AGOCG logo
Graphics Multimedia VR Visualisation Contents
Training Reports Workshops Briefings Index

Next Back
Next: Explanatory Notes on Facilities Matrix.

Selecting an Output Device

We look in this section at the various options available when printing graphics images on hardcopy devices (that is, devices which can print graphics images on various types of paper, transparencies, 35mm slides, video, and so on). We do not delve deeply into the detailed mechanisms which produce the hardcopy, rather we examine the available options at a level sufficient to enable the user to make sensible judgements about the sort of output device to use. For readers interested in knowing more about how hardcopy graphics images are produced, there is an introductory overview of colour output device technology in the Tektronix booklet Picture Perfect, referenced at the end of chapter 4; for more detailed information about all types of hardcopy devices, the book Output Hardcopy Devices by Durbeck and Sherr (Academic Press, 1988) provides a wide-ranging set of articles about printer technology; the textbook Computer Graphics: Principles and Practice, referenced in chapter 4, also has a section on hardcopy technologies. Readers who want to know more about what colour printer options are currently available should look at Roy Middleton's Colour Printer Review (AGOCG/EUCS, September 1992). For general advice about graphics printers locally refer to the supplement about local arrangements at the end of this document.

The images that you create on your computer display are virtually certain nowadays to be based on raster technology. However, when you want to produce a hardcopy version of a particular screen-based image, there is a considerable range of potential output devices for you to choose from. The range of equipment available can be bewildering, and the cost of producing a hardcopy picture varies considerably depending on the particular print technology employed, so it is important to decide on the sort of hardcopy that you require in advance of any production runs.

Generally speaking, graphics packages have to support a very wide range of output devices. The way in which they do this, usually, is to provide a special piece of code, called a device driver, for each output device supported. Whenever the need arises to produce output on a particular device, the appropriate device driver is selected. (Note: systems, such as Microsoft Windows and Macintosh, come complete with a set of device drivers which can be selected by the graphics package rather than the package itself having to provide the device driver).

Faithful reproduction

A particularly important point to note at the outset when choosing an output device is that it is difficult, indeed impossible in most cases, to produce exactly the same image on some hardcopy device as the one you have produced on your computer display. This is particularly true where colour images are involved since it is notoriously difficult to produce the same colour on two different types of output device (see, for example, Hopgood' s technical report, as referenced in the section Aspects of Design of the previous chapter). The basic reason for this difficulty is that different output device types employ different underlying technologies.

Other important points to bear in mind are:

Device Classification

Before looking at these aspects, it is useful to classify output devices in some way. One way is to group them into impact versus non-impact, and vector versus raster. Impact devices rely on the use of force to transfer dyes from a ribbon to paper or transparency. Typewriters, daisywheel printers and dot-matrix printers are examples of impact devices. Non-impact devices make no use of force to transfer the colour. Inkjet, thermal-wax transfer and laser printers are examples of non-impact devices.

Vector devices typically use pens to draw lines ('vectors') on the paper or transparency. Pen plotters are the commonest example of a vector device. Raster devices make use of a regular matrix or grid containing thousands of tiny dots (which can be switched on or off, or otherwise coloured in some way) to form the image. Inkjet, laser, and thermal-wax transfer printers, and 35mm slide devices are examples of raster output devices.

Vector devices are generally good at producing high line quality, but raster printers are much more effective at producing filled areas of colour and varied colour shading. Most high- quality computer graphics printers are nowadays non-impact, raster devices.

Device Assessment

Returning to the matters of cost, resolution, quality, etc, which were noted above:

If cost is an issue, then both the initial cost of procuring an output device (along with maintenance costs) and the cost of producing hardcopy from the device must be taken into account. It may be that a suitable output device is already available, either on a networked basis, or within your department or some other department. It is therefore important, once you have decided on the sort of hardcopy output that you require, to find out if the sort of device needed is already available to you within your organisation. (The types of graphical output device available locally may be summarised in the Supplement at the end of this document)

The issue of output quality is tied up with a number of factors. Firstly, the device resolution - or addressability - which is typically measured in dots-per-inch (dpi), should normally be 300 dpi and above to qualify as being of medium to high quality. However, colour can also play an important part in determining image quality, and it is possible to achieve impressive hardcopy images on relatively low resolution devices which have a large range of colours. Another critical factor in the quality equation is that of dot size. A high dpi resolution will only be really effective if the printer can also produce a small dot size. Equally, the consistency of the size, shape, and density of the dots, and the accuracy with which they are positioned on the output medium, all contribute to the overall quality of the final image.

Finally, it is important to decide on the actual size of picture required. Large images (beyond A3 size) are generally only handled by the so-called electrostatic output devices or larger pen plotters.

Types of Device

Let us now take a look at the most commonly encountered types of output device and note some relevant facts about each:

Dot-matrix printers

These are low cost devices in the impact category which rely on the use of small wires in the print-head to transfer ink from a ribbon on to paper. Consumable costs are cheap, but the devices are typically noisy because of the use of impact technology. Speed, resolution and colour quality are generally poor, but nonetheless, dot-matrix printers are often satisfactory for drafts or in-house presentations.

Laser printers

A4 Mono Laser printers

Nowadays this is without doubt the workhorse of the hardcopy business, generally replacing the impact printers in the office and relieving the load on pen plotters. It produces good quality paper and OHP transparency output at 300 dpi, reliably and at a reasonable speed, with cost per copy merely a few pence. Being a raster device it can integrate text (permitting many typefaces) with graphics and raster images. A variety of models exist with facilities to accept PostScript or HPGL. Some models emulate the HP Laserjet, a very popular printer from Hewlett-Packard. HPGL emulation is useful to enable laser printer output to be obtained from software that otherwise would only output to a pen plotter. PostScript printers tend to be more expensive than Laserjet types, but offer greater versatility and are effectively a de facto standard. Extra memory is usually required for graphics applications. Software PostScript interpreters may also be deployed to allow .PostScript files to be printed on non-PostScript printers. Capital cost ranges from less than 1,000 pounds upwards.

Large Format Mono Laser printers

A variety of models exist with facilities to accept PostScript or HPGL. The capital cost is very high (50,000 pounds plus), and the consumable cost varies from around 1 pound (for in-house use) up to 10 pounds (commercial use).

Colour Laser printers

The capital cost of this type of printer is currently very high, but in due course they are likely to become as popular as their mono counterparts, especially when considering the speed and quality of output (300 dpi) and the relatively low consumable cost (about 25p per copy in-house, ranging up to 250p commercially).

Pen Plotters

Until the introduction of the laser printer this was the mainstay of the graphics hardcopy business P excellent for producing smooth lines on paper and OHP transparencies where few colours are required. They can be driven by a wide range of software especially for those plotters adopting the industry standard language HPGL, 'However plotters are vector devices: they are not suited for area fill applications and are slow for complicated line drawings. The capital cost is fairly low (of the order of 1,000 pounds for an A4 plotter), and the cost per copy is less than 10p.

Electrostatic plotters

These are raster devices (both mono and colour), but have traditionally been driven from vector-based software. Hence they are usually provided with a rasterisation controller which nowadays tends to emulate the HPGL standard. It is also possible to drive these devices via a software PostScript interpreter. This is the only device capable of producing fast, high resolution (400 dpi) wide-bodied (A0) output in more than eight colours and extensive area fill. It is a multipass device and can produce more than 1000 dithered colours, but at reduced resolution. Both capital and consumable costs are very high for this type of device. Typical consumable costs would be from around 250p (in-house)'up to 50 pounds (commercially).

(Note: dithered colouring is the technique of generating different colours by grouping patterns of coloured dots, the effect on the human eye being that of a single colour. The use of dithering greatly increases the range of colours available, but also effectively reduces a device's resolution.)

Thermal Transfer Printers

A colour raster device that is becoming ever more popular, which creates images by transferring coloured wax to paper. It is a high quality hardcopy device supported by many software packages in the areas of presentation graphics, graphics design, image processing, solid modelling and scientific visualisation. In fact this is the only device that can speedily produce output on paper and OHP transparencies containing up to 4,000 dithered colours from a palette of 16.7 million, at a tolerable cost. Depending upon the interface connection, large image files can take a considerable time to transmit and process.

It is a multipass device with a nominal resolution of 300 dpi which falls when dithering is used: this may unfortunately become noticeable as 'banding' on smooth shaded objects and graduated fills. Since these devices have more recently been coupled with PostScript interpreters they have become more readily accessible by lots of software. The capital cost is fairly high (5,000 pounds upwards), but is more than compensated for by the quality of output. Another drawback is the relathely high cost of consumables, which ranges from about 70p (in-house) up to 10 pounds (commercially).

Ink Jet Printers

Generally, these devices are not capable of reproducing the output quality, versadlity or efficiency of laser and thermal printers, but are popular with personal computer users for immediate hardcopy output. The capital cost is low and the consumable costs are tolerable: somewhat more expensive than the laser printer for mono, but less than the thermal printer for colour. The number of colours varies from around 300 for low-cost devices to full colour for high-end 'prepress' printers.

Thermal Dye Subilmation Printers

This hardcopy device offers full colour output similar to photographic quality. Capital cost is high, and consumable cost is also high, ranging from 2 pounds (in-house) up to 20 pounds (commercially).

Film Recorders

Very much in demand for producing 35mm slide output from a wide range of software. Film offers very high resolution (at least 4,000 dpi) and excellent colour representation. Cost per 35mm slide ranges from 50p (in-house) up to 5 pounds (commercially).

Unsupported Devices

If the package does not support the particular device to which you wish to send output, but does support, for instance, CGM - then it should be possible to get hard copy output by having the CGM interpreted by other software (such as, for example, UNIRAS) which can produce the required printer output

Next Back
Next: Explanatory Notes on Facilities Matrix.

Graphics     Multimedia      Virtual Environments      Visualisation      Contents