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A plotter is a computer printing device for printing vector graphics. In the past, plotters were widely used in applications such as computer-aided design, though they have generally been replaced with wide-format conventional printers, and it is now commonplace to refer to such wide-format printers as "plotters," even though they technically aren't.



Pen plotters print by moving a pen or other instrument across the surface of a piece of paper. This means that plotters are restricted to line art, rather than raster graphics as with other printers. Pen plotters can draw complex line art, including text, but do so very slowly because of the mechanical movement of the pens. Pen plotters are often incapable of creating a solid region of color, but can hatch an area by drawing a number of close, regular lines. This was often the fastest way to efficiently produce very large drawings or color high-resolution vector-based artwork when computer memory was very expensive and processor power was very limited

Traditionally, printers were primarily for printing text. This made them fairly easy to control; simply sending the text to the printer was usually enough to generate a page of output. This is not the case of drawing line art on a plotter, where a number of printer control languages were created to send the more detailed commands like "lift pen from paper", "place pen on paper", or "draw a line from here to here". The two common ASCII-based plotter control languages are Hewlett-Packard's HPGL2 or Houston Instruments DMPL, with commands such as "PA 3000,2000; PD".[1]

Programmers using FORTRAN or BASIC generally did not program these directly, but used software packages, such as the Calcomp library, or device independent graphics packages, such as Hewlett-Packard's AGL libraries or BASIC extensions or high end packages such as DISSPLA. These would establish scaling factors from world coordinates to device coordinates, and translate to the low level device commands. For example, to plot X*X in HP 9830 BASIC, the program would be

10 SCALE -1,1,1,1
20 FOR X =-1 to 1 STEP 0.1
30 PLOT X, X*X
50 PEN
60 END

Pen plotters have essentially become obsolete, and have been replaced by large-format inkjet printers and LED toner based printers. Such printers are often still known as plotters, even though they are raster devices rather than pen based plotters by the definition of this article. The newer plotters still understand vector languages such as HPGL2. This is because the language is an efficient way to describe how to draw the file using just text commands. A technical drawing in HPGL2 can be quite a bit smaller file than the same drawing in a pure raster form.

Early pen plotters, e.g., the Calcomp 565 of 1959, worked by placing the paper over a roller that moved the paper back and forth for X motion, while the pen moved back and forth on a track for Y motion. The paper was supplied in roll form and had perforations along both edges that were engaged by sprockets on the rollers.

Another approach, e.g. Computervision's Interact I, involved attaching ball-point pens to drafting pantographs and driving the machines with motors controlled by the computer. This had the disadvantage of being somewhat slow to move, as well as requiring floor space equal to the size of the paper, but could double as a digitizer. A later change was the addition of an electrically controlled clamp to hold the pens, which allowed them to be changed, and thus create multi-colored output.

Hewlett Packard and Tektronix produced small, desktop-sized flatbed plotters in the late 1960s and 1970s. The pens were mounted on a traveling bar, whereby the y-axis was represented by motion up and down the length of the bar and the x-axis was represented by motion of the bar back and forth across the plotting table. Due to the mass of the bar, these plotters operated relatively slowly.

In the 1980s, the small and lightweight HP 7470 introduced the "grit wheel" mechanism, eliminating the need for perforations along the edges, unlike the Calcomp plotters two decades earlier. The grit wheels at opposite edges of the sheet press against resilient urethane-coated rollers and form tiny indentations in the sheet. As the sheet is moved back and forth, the grit wheels keep the sheet in proper registration due to the grit particles falling into the earlier indentations, much like the teeth of two gears meshing. The pen is mounted on a carriage that moves back and forth in a line between the grit wheels, representing the orthogonal axis. These smaller "home-use" plotters became popular for desktop business graphics and in engineering laboratories, but their low speed meant they were not useful for general printing purposes, and different conventional printer would be required for those jobs. One category, introduced by Hewlett Packard's MultiPlot for the HP 2647, was the "word chart", which used the plotter to draw large letters on a transparency. This was the forerunner of the modern Powerpoint chart. With the widespread availability of high-resolution inkjet and laser printers, inexpensive memory and computers fast enough to rasterize color images, pen plotters have all but disappeared. However, the grit wheel mechanism is still found in inkjet-based, large format engineering plotters.

Plotters were also used in the Create-A-Card kiosks that were available for a while in the greeting card area of supermarkets that used the HP 7475 six-pen plotter.

Plotters are used primarily in technical drawing and CAD applications, where they have the advantage of working on very large paper sizes while maintaining high resolution. Another use has been found by replacing the pen with a cutter, and in this form plotters can be found in many garment and sign shops.

If a plotter was commanded to use different colors it had to replace the pen and select the wanted color and/or width.

A niche application of plotters is in creating tactile images for visually handicapped people on special thermal cell paper.

Unlike other printer types, pen plotter speed is measured by pen speed and acceleration rate, instead of by page printing speed. A pen plotter's speed is primarily limited by the type of pen used, so the choice of pen is a key factor in pen plotter output speed. Indeed, most modern pen plotters have commands to control slewing speed, depending on the type of pen currently in use.

There are many types of plotter pen, some of which are no longer mass produced. Technical pen tips are often used, many of which can be renewed using parts and supplies for manual drafting pens. Early HP flatbed and grit wheel plotters used small, proprietary fiber-tipped or plastic nib disposable pens.

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