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DUAL MONITOR VIEWING OF DIGITAL STEREO PAIRS Prof. John Hart Program in Atmospheric and Oceanic Sciences University of Colorado Boulder, CO 80302 nimbus.colorado.edu/hart/science.htm
INTRODUCTION We have developed a simple method for viewing 3D content. It is our intent to display 3D simulations of atmospheric and oceanic phenomena to students using a display case in the Duane Physics building of the University of Colorado.
Files containing 3D content from computer simulations or from digital cameras have historically been viewed in a number of ways: 1) small parallel (or "wide-eyed") pairs 2) small "cross-eyed" pairs 3) B&W or color anaglyphs 4) Polarized glasses with special (.e.g micro-polarized) screen or a beam splitter plus two (L & R) monitors. 5) Auto-stereo (no glasses): parallax-barrier or lenticular-screen monitors 6) Shutter glasses None of these methods is ideal. 1 and 2 are limited to small (~3" per eye) pictures, and so contain relatively few pixels. 3 and 4 require special glasses, and may suffer from ghosting, dimming, or lost resolution (because, in the case of micro-polar panels, one screen must carry both the left-eye and right-eye images). 5 also loses resolution for the same reason, and is dim. It is also usually strongly dependent head position, and expensive. To date, shutter glasses have been considered to give the highest quality in digital viewing. Full-frame left and right images are sent sequentially to a monitor in what is called a "page flipping" mode. Left and right shutters in the glasses synchronize the beaming, blocking out the wrong eye (so the left eye is blanked when the right image is on the screen, and vice versa.). While permitting full screen viewing, shutter glasses suffer from several problems: a) They are very dim (losing almost 2 stops in the glasses and 1 stop because each eye is only on less than half the time). b) You must wear (and purchase) the glasses. At less than $100 per pair, these are not the greatest optical devices. As a result image quality is usually lost. c) Software for shutterglasses is often problematic. The page flipping and synchronization must be precise or ghosting is significant and annoying. Special application drivers are usually required. d) High monitor refresh rates must be used. 120Hertz is recommended. Very few high resolution (1600 x 1200 or 2048 x 1556) monitors will go this fast. Those that will are very expensive! NO LCD panels will work with shutterglasses!! DUAL HEAD GRAPHICS CARDS In 2001 and 2002, inexpensive "dual-head" graphics cards have become readily available. These are capable of sending wide-screen images out to two monitors that span the width. The signals come from two independent digital to analog converters, or RAMDACs, on the graphics card. The newer units can drive two very high resolution displays (e.g. 2048 x 1556 each, at 75Hertz refresh, giving a total display of 4096 x 1556). Some examples are: Matrox Parhelia, Gainward MX440, Xtasy MX440. These marvelous devices are inexpensive, the latter two going for around $130. Look for "dual head display", with "dual Ramdac" (at 350MHz or greater). DUAL MONITOR VIEWING OF STEREO PAIRS The idea is simple and extremely effective. 1) Construct a stereo pair by horizontally flipping the left image of a Left-Right parallel pair. The latter can be made by 3D Image Factory, Pokescope, Photoshop, PaintshopPro, or other stereo authoring tools. An example of what you want is:
Fig. 1. Parallel pair (left eye on left, right eye on right).
Fig. 2. Left side is flipped horizontally (e.g. in Photoshop: select the left part of the stereo pair, cut and paste to a temporary image, flip that image, re-paste back into original). 2) Output the modified pair (as in figure 2) to the full screen dual display (set up to drive two side-by-side screens). The image can be as big as 4096 x 1556! (with current 2002 technology). 3) Set up the two monitors for viewing with a single front surface mirror system as diagramed below.
Fig. 3. Top view of the dual monitor system. The composite image is viewed by a simple mirror system. The mirror is placed on the diagonal between the monitors as shown. The viewer looks directly at the right screen, with the right eye just to the right of the leading edge of the mirror. Typical values are: Monitor width W ~ 14" (e.g. a standard 1600 x 1200, 19" monitor), D ~ 16 ", q = 45 degrees. This is a variant of the large-print "stereo-tower" viewing system for display of 3D scientific results on posters at conferences. You can read up on the underlying mathematics, or just carry on here for construction tips. But before going on to the construction details, let's list the advantages of this system (in no particular order): a) There are no ghosts! There is no flicker! None!! b) The images are bright and at full resolution! c) The angle of view can approach orthostereo (for a normal taking lens). Even at D ~ W, a 1600x1200 monitor's pixels are almost invisible (at least to Prof. Hart's tired old eyes). d) The quality of the experience (with 2048 x 1556 monitors) approaches the best a hand-held slide-viewer with 35mm frames can offer! Of course the potential image sources are much more numerous than just carefully hand-mounted slide transparencies. Your images can come from digital cameras, computer generated graphics, the web, email sharing.....) e) Digital effects (transitions, animations, videos, etc.) can be done (though, in practice, handling 4096 x 1556 files will require some horsepower and innovation in programming perhaps). To do full screen movies, you can just set your display resolution to 640 by 480 and go for it. f) The cost is not terribly unreasonable. If you already have one good monitor, another (at 1600x1200, 75Hz, say), along with a dual-head graphics card will set you back less than $500. Shutterglasses, a compatible graphics card, software, and a 120Hz 1280x1024 19" monitor, will cost just about the same. Note: It is best to have two identical monitors. g) Contrary to intuition, there is no keystone effect in the mirrored print (provided the mirror position is adjusted properly). Angling out the left monitor is important because it allows the viewer to look at the images on center (this is obvious for the right one, see figure 3). Previous incarnations of the single-mirror, flipped-image stereoscope go back in history over 150 years. One use is to view flat material (like a book spread out on a table) with a perpendicular mirror. This forces the viewer to look slantwise at the images (like standing in an art gallery and looking at all the pictures from the left edge over. To verify the absence of keystoning in the left image for our setup, there is a mathematical proof at the bottom of this link. h) Setup is relatively easy. Head position is important BUT NON-CRITICAL. Depending on mirror size (larger is better), variations of one to two inches side to side and front to back are OK. Vertical displacement is no problem at all. However it is best to adjust the seat so that your eyes are at mid monitor height. i) Illumination (being two identical monitors) is uniform. When this method is applied to prints, it is important to guarantee equal uniform illumination on both images. This may be problematic if the images are really large gallery sized exhibit prints. Here, bright uniform lighting is automatic. j) Unlike the situation with most hand-held slide viewers, the images are sharp corner to corner. There are no intermediate-lens aberrations (usually manifest at the edges in hand-held twin-lens viewers), magnified dust or scratches, non-uniform lighting, etc. Viewing stereo pairs digitally does not get any better than this!! (IMHO)
CONSTRUCTION TIPS Pictures are better than words......
Fig. 4. Dual monitor setup. The mirror is attached to a 3-way tripod head (or a ball head). This setup has the mirror support extending out from the head, leaving room for the keyboard. A front-surface mirror is recommended (else you will get a little fuzziness in the left image). A scientific grade front surface mirror is not required because the magnification is minimal. You just want to eliminate the slight ghost reflection that you get from the first surface of a normal back-coated mirror. One source for mirrors is the Stained Glass Warehouse. For example, they sell a 16" x 12" front-surface mirror (which is just about right) for ~ $20.
Fig. 5 Viewing 3D. Look directly at the right screen. Eyes straddle the leading edge of the mirror.
Fig. 6 It is best if the monitor center is at eye level. The screens are first set perpendicular to the table. Because monitor swivels don't usually tip side to side, it is hard to mount the monitors correctly if you look down on them. That is, the two displays must fold like a book. If you are looking down, the book must be tipped back at the top. You can only do this by tipping the whole platform, swivels and all (possible, but not for my first try).
Fig. 7. Adjust the left screen to be 45 degrees w.r.t. the right. Angle is not critical.
Fig. 8. Mount a wood or plastic sheet on the backside of the mirror with silicone rubber. Drill holes at the bottom so that a small aluminum angle bracket may be attached. This then permits the mirror to be held upright by the tripod head (see figure 4).
Fig. 9 Riser box, with tripod head.
Fig. 10 A weight counterbalances the offset mirror (figure 4).
Fig. 11. Positioning is important. Rotating the mirror about the vertical axis, and moving the box horizontally on the table allows you to get things pretty well aligned. Such movements are carried out with an image or grid on the screens. With a little practice it doesn't take long to get the top-bottom and side-to-side borders to overlap almost exactly. A final, AND VERY USEFUL, adjustment may be made by using the left (or right) monitor's horizontal and vertical position controls to tweak the overlay.
Fig. 12. When not being used for viewing 3D's, the mirror can be carefully swung aside, and the display reset to left-only. If you don't whack anything, swinging the mirror back until the edges line up will enable a quick return to virtual reality. Summary of Equipment Recommendations: Two 1600 x 1200 FLAT SCREEN color monitors (75Hz) or LCD panels. The system works pretty well with 1280x1024, and is even better with 2048x1556.... For ease of alignment and good stereo quality the two monitors should be same make and model. A Dual Head (dual VGA connectors) graphics card (Matrox Parhelia, Gainward MX440, Xtasy MX440, etc.) Note: Matrox G450, while under $100, won't do 1600 x 1200 on the second screen. Front-surface mirror (commercial or hobby grade). Try Stained Glass Warehouse and search on mirrors. Adjustable support for mirror (3-way head, ball head).
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