HPGL, sometimes hyphenated as HP-GL, was the primary printer control language used by Hewlett-Packard plotters. The name is an initialism for Hewlett-Packard Graphics Language. It later became a standard for almost all plotters. Hewlett-Packard's printers also usually support HPGL in addition to PCL.
The language is formed from a series of two letter codes, followed by optional parameters. For instance an arc can be drawn on a page by sending the string:
This means Arc Absolute, and the parameters place the center of the arc at absolute coordinates 100,100 on the page, with a starting angle of 50 degrees measured counter-clockwise. A fourth optional parameter (not used here) specifies how far the arc continues, and defaults to 5 degrees.
Typical HPGL files start with a few setup commands, followed by a long string of graphics commands. The file was in ASCII (text file) format, for instance:
The coordinate system was based on the smallest units one of the HP plotters could support, and was set to 25 µm (i.e. 40 units per millimeter, 1016 per inch). The coordinate space was positive or negative floating point numbers, specifically ± 230.
The original HP/GL-Language did not support definition of line width, as this parameter was determined by the pens loaded into the plotter. With the advent of the first inkjet plotters, line width for the "pens" specified within the HP/GL-files had to be set at the printer so it would know what line width to print for each pen, a cumbersome and error-prone process. With HP/GL-2, definition of line width was introduced into the language and allowed for elimination of this step. Also, among other improvements a binary file format was defined that allowed for smaller files and shorter file transfer times, and the minimal resolution was reduced.
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