HP-GL syntax, units and encoding

HP-GL is the language of Hewlett-Packard’s pen plotters; HP-GL/2 is its successor for plotters and printers, and is also built into PCL 5. HP-GL/2 is defined in HP’s The HP-GL/2 and HP RTL Reference Guide, which this page follows; HP-GL was described in each plotter’s programming manual. A plot file is the stream of instructions a program sends to the plotter.

Instructions

An instruction is a two-letter mnemonic, optional parameters, and a terminator:

PA 1000,2000;   PD3000,2000,3000,4000;   LBHello<ETX>

(<ETX> stands for the single byte 0x03.)

Mnemonic: two letters, written in upper case.
Separators between parameters: a comma (recommended), a space, or, for numbers, a + or - sign starting the next one. DT, LB, PE and SM take no separators.
Terminator: HP recommends none: the next mnemonic ends the instruction (PDPU10,20). A semicolon, or any non-numeric character, also ends one. PE and PG and the output instructions must end with a semicolon, LB ends with the label terminator, the CO text is in double quotes, and a semicolon is required after the last instruction before leaving HP-GL/2.
An optional parameter can only be left out together with all the parameters after it (DT is the exception).

Parameter types

Type	Range in HP-GL/2	Notes
Integer	−8,388,608 to 8,388,607 (±2²³)	A fraction is rounded to the nearest integer; values outside may give unexpected results.
Clamped integer	−32,768 to 32,767	Values outside are clamped to the limits; some parameters allow less.
Real	−8,388,608.0000 to 8,388,607.9999	At least six significant digits; the decimal point may be left out; no exponent form.
Clamped real	−32,768.0000 to 32,767.9999	As above, clamped.
Label text	any characters	Up to the label terminator: ETX (0x03) by default, changed with `DT`.
Quoted string	text in `"`	Used by `CO` and `BP`; `""` is a quotation mark inside it.

These ranges are minimums; a device may accept more. Coordinates are in current units: user units when scaling is on, plotter units when it is off.

Units and coordinates

The plotter unit is 0.025 mm: 40 to the millimetre, 1,016 to the inch. Unscaled coordinates are in plotter units.
The origin is a corner of the plot area that depends on the device; in PCL 5 printers it is a corner of the PCL picture frame.
P1 and P2 are two scaling points. On printers they default to the lower-left and upper-right corners of the hard-clip limits (the physical boundary of printing); on plotters the defaults depend on the device. IP and IR move them.
User units: SC maps P1 and P2 to user coordinates, so drawing commands can use any units; with SC off, plotter units apply.
RO rotates the coordinate system by 90° steps; IW clips to a window.
Pen widths and some sizes are in millimetres or as a percentage of the P1–P2 diagonal, as set by WU.

PE: polyline encoded

HP-GL/2’s PE instruction packs a run of coordinates into printable characters, about half the size of plain numbers. After PE come flags and numbers, up to ;:

Flag	Meaning
`:`	Select pen: the next number is a pen number.
`<`	Pen up: move to the next pair with the pen raised (in rectangle mode, also return to line mode).
`>`	Fractional data: the next number gives how many binary fraction bits the coordinates have (default 0).
`=`	The next point is absolute; others are relative to the previous point.
`7`	Seven-bit mode: numbers use base 32 rather than base 64 for the rest of the instruction, and eighth bits are ignored.
`9`	Rectangle mode: each following quadruplet is a pen-up move and then a relative rectangle filled with the current fill type.

Each number is encoded as follows. A signed value v is first made unsigned: 2v if positive, 2|v| + 1 if negative (the sign goes in the lowest bit). It is then written in base 64 (or 32), least significant digit first. In base 64, every digit but the last is the byte 63 + d (63–126) and the last is 191 + d (191–254). In base 32 every digit but the last is 63 + d (63–94) and the last 95 + d (95–126). For fractional data the value is first multiplied by 2ⁿ and rounded, where n is the number of fraction bits given by >. Flags are plain characters (their eighth bit is ignored); numbers after : and > are encoded like coordinates. Coordinates come in x, y pairs, with the pen down unless < precedes the pair.

What a file looks like

A plot normally starts by setting known conditions with IN (all defaults) or DF (most defaults), then selects a pen with SP.
On devices with the Technical Graphics extension, a plot starts with BP, which also performs IN; MT, PS and QL must come in this picture header, before any drawing. A picture ends at PG, RP or the next BP.
In a PCL 5 job, Esc%0B or Esc%1B enters HP-GL/2 and Esc%0A or Esc%1A returns (see the PCL page). Devices with HP RTL use the same commands to switch between vector and raster data.

Sources

HP, The HP-GL/2 and HP RTL Reference Guide (third edition, 1997) – HP-GL/2 syntax, parameter formats, units of measure, the PE instruction and its encoding, the picture header
HP, PCL 5 Printer Language Technical Reference Manual, part II (bpl13211) – HP-GL/2 inside PCL 5