A Digistar II VLA file is supported by a number of utilities supplied with the Digistar system, it facilitates the migration of geometry from third party applications to the Digistar planetarium dome projection. The files are text (ascii) only, they consist of a header section followed by the coordinate information. The text in a VLA file is case insensitive. A line starting with a semicolon ";" is taken to be a comment, these may appear anywhere and the comment continues until the end of the line.Header
The header consists of a number of keywords indicating different header information, each line is of the following form.
set keyword data_for_this_keyword
The keywords can be from the following list, they can appear in any order, not all are required eg: author, and some can occur many times, eg: comments.
Any comment follows to the end of the line. Note that unlike ";" comments, these are intended to convey higher level information about the model while the ";" comments are lower level and related more to the actual geometric elements.
Intended to be used for the author of the file, may be a program name.
Intended for the location the file is created at or where it is intended for.
This may either be RIGHT (default) or LEFT, this not only sets the coordinate system to a right or left handed one but also sets how the coordinates are interpreted. Left assumes meters, right assumes light years.
Either EXPLICIT or FULL, the later ignores the intensity setting provided with the geometry.
Options: stellar, obstinate, galactic and precess. Obstinate files are not affected by any motion of the observer. Galactic files are affected by viewer rotations but not translations. Precess files are the same as galactic, except that over time they precess with the Earth.
Either OLD or NEW, this relates to the version of vla file. The format discussed here is the "new" format, the older format was used for the Digistar I.
This can be either DOTS or LINES, the former indicates that only dots are used, the later allows both dots and lines.
This is set to either PARAMETRIC or NON_PARAMETRIC. A parametric file has an offset for each point in the file, this is the step size that can be used to linearly interpolate points in a line, possibly morphing from one form to another.
This expects a number as an argument and sets a intensity falloff with distance from the viewer. The intensity falloff is 1/rn, so "set depthcue 0" results in no intensity drop with distance, 1 is a 1/r dependence, etc.
Each line contains one geometric element consisting of the following fields.
datatype x y z intensity
The datatype can be one of the following, it is based upon the notion of moving a beam in either an on or off state in a similar way to pen plotters for example.
Draw a dot at the position specified by the (x,y,z) coordinate and with the intensity supplied. The intensity ranges from 0 (no light) to 1 (brightest).
Move the beam to the position specified (the intensity is ignored). The (x,y,z) coordinates in any of these geometry types can be specified in scientific notation where "e" represents powers of ten, for example: 1.23456e5 is the same as 12345.
Draw a line segment from the current beam position to the position specified on this line, use the intensity specified. So to draw a single line segment one needs to ussue a "P" followed by a "L" command. Connected line segments can be drawn with subsequent "L" commands.
This is the step size along each coordinate (dx,dy,dz) for parametric files. For some parameter "u" between 0 and 1, the next point will be (x + u dx,y + u dy,z + u dz). This only applies to parametric files and this variation must be defined for every point in the geometry section. See parametric header keyword.
The following is a simple example of a unit cube sitting on the x-y plane. Note that this could be made much more efficient by reducing the number of "P" commands and drawing longer sequences of connected lines, this is an important consideration for more complicated models where the vector throughput may limit the datarate.
set comment cube.vla created by IVCON. set intensity EXPLICIT set parametric NON_PARAMETRIC set filecontent LINES set filetype NEW set depthcue 0 set defaultdraw STELLAR set coordsys RIGHT set author IVCON set site Buhl Planetarium set library_id UNKNOWN P -0.500000 -0.500000 1.000000 1.00 L 0.500000 -0.500000 1.000000 1.00 L 0.500000 0.500000 1.000000 1.00 L -0.500000 0.500000 1.000000 1.00 L -0.500000 -0.500000 1.000000 1.00 P 0.500000 -0.500000 0.000000 1.00 L -0.500000 -0.500000 0.000000 1.00 L -0.500000 0.500000 0.000000 1.00 L 0.500000 0.500000 0.000000 1.00 L 0.500000 -0.500000 0.000000 1.00 P -0.500000 -0.500000 0.000000 1.00 L -0.500000 -0.500000 1.000000 1.00 L -0.500000 0.500000 1.000000 1.00 L -0.500000 0.500000 0.000000 1.00 L -0.500000 -0.500000 0.000000 1.00 P 0.500000 -0.500000 1.000000 1.00 L 0.500000 -0.500000 0.000000 1.00 L 0.500000 0.500000 0.000000 1.00 L 0.500000 0.500000 1.000000 1.00 L 0.500000 -0.500000 1.000000 1.00 P -0.500000 -0.500000 0.000000 1.00 L 0.500000 -0.500000 0.000000 1.00 L 0.500000 -0.500000 1.000000 1.00 L -0.500000 -0.500000 1.000000 1.00 L -0.500000 -0.500000 0.000000 1.00 P -0.500000 0.500000 1.000000 1.00 L 0.500000 0.500000 1.000000 1.00 L 0.500000 0.500000 0.000000 1.00 L -0.500000 0.500000 0.000000 1.00 L -0.500000 0.500000 1.000000 1.00From the Digistar technical overview
Evans & Sutherland's star projection system, Digistar II, is a real-time, computer graphics based system intended to entertain and educate large audiences. Show producers can create environments that completely immerse audiences in 3D real-time computer generated images. Digistar II is a special-purpose computer graphics system that can do everything a conventional planetarium system can do in addition to other advanced features. Besides simulating traditional planetarium objects, Digistar II generates new special effects, such as dynamic proper motion and travel through three-dimensional space.Further examples