repertory was used to develop a monowidth type font suitable for providing renditions of computer card listings (See figure 13). Another routine has been written to generate vue-graphs using the Hershey character set (See figure 14). The vue-graphs are prepared by first making an ink drawing of the vue-graph material with a digital plotter, then making a reduction on an office copying machine. From this the final vue-graph can be made using a thermal printer. Other applications will be limited only by the ingenuity of the user.
5. Operational Considerations
In order for a digital character set to be useful, there must be a way of transforming the digital characters to a graphic output medium, be it microfilm or hard copy. While in principle it would be possible to do this on-line with a main frame computer, in practice it is more convenient to generate an intermediate file on magnetic tape, and then to use this magnetic tape to drive the graphic output device. The computer program used to generate the intermediate tape file depends on the output characteristics desired. We have purposely used a variety of programs in order to indicate the flexibility of the digital character set. For vue-graphs (figure 14), or computer-listings (figure 1) for which no typography (justification, hyphenation, paging) is required, a simple FORTRAN program is all that is needed. Where more typographic features are desired as in composing tables or pages of formulae or equations, a typographic program must be used. Two typographic programs were used to prepare this publication The first is Dr. A V. Hershey's Typographic System' which was used as indicated above. The second approach, by means of which the body of this report was generated, was to use an existing NBS typography program which performed the line justification and page makeup. That program was modified to use the Hershey character set and associated character widths.
For the highest quality output a large flat-bed pen plotter with multi-pen capability should be used. This is the technique used by Hershey to produce his reports (Ref. 3-10). Fairly good quality can also be obtained on a drum plotter, but variable line widths are difficult to obtain if a turret pen is not available. The speed of a pen plotter is one to two characters per second depending on the size and complexity of the character.
The next best quality is obtained with a 35mm graphics COM recorder with 16,000 addressable rasters in each of the x and y directions. This is the resolution of the COM device on which the bulk of this publication was produced
A graphics COM recorder customarily consists of a tape drive, a programmable mini-computer, perhaps a disc drive, a cathode ray tube on which the images are drawn by the CRT electron beam, a 35mm camera which photographs the CRT tube face, and a film transport mechanism which advances the film between frames. The beam on the CRT tube face can be directed to one of approximately 12,000 x 16,000 addressable positions. The minimum separation between addressable positions is called a raster unit. The minimum spot size is larger than a raster unit - in the FR80 it is approximately seven raster units. The 35mm film can be developed to produce either a positive or a negative image. The 35mm film is then used to produce hard copy or printing plates by photographic techniques.
To use the vector characters for different type sizes, it is essential that a method be available for varying the width of the strokes on the output device. On
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