SIMPLE PRINT or COMPUTER SCREEN VIEWING

The simplest way to present a stereo image is to make a print (or a computer screen display) of the two images in the pair and set them side-to-side.  Fig. 2.1 shows a shot of a kayaker in wild water.  The left image is on the left and the right image is on the right.  This is called a parallel-pair.  The print was made (or displayed here on the web) by abutting the two digitized images using an image processing program.  Photoshop, Paintshop Pro, or similar work, but by far the best application is the freeware STEREO PHOTO MAKER (SPM) which can be downloaded from here.

Fig. 2.1  Making a Side-by-Side Pair with normal image processing programs: Open the right image in Photoshop or Paintshop Pro.  Expand the canvas size, slightly surrounding the right picture and providing room for the left image.  Open the latter and copy it into the left canvas space.  Since it is pasted as a layer you can move it relative to the right image in order to register the stereo pair.  Crop off the uneven parts and save.

In order to fuse such a pair in your brain, your eyes have to look at the image straight ahead.  It's like imaging a point far behind the image-plane so your view is parallel, but then at the same time focusing on the near object.   Unfortunately, without a little assistance, few people can do this.  The right picture confuses the left eye, and you have to fight off the natural tendency  to toe in your view to look at this image pair as a close-up.  The principle of print viewers or prism glasses is to

a) reduce the cross influence,

b) magnify the images, and

c) relieve the strain from having to force  your eyes to look at a nearby object "wide-eyed."

Fig. 2.2  Wide-Eyed Viewing.   Because the images must be placed side by side, they cannot be made too large.  For example, a pair of 6" x 6" images would have a center-to-center separation of 6", which is much too far apart to accommodate, even with prism glasses.  The limit on image size is typically 3 to 4 inches.  The American National Stereographic Association's standard is that a pair for competition should be printed on a 3.5" x 7" card, so that with some borders you get two 3x3 images in a 3 x 6 pair.  Some people put  a small space between the pictures, others do not since this just pushes them wider, making them harder to fuse.

Fig. 2.3  Prism Glasses. The prism-lens combination both magnifies the image and re-directs the rays to the outside slightly.  The blankout panel reduces interference from the left picture into the right eye, and vice versa.  These plastic glasses cost about $2, and can be obtained from Steve Berezin  in California.  A fancier variant  is  the "Pokescope" .

Most classic print viewers are made up of straight ahead lenses aimed at a stereo card.  They have the advantage of holding the print and the lenses in alignment.  

The main problem with stereoscopes, viewers, prism glasses, and the like is that the individual image size in the parallel viewing mode is limited to a few inches (say 3",  to 5" at the most, since the latter is about the maximum anyone can fuse, even with the little glasses).  Under magnification (via a rigid frame print viewer, or via the glasses) the picture will look bigger, but any flaws in the presentation will come out.  This means that you should:

a)  Prepare prints (stereo postcards, greeting cards, pictures to astound your friends, competition stereograms, etc.) using as high a photographic standard as you can.  Start out with 3" x 3" individual images set side by side.  If doing this on a digital inkjet printer, use premium glossy paper, the highest printer "dpi" setting, and about twice the recommended dpi feed (e.g. 600 dpi).  Clean the print head (usually by a utility program) as many times as needed to get a high quality product with no artifacts, horizontal dropout lines, or other flaws.  Flaws become more objectionable in 3D viewing because it is unlikely that they will be equal in the each image making up the stereo pair.

b)  If using a computer monitor to preview or exhibit stereos, employ the highest  pixels-per-inch setting available.  This is usually limited by the aperture grill pitch of the monitor.  The best computer monitor screens have a  0.22mm pitch (a little less than  1/100 inch).  This means that a 3" square stereogram's  L (left) image will have about 300 "lines of resolution" on the screen.  This is comparable to the full screen of a standard TV (NTSC, not HDTV of course).  More typical computer monitor images of rational size (3 to 4 inches) can contain about 200 lines of information.  

Of these two methods for showing stereograms, the digital printer is somewhat better, since a high quality ink jet print, even on a cheap printer (like a $70 Epson Stylus Color 777), will have a resolution of about 450 lines per 3 inches of print size.  Note that inkjet dpi is not the same as print resolution, as you will easily find out if you try and print fine parallel lines on your inkjet printer.  

SLIDE VIEWERS (primarily for 35mm transparencies, not digital camera output)

Fig. 2.5a   "Pinsharp" (left) and Franka (right) Viewers for twin 35mm slides.  Available from   Berezin 3D (for achromatic lenses on the Franka).  Pinsharp w/o achromats is poor.  For very high quality but expensive viewers see Jon Golden's 3D Concepts.	Fig. 2.5b  StarD (top)  and LifeLike.  Available from Berezin 3D.   These viewers will not do twin 35's.  They are for Realist type stereo pairs mounted on single cards, as illustrated in Figure 1.11 of this Guide.

35mm slide viewing is not quite as constrained as print viewing, because interference between the images can be eliminated by focusing the eyes directly on individual slides using lenses.   Lenses are of varying quality inside commercially available slide viewers.  For serious work (critical editing, extended sessions, getting the highest detail), look for a viewer with achromatic (color corrected) lenses.  The slides are hand loaded into the viewer in pairs.  This takes a little longer than shuffling prints of stereo pairs.  For orthoscopic viewing, where the image you look at has the same perspective as when it was taken, the focal lengths of the taking and viewer lenses should be the same (ex.  a 35mm lens on the camera and a 35mm lens in the viewer).

The size limitation on prints for stereoscopes and prism glasses may be removed by  

VIEWING LARGE PRINTS OR COMPUTER SCREEN IMAGES

To get around the ~ 3" wide size restriction ( ~ 6" total) on viewing prints, it is necessary

a) not to mount the images side by side, or 

b) to use a more advanced optical method to view them (see the Print Display  chapter).

To get around these problems, several  schemes have been proposed.  They were precursors to modern virtual-reality head-mounted displays (HMD's)    

Fig. 2.6 The Wearing of Mirrors. Mirrors can be used to redirect  the rays from large images to the eyes.  A head clamp is not always required.  The horizontal version (a) comes in various sizes, some that can be hand held for viewing prints of modest size, up to 11" per image. Version (b) was  patented for 3D computer gaming, where L and R are monitors displaying the game.  Never produced AFAIK.   Both methods are quite sensitive to head movements, which make the images shake.   I consider them difficult to use.

Those who want to follow up on large print viewing might take a look at the lightweight  "Hyperview" mirror box, that you hold up with both hands and look at images as large as 11"W x 13"H  (13"H x 22"W for the pair).   The device uses the mirror arrangement of figure 2.6a but is not attached to your head.  It is not as cumbersome as I expected, but you have to hold it level with the print or it hurts (your brain).  Pretty good for prints of this size, but after a while your arms get tired.

	Fig. 2.7  The Hyperview Mirror Box (by David Lee) Top left: Looking into the device with large mirrors at the outside. Bottom left:  The eye and nose side. Above:  John H trying the device on a parallel pair of of Antelope canyon.

Another version is to try a mirror or prism rig that attaches to the computer, rather than to your head, a.k.a.  The ScreenScope.   The optics and baffling in this system are decent but not great  (IMHO).

Well, imagination is on the run!  But two standard methods are still the most used and most reliable (less subject to mirror shake, for example).  And at least one new development for computer junkies looks promising.

ANAGLYPH AND POLARIZING GLASSES

The classic "anaglyph" method requires each viewer to wear red and cyan color filtered glasses.  Red light enters the left eye only (in principle), while blue-green light enters the right.  If the image pair is color coded accordingly, registered appropriately, and overlaid, then a person equipped with anaglyph glasses can view as-large-as-you-like "anaglyphs" in 3D.   In practice, there is some color leakage so that the red (Left) eye may see a little of the cyan signal, and vice versa.  This leads to GHOSTING, which can become a major problem for images that are constrasty (as in the white flower on a black background as shown below).

Fig. 2.8a   Anaglyph Glasses	Fig 2.8b  Color Anaglyph of Avalanche Creek

Anaglyphs are usually quite effective with B&W images.  Color anaglyphs are feasible, and for certain scenes can work reasonably well.  They are prone to color errors, ghosts, and can be difficult to view due to "retinal rivalry" wherein the signals from the left eye cannot be merged with those from the right.  For a good color anaglyph, subject material must be carefully chosen and handled properly.   The Anachrome Company sells high quality anaglyph glasses and their website has useful tutorials.  Glasses are also available at Berezin's 3D shop.

POLARIZING GLASSES

Most  projection 3D shows ( chapter 7 ) are done with polarizing glasses.    Instead of using color to separate between L and R, the left and right images are polarized orthogonally.  Viewers wear special glasses in which the polarization axes are perpendicular and aligned with those of the appropriate projected.

SHUTTER GLASSES

Along with using color or polarization to arrive at L & R distinction of the stereo pair, time sequencing can also be employed.  This is only feasible with computer display viewing. 

Images on a cathode ray tube display (CRT), like a TV or a computer monitor, are sequenced at 60Hz (cycles per second) or faster (computer monitors can go up to ~ 180 Hz).  LCD shutter glasses alternately block out light coming to the left and right eyes. This is done at high frequency in order to eliminate flicker. The shutters are synchronized with the sequential presentation of the left and right images to the monitor by your graphics board.  This sync is effected using a wire or a remote IR transmitter.  Various methods of sending the images to the monitor or TV may be employed. An interlace scheme is sometimes required (e.g. for the inexpensive VRSurfer glasses).  The interlace scheme will work with your TV and you can play 3D VHS tapes.  Higher quality is obtained on a computer screen.  Modern graphics cards often do not support the interlaced display mode, which has the disadvantage that the individual picture resolution is effectively halved in the vertical. Currently, the most accurate computer display of 3D images involves "page flipping”.  In this method, full-screen left and full-screen right images are alternatively transferred to the monitor by the video card itself. High refresh rates (> 100 Hz) are recommended to avoid tiresome flicker.  Most shutter glasses come with software to perform the decomposition of a left-right stereo-jpeg (.jps) pair into the two pages that are subsequently flipped by your graphics adapter.  Software plugins for web-browsers enable you to directly view .jps stereo pair images sent by out by a server.  Only certain graphics cards support such 3D display, but the number is already impressive, prices are reasonable, and the results are about as good as it gets, apart from  the Print Display and 3D Projection setups described later.   Reviews of shutterglass and other technologies may be found at http://www.stereo3d.com/3dhome.htm.

CRITIQUE OF SHUTTERGLASSES:

POSITIVES:  

1)  Shutterglasses enable full screen high resolution viewing on a computer monitor.

2)   A shutterglass set plus a compatible graphics card (if you need one) will set you back about  $150.  Glasses alone can be found for less than $50. 

3)   The full screen 3D can be very good!

NEGATIVES:  

1)   The field is evolving and there is no standard method for triggering the shutterglasses.  Each vendor has its own scheme. Software with stable problem-free drivers is THE PROBLEM.  These programs are difficult to install and de-bug!! They usually will fail as soon as a new Operating System is installed on your computer.   Professional technical help is virtually non-existent.  Get the most recent updates from the vendor's website and have patience!   The result can be worth the work, but implementing shutterglasses is not easy.

2)   Shutterglasses require high refresh rates (>100Hz).  Before jumping in, check your monitor.  LCD flat panel monitors will not work!  Only high speed CRT's are effective.  These are getting hard to find.  Only certain graphics cards can drive shutterglasses.  Ascertain compatibility before ordering glasses.  

3)   Shutterglasses are dim.  They use liquid crystal technology to switch on and off.  This technology involves alternating the polarization of one layer over another.  Thus there is loss, even for the eye for which the shutter is "open."  For the high quality Elsa Revelator glasses I measured this loss using a Minolta spot-meter.  It is slightly more than 1.5EV, or 1 1/2 stops, or a factor of about 3.  Coupled with the fact that each eye only sees an image half the time (actually less because the switching is not perfect), the net light loss is at least a factor of 6.  This is noticeable.  You have to use this with a good bright monitor in a dark room.  Shutterglasses will not work with LCD flat-panel displays or computer laptop screens.

DUAL MONITOR DISPLAYS

A very effective method of computer viewing of 3D pairs the single mirror dual display (SMDD) system.  In figure 2.10 the left monitor holds a horizontally reversed version of the left image, while the right display carries the normal right image.  A person looks straight at the right monitor, while a front-surface mirror reflects the left image into the left eye.  This method is illustrated below.  The whole arrangement can be flipped with the right monitor holding the mirrored image, etc.

Fig. 2.10  Dual display system.

The author has found this to be the BEST WAY to look at stereo pairs on a computer.  There is a little more hardware involved than for shutterglasses but the software issues are trivial.  The results rival the best than a hand-viewer can do.  This is glasses-free too!

A detailed description is posted on the TechNotes Page (i.e. Dual Monitor Digital Viewing of Stereo Pairs).

A dual monitor solution using polarized glasses and a beamsplitter has been developed.

AUTOSTEREO DISPLAYS

The computer industry is also actively pursuing the Holy Grail of virtual reality, the construction of an auto-stereo display. “Auto-stereo” means unassisted viewing, without any anaglyph glasses, polarizing glasses, LCD shutters, mirrors, or other aids attached to your body.  Head-mounted displays and glasses that contain a TV (or LCD panel) for each eye are available.  Some 3D autostereo computer (single) monitors have already appeared (Click to see list).  Most are based on parallax barrier methodology (see Chapter 6).  They are also expensive, but the manufacturers' stated goal is to sell 3D displays for less than twice the cost of a similar normal display by 2003.  For example, a 15" display that cost $8600 in 1999 now costs $1600.  Another halving or two and the price starts to become reasonable.  If 3D auto-stereo viewing becomes practical, source material will be in great demand. The potential for constructing high-impact teaching modules and research presentations will increase quickly.  It is unlikely that any autostereo single-monitor display will match the dual monitor display system mentioned above in terms of quality.

RECOMMENDATIONS:

Start with a Pokescope and a pair of anaglyph glasses for viewing web material and prints or computer-pairs you make yourself. 

References:

A great site for 3D information, with product reviews, technical details, etc., is http://www.stereo3d.com. Look in the index for “formats” and “shutter glasses for computers” for a discussion of methods and products. Another site with useful information and links is http://www.stereoscopy.com. For the ultimate high end stuff used by companies like SGI see http://www.stereographics.com/products/synthagram/synthagram.htm.