Real-Time, Color, Stereo, Computer Displays Richard I. Land and Ivan E. Sutherland

Harvard University, Cambridge, Massachusetts 02138. Received 22 July 1968.

Goethe said, " I personally should like to renounce speech altogether and, like organic nature, communicate everything I have to say in sketches." The urgent need for computers to communicate large volumes of information unambiguously has motivated recent developments in graphic display techniques. Many computers are now able to plot points and generate lines on CRT displays and can thus sketch black and white line drawings in two dimensions. This letter describes a simple attachment which can make such a display system capable of producing colored stereo pictures in real time.

Previous color systems have been based on one of two tech­niques. Some on-line colored computer displays have used shadow-mask color TV tubes which provide color by using three independent electron guns to excite independent spots of different colored phosphors.1 Other colored pictures have been made from color-separated black and white display pictures by photographic processing.2 Previous stereo systems have been formed by displaying two images on different parts of the CRT face. The two images are then presented separately to the observer's eyes by a viewing system, usually consisting of some mirrors.3 The stereo color display described here uses a small filter disk held just in front of the eyes to interpose different color filters in front of each eye separately. Rotation of the disk is synchronized with the presentation of the stereo pairs and color-separated images.

The basic system consists of a filter wheel whose six segments are successively: red, opaque, green, opaque, blue, and opaque. The computer display tube used has a white phosphor which is reasonably balanced in red, green, and blue output and has fast decay. The red right-eye image is drawn while the right eye is looking through the red filter segment; it is followed by the blue left-eye image which is drawn while the left eye is looking through the blue segment, etc. The rotation speed of the filter is sufficiently fast to offer negligible flicker of the integrated image, but slow enough so the phosphor output from one image has decayed before the color wheel reaches its next position (Figs. 1 and 2) .

We have experimented with our device using pictures produced with a Digital Equipment Corporation PDP–1 computer using type 340 display hardware. Within a display area 25 cm square, image points can be positioned in two dimensions to within 0.3 mm. Control of the relative brightness of the color separation images is provided both by repetition of particular images and a limited range of intensity control. We are using a P -4 phosphor which has a decay time to 10% intensity of less than 100 µsec. We have found the P -4 phosphor to have good blue, adequate green, somewhat weak red response. The colored segments of our wheel are Cinabex (theatrical color media) numbers 6 (primary red), 19 (dark blue), and 39 (primary green). We have also used other filter wheels which

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Fig. 1. Line-synchronous filter wheel for stereo color display in use. Switches select colors, knobs move central objects.

Note spare stereo wheel without color filters on paper.

Fig. 2. Color, stereo, computer display. Photographs of display screen through left- and right-eye holes of rotating filter. Kodak EHB-135 film, ƒ/2.8, 30-sec exposure. (Rotate page 90° and force ocular divergence or convergence to view stereo image.)

are half clear and half opaque to give only stereo effects, or have other filter colors to give a different palette, or which have the blanks replaced with colored segments so both eyes see the picture through the same color for colored nonstereo viewing.

We have tried two techniques of synchronizing the rotating wheel and the display. In one system, the wheel was turned by a computer-driven stepping motor. In the other, the wheel was driven with a power-line synchronous motor and the display clock was also matched to the line frequency. The synchronous technique, being inherently simple, is to be preferred for com­puters which have a line-synchronous clock or interrupt. Mul­tiple viewers can be accomodated if each is provided a wheel.

In one demonstration, the system displayed a hexagon with each side a different color: red, yellow, green, cyan, blue, and magneta. Within this reference object, a rectangle and two diamonds were drawn. Switches on the computer permitted the selection of white or any of the other six colors for the central objects. Knobs provided continuous control of the stereo separation of the diamonds or rectangle, offering the viewer control over the apparent depth of these objects. Thus the rectangle could be made blue, the diamonds yellow, and both be placed at various depths in or out of the display surface. One interesting observation has been that all who experienced the stereo phenomena have been able to make the objects apparently coplaner (to within 0.3 mm separation), regardless of color choice for either object and independent of both being in front or behind the actual display plane. The artistic sense of red advancing and blue receding is apparently more subtle than produced by this experiment.

With three color filters and a white phosphor display tube, any additively produced color is available with allowance made for the spectral character of the viewer or photographic film. Box patterns have been displayed with more than thirty identi­fiable colors present at a time. Complex patterns essentially meaningless in black and white have been given understandable significance by the addition of color. For example, any of the color solids or trees could be represented with the options of moving it to suit the observer, but the actual colors seen would be dependent on both the filter colors and the phosphor spectrum, a variable only poorly controlled at present.

We made a simple at tempt to test the color perception theory of Yilmaz.4 For this test we used a filter wheel with clear and red sectors only, much like the E . H. Land demonstrations.5

In spite of having only red and clear segments, colors other than red can be seen. Unfortunately, the steps of intensity control available to us at present are too crude to offer more than a blue-green, white, several reds, and yellow.

This rotating filter device has proven to offer a considerable advance in the unambiguous display of real-time computer output. I t may also offer an interesting tool for examination of various phenomena of vision. The simplicity, low cost, and adaptability to existing display systems recommends this device as a technique for real-time, stereo viewing of full color displays.

The work reported in this paper was supported in part by the Advanced Research Projects Agency of the Department of Defense under a contract and partly with funds generously provided by the Bell Telephone Laboratories under a long standing relationship with the Harvard Computation Laboratory.

References 1. Sci. Amer. 215, September (1966), Cover photograph from

IBM also DX–1 Color Equipment at Air Force Cambridge Research Labs.

2. I . E . Sutherland, Sci. Amer. 215, 86 (September, 1966); A. S. Oettinger, Sci. Amer. 215, 160 (September, 1966).

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3. S. Poley and C. M. Strauss, "3DPTP—A Three-Dimensional Typing Design Program"—IFIPS Congress (1968).

4. H. Yilmaz, in Proceedings of the International School of Physics Enrico Fermi (Academic Press Inc., New York, 1962), Course 20, pp. 239-51.

5. E. H. Land, Sci. Amer. 208, 200 (1959).

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