HP-GL


HP-GL, short for Hewlett-Packard Graphics Language and often written as HPGL, is a printer control language created by Hewlett-Packard. HP-GL was the primary printer control language used by HP plotters. It was introduced with the plotter in 1977 and became a standard for almost all plotters. Hewlett-Packard's printers also usually support HP-GL/2 in addition to PCL.

Design

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:
AA100,100,50;
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 specifies how far the arc continues, and defaults to 5 degrees.
When first introduced, HP-GL contained the following commands:
Formats:
Typical HP-GL files start with a few setup commands, followed by a long string of graphics commands. The file was in ASCII format, for instance:
CommandMeaning
initialize, start a plotting job
set the scaling points to their default positions
select pen 1
lift Pen Up and move to starting point for next action
put Pen Down and move to the following locations
Pen Up and move to X,Y coordinates 50,50
draw a circle with radius 25
select the standard character set
set the text delimiter to the asterisk, and do not print them
lift the pen and move to 20,80
draw a label

The coordinate system was based on the smallest units one of the HP plotters could support, and was set to 25 µm.
The coordinate space was positive or negative floating point numbers, specifically ±230.

HP-GL/2

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 Hewlett-Packard Graphics Language/2 aka 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..
CommandMeaning
NPxnumber of pens; x=1..256
PCx,r,g,bpen color; x=pen, r=red, g=green, b=blue, 0..255
PWw,xpen width; w=pen width in mm with decimal point, x=pen

AGL

HP-GL is related to AGL, an extension of the BASIC programming language. AGL was implemented on Hewlett-Packard minicomputers to simplify controlling a plotter. AGL commands describe the desired graphics plotting function, which the computer relays as several HP-GL instructions to the plotter.