280 by about 500 pixels per eye, which looks good.  Even from the workstation viewing distance of two feet, the image is extraordinary, and most people would agree that this is near photographic quality.

Stereo-Ready Computers

These days most graphics computers, like those from Silicon Graphics (SGI), Sun, DEC, IBM, and HP, use a double buffering technique to run their machines at a true 120 fields per second rate.  Each field has a vertical blanking area associated with it which has a synchronization pulse.  These computers are intrinsically outputting a high field rate and they don't need the above-and-below solution to make flicker-free stereo images, so they don't need a sync-doubling emitter to add the missing sync pulse.  These computers are all outfitted with a jack that accepts the StereoGraphics standard emitter, which watches for sync pulses and broadcasts the IR signal with each pulse.  Most of these machines still offer a disproportionately higher pixel count in the horizontal compared with the vertical.

Some aficionados will insist that square pixels are de rigueur for high-end graphics, but the above-and-below format (or most stereo-ready graphics computers) produces oblong pixels - pixels which are longer in the vertical than they are in the horizontal. The popular 1280x1024 display produces a ratio of horizontal to vertical pixels of about 1.3:1, which is the aspect ratio of most display screens, so the result is square pixels.  But in the above-and-below stereo-vision version for this resolution, the ratio of  horizontal to vertical pixels for each eye is more like 2.6:1, and the result is a pixel longer than it is high by a factor of two.

SGI's high-end machines, the Onyx : and Crimson : lines, may be configured to run square-pixel windowed stereo with separate addressable buffers for left and right eye views, at 960x680 pixels per eye (108 fields per second). This does not require any more pixel memory than already exists to support the planar 1280x1024.  Some SGI  high-end computers have additional display RAM available, and they can support other square-pixel windowed stereo resolutions, though higher resolutions come at the expense of field rate. For example, such high-end high-display-memory SGI systems support 1024x768 pixels per eye but only at 96 fields per second, which is high enough to have a flicker free effect for all but very bright images.

 For most applications, having pixels which are not perfectly square can result in a good-looking picture.  The higher the resolution of the image, or the more pixels available for image forming, the less concern the shape of the pixels is.

Side-by-Side

We developed the side-by-side technique to cure a significant problem of the above-and-below technique as applied to video: not enough raster lines.

While the above-and-below solution is a good one for computer graphics applications because computer displays tend to output more raster lines than television, our video product uses different techniques to create stereo formats.  First, for real-time viewing this is what we do:  The left and right images from the two video camera heads making up a stereoscopic camera are fed to our View/Record box, and for viewing real-time the images are stored and then played back at twice the rate at which they were read.

 In addition, the fields are concatenated or alternated to achieve the necessary left-right pattern.  The result is an over-30-KHz or twice normal video bandwidth signal which preserves the original image characteristics but, in addition, is stereoscopic.  What we've described here is for real-time viewing using a graphics monitor with 120 fields per second capability, and it is the function of the View section of the View/Record box to produce such a signal.

Once it becomes necessary to interface with the existing television infrastructure, a problem arises:  NTSC recorders must be used if we are to have a generally useful system, and that means containing the image to within the NTSC specification of about a 15 KHz line rate.  Thus the Record section of the View/Record box serves to compress the left and right images so they occupy the normal NTSC bandwidth. It does this by squeezing the images horizontally so that they occupy a single standard field.  The resultant signal is in fact an NTSC signal which may be recorded on an NTSC recorder (we also make a PAL version).  When played back the side-by-side image is digitized by our Playback Controller, and formatted for stereo viewing.  The result is an image which has characteristics which are similar to the real-time image described above.

White-Line-Code

The White-Line-Code (WLC) system is used for multi-media PCs and it offers a high-quality but low-cost solution to the problem of stereo-vision imaging.  This format doesn't care if the left and right fields are in interlace or progressive scan modes, and it doesn't care about the field rate.  WLC was created to offer the most flexible possible stereo-vision system for the content providers, developers, and users. The hardware components of the WLC allow for rapid user installation.

On the bottom of every field, for the last line of video, white lines are added to signify whether the field is a left or a right, as shown in the illustration.  The last line of video was chosen because it is within the province of the developer to add the code in this area immediately before the blanking area (which is not accessible to the developer).  When our electronics see the white line, it is prepared to shutter the eyewear once the vertical sync pulse is sensed.

The WLC is universal in the sense that it simply doesn't care about interlace, or progressive scan, or field rate, or resolution.  If the WLC is there, the eyewear will shutter in synchrony with the fields and you'll see a stereoscopic image.  Our SimulEyes VR product is used with the WLC in PCs, and the WLC is an integral  part of the SimulEyes VR product. SimulEyes VR chips are used on video boards for driving our new low-cost eyewear; for users who wish to retrofit their PCs for stereo-vision, we have a solution with our SimulEyes VR controller.

The two most popular modes of operation for WLC are the page-swapping mode, which is used most often for DOS action games running at either 70 or  80 fields per second (we've worked out a driver that allows for 80 fields per second which reduces flicker considerably); the other mode is that used most often for multi-media in Windows, the IBM standard 8514A, which is a 90 fields per second interlace mode.

New video boards are coming along for the PC and these use a technique which is similar to that used in stereo-ready workstations.  They assign an area in the video buffer to a left or right image with a software "poke."

With the passage of time it's our expectation that the board and monitor manufacturers will allow their machines to run at higher field rates, say 120 fields per second, as is the case for workstations.  When this happens the difference between PCs and workstations will entirely vanish vis-a-vis stereo-vision displays.  Until that time, the lower 80 and 90 fields per second rates serve nicely to provide good-looking images in most situations.

Summing Up

We have seen that there is a wide variety of stereo formats that are used on different computer and video platforms.  All of the time-multiplexed formats discussed here are to a greater or a lesser extent supported by StereoGraphics.  For example, our high-end video camera may be used to shoot material for the interlace format.  PC users may find themselves using either  above-and-below format with the  sync-doubling emitter and CrystalEyes , or the WLC format with SimulEyes VR .

Users preparing computer generated animation to be used in videos may find themselves viewing in above-and-below or the workstation's stereo-ready format.  However, for recording on a standard VCR, the user may actually format the image in the side-by-side format.

Format Attributes

Format

Fields/Sec1

Medium

Viewing Hardware

Interlace, NTSC

60

NTSC2

Eyewear or HMD3

Interlace, PAL

50

PAL2

Eyewear or HMD3

Side-by-side, NTSC

120

NTSC2

Playback, Emitter, CE24

Side-by-side, PAL

100

PAL2

Playback, Emitter, CE24

Above-and-Below

120

PC

Sync-Doubling Emitter, CE-PC4

Stereo-Ready

120

Workstation

Emitter, CE24

White-Line-Code

70-90

Multi-Media PC

SimulEyes VR

Footnotes:

  1. Divide by two for field rate for one eye.
  2. For broadcast, VCR, laser disc, or DVD.
  3. StereoGraphics manufactures OEM eyewear for this application.
  4. CE 2 is the latest model of our CrystalEyes product. CE-PC is our product for the PC.

All materials © Copyright 1996-97, StereoGraphics Corporation

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