Digistar is the first computer graphics-based planetarium projection and content system. It was designed by Evans & Sutherland and first released in 1983. The technology originally focused on accurate and high quality display of stars, including for the first time showing stars from points of view other than Earth's surface, travelling through the stars, and accurately showing celestial bodies from different times in the past and future. Beginning with the Digistar 3 the system now projects full-dome video.
Computer graphics is the discipline of generating images with the aid of computers. Today, computer graphics is a core technology in digital photography, film, video games, cell phone and computer displays, and many specialized applications. A great deal of specialized hardware and software has been developed, with the displays of most devices being driven by computer graphics hardware. It is a vast and recently developed area of computer science. The phrase was coined in 1960 by computer graphics researchers Verne Hudson and William Fetter of Boeing. It is often abbreviated as CG, or typically in the context of film as CGI.
A planetarium projector is a device used to project images of celestial objects onto the dome in a planetarium.
Evans & Sutherland is a pioneering American computer firm in the computer graphics field. Its current products are used in digital projection environments like planetariums. Its simulation business, which it sold to Rockwell Collins, sold products that were used primarily by the military and large industrial firms for training and simulation.
Unlike modern full-dome systems, which use LCD, DLP, SXRD or Laser projection technology, the Digistar projection system was designed for projecting bright pinpoints of light representing stars. This was accomplished using a calligraphic display, a form of vector graphics, rather than raster graphics. The heart of the Digistar projector is a large cathode-ray tube (CRT). A phosphor plate is mounted atop the tube, and light is then dispersed by a large lens with a 179 degree field of view to cover the planetarium dome.
Digital Light Processing (DLP) is a set of chipsets based on optical micro-electro-mechanical technology that uses a digital micromirror device. It was originally developed in 1987 by Larry Hornbeck of Texas Instruments. While the DLP imaging device was invented by Texas Instruments, the first DLP-based projector was introduced by Digital Projection Ltd in 1997. Digital Projection and Texas Instruments were both awarded Emmy Awards in 1998 for the DLP projector technology. DLP is used in a variety of display applications from traditional static displays to interactive displays and also non-traditional embedded applications including medical, security, and industrial uses.
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for "light amplification by stimulated emission of radiation". The first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.
Calligraphic projection is a system for displaying or projecting an image composed of a beam of light or electrons directly tracing the image, as opposed to sweeping in raster order over the entire display surface, as in a standard pixel-based display. Calligraphic projection is presently often used for laser lighting displays, whereby one or more laser beams draws an image on a screen by reflecting the laser beam from one or more mirrors attached to a deflecting mechanism.
The coordinates of the stars and wire-frame models to be displayed by the projector were stored in computer RAM in a display list. The display would read each set of coordinates in turn and drive the CRT's electron beam directly to those coordinates. If the electron beam was enabled while being moved a line would be painted on the phosphor plate. Otherwise, the electron beam would be enabled once at its destination and a star would be painted. Once all coordinates in the display list had been processed, the display would repeat from the top of the display list.
A wire-frame model, also wireframe model, is a visual representation of a three-dimensional (3D) physical object used in 3D computer graphics. It is created by specifying each edge of the physical object where two mathematically continuous smooth surfaces meet, or by connecting an object's constituent vertices using (straight) lines or curves. The object is projected into screen space and rendered by drawing lines at the location of each edge. The term "wire frame" comes from designers using metal wire to represent the three-dimensional shape of solid objects. 3D wire frame computer models allow for the construction and manipulation of solids and solid surfaces. 3D solid modeling efficiently draws higher quality representations of solids than conventional line drawing.
Thus, the shorter the display list the more frequently the electron beam would refresh the charge on a given point on the phosphor plate, making the projection of the points brighter. In this way, the stars projected by Digistar were substantially brighter than could be achieved using a raster display, which has to touch every point on the phosphor plate before repeating. Likewise, the calligraphic technology allowed Digistar to have a darker black-level than full-dome projectors, since the portions of the phosphor plate representing dark sky were never hit by the electron beam. As it is only one tube, with no pixelated color filter screen, the Digistar projector is monochromatic. The Digistar projects a bright, phosphorescent green, though many (including both visitors and planetarians) report they cannot distinguish between this green and white.
Additionally, unlike a raster display, the calligraphic display is not discretized into pixels, so the displayed stars were a more realistic single spot of light, without the blocky or ropy artifacts that are hard to avoid with raster graphics. Due to the use of vector graphics, as opposed to raster imaging, the Digistar does not have the resolution issues that many full-dome systems have. Thanks to this, and the brightness of the CRT, only one projector is needed to project on the entire dome, whereas most full-dome systems require up to six raster projectors, depending on dome size.
Anti-aliasing may refer to any of a number of techniques to combat the problems of aliasing in a sampled signal such as a digital image or digital audio recording.
The projector in the original Digistar was housed in a square pyramid-shaped sheathing. When powered on, the four sides at the tip of the pyramid would recede into the housing, exposing the lens and appearing as a cut-off pyramid.
As Digistar II was being developed, many planetaria were sold Digistar LEA projectors. The LEA, called Digistar 1.5 by many users, was effectively a prototype of the D2 projector, compatible with Digistar and upgradable to Digistar II. There are no significant differences in performance between the LEA and the true D2.
History
Digistar was developed by a small team of engineers within the Simulation Division of Evans and Sutherland. The system was the brainchild of Stephen McAllister and Brent Watson, both of whom were long-time amateur astronomers and computer graphics engineers.[1][2] Work began in approximately 1980. The primary goal of the system was to use computer graphics to overcome the limitation of traditional star ball technology that only allowed display of star fields from the point of view of Earth's surface. By using computer graphics the stars could be displayed from viewpoints in space, including simulating the appearance of space flight. Likewise, planets and moons within the solar system could be displayed accurately for any time in history, from any point of view. The system used the location of real stars from the Yale Bright Star Catalogue, as well as random stars.
The Bright Star Catalogue, also known as the Yale Catalogue of Bright Stars, Yale Bright Star Catalogue, or just YBS, is a star catalogue that lists all stars of stellar magnitude 6.5 or brighter, which is roughly every star visible to the naked eye from Earth. The catalog lists 9,110 objects, of which 9,095 are stars, 11 are novae or supernovae, and 4 are non-stellar objects which are the globular clusters 47 Tucanae and NGC 2808 (HR 3671), and the open clusters NGC 2281 (HR 2496) and Messier 67 (HR 3515).
A laboratory prototype of Digistar was used to generate the star fields and tactical displays in the science fiction film Star Trek II: The Wrath of Khan. Filming was done directly from the Digistar display in the lab.[3]:1038[4] The Digistar team members are credited in the film.
Star Trek II: The Wrath of Khan is a 1982 American science fiction film directed by Nicholas Meyer and based on the television series Star Trek. It is the second film in the Star Trek film series, and is a sequel to Star Trek: The Motion Picture (1979). The plot features Admiral James T. Kirk and the crew of the starship USS Enterprise facing off against the genetically engineered tyrant Khan Noonien Singh, a character who first appeared in the 1967 Star Trek episode "Space Seed". When Khan escapes from a 15-year exile to exact revenge on Kirk, the crew of the Enterprise must stop him from acquiring a powerful terraforming device named Genesis. The film is the beginning of a story arc that continues with the film Star Trek III: The Search for Spock (1984) and concludes with the film Star Trek IV: The Voyage Home (1986).
After prototyping in labs at Evans and Sutherland the team worked out a deal with Salt Lake City's Hansen planetarium to beta test the system at the planetarium at night. In exchange, the planetarium received an improved prototype Digistar to replace "Jake", the planetarium's aging Spitz planetarium projector.[5]
The first customer installation was to the newly constructed Universe Planetarium at the Science Museum of Virginia in 1983, the largest planetarium dome in the world at the time.[6]
Digistar was driven by a VAX-11/780 minicomputer, with custom graphics hardware related to the E&S PS/300.
The original Digistar had a physical control panel that was used for running the star shows. This control panel was approximately 3' x 4' and contained a keyboard, a 6 DOF joystick, and a large array of back-lit buttons. One button that was used for moving the viewpoint forward in space was labeled "Boldly Go".
Later iterations of Digistar replace the physical control panel with a common graphical user interface.
Digistar 3 was the first Digistar system to offer full-dome video in 2002, using six projectors. Digistar 4 was able to cover the dome using only two projectors.
System limits
Though technologically advanced in its day, and the closest system to true full-dome at the time of its release, the D2 is a limited system. The D2 can only project dots and lines—meaning only wireframe models can be projected. To compensate for this, the projector is capable of "defocusing" specific models, blurring lines and dots together. An example of this is in the D2's built-in milky way model. The model is a circle of parallel lines that, when defocused, appear as the continuous band of the milky way across the sky. On more complex models, especially three-dimensional ones, brightness and details may be lost in this process, so it is not useful in all situations.
The D2 also suffers focus limitations. Because it uses a single lens to cover the entire dome, it is difficult to gain perfect focus across the dome. Coupled with this, stars greater than a certain brightness are "multihit" points, meaning the projector draws two dots at the given position to accommodate for the brightness of the star. As in the Digistar, errors in the projector can lead for the second dot to be slightly out-of-place with the first one. These two issues together, along with other issues that can occur within the projectors focus system, give the stars a blobby look. Many planetarians, used to the pinpoint opto-mechanical projector stars ubiquitous in the day, rejected the Digistar and D2 because of this, ignoring the other advantages of the system.
The CRT in the Digistar and D2 begins to burn out and lose brightness after roughly 1000 hours of use. This means most planetariums must change out the tube after every year or year-and-a-half.
File types
While Digistar ran off large VAX computers, Digistar II runs off the much more compact and advanced Sun MicrosystemsSPARCstation 5. D2 uses two primary file types, .vl and .sf. .vl files are binary models files, while .sf files are binary show data files. Model files contain vector, line and dot data, as well as parametric changes to data within the file, show files contain commands to the system, regarding the manipulation of the observer and models declared within the file. Several show files are often strung together underneath each other in show production. Both .vl and .sf have ASCII equivalents for editing--.vla and .sfa respectively. These are converted to their binary equivalents by a utility built into the Digistar system, which also checks for errors within the file. Digistar II show files are programmed in a language related to Pascal.
Further, Digistar II can run animation files, .af, with the ASCII format .afa. An animation file consists of several model files, grouped together and loaded as one object. The Digistar II can either select frames individually, or animate the entire file.
Digistar II is able to convert Digistar show and model files. Similarly, Digistar 3 is can convert Digistar II model files, though it cannot, at this time, convert show files.
Popularity
Despite its limits, the original Digistar was well received by many planetarians, and has been distributed worldwide. Though it lacked the pin-point stars of opto-mechanical projectors, and the full-dome rendering abilities of the later Digistar 3, many planetarians consider it a good balance between the two, especially considering the novel capabilities of seeing heavenly bodies from any point in space and time. The Digistar line has an installed base of over 550 planetaria as of 2019.[8]
Terence Murtagh, past president of the International Planetarium Society, stated in 2000, "I think the next ten years will see the most dramatic advances in all-dome presentations since the invention of the projection planetarium in the 1920s and the arrival of the electronic Digistar in the 1980s."[9]
The Digistar Users Group has been operating since the mid-1980s and consists of several hundred facilities that have installed Digistar systems.
Related Research Articles
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↑ Smith, Alvy Ray (October 1982). "Special Effects for 'Star Trek II': The Genesis Demo Instant Evolution with Computer Graphics". American Cinematographer.
↑ Smith, Dale W. (March 1, 2000). "President's Message"(PDF). Planetarian: Journal of the International Planetarium Society. International Planetarium Society. Retrieved October 30, 2019.
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