In this section we will discuss the common sources of electronically produced illustration material. This in no way is intended to be a complete analysis of commercially available systems. It is an attempt to characterize these systems for the potential designer or expert user of software for illustrations. The analysis will focus on four areas: the user model, computer graphics techniques, hardware/system requirements, and graphics arts quality.

Graphic arts quality means that we want to judge the output of the system by the existing standards of the graphic arts industry. The image should be interesting independent of its origin as a digitally produced illustration. This means no unintended jaggies, sophisticated, well produced fonts, smooth lines in a variety of weights with smooth or mitered corners as appropriate, elegant arrowheads and symbols, tasteful use of colors and textures, and commercial quality reproduction of continuous tone images. Page description languages have been designed to describe two dimensional illustrations such that these quality issues can be addressed on a wide range of devices, so our analysis will relate the semantics of the image description presented by the illustration system to that provided by a page description language.

We group commercially available systems that are used for illustration into five areas: bitmap/pixel painting, drafting/CAD (computer aided design) systems, geometric illustration systems, business graphics and full color prepress systems. These areas are not exclusive; some products span multiple areas, and the trend towards integrated software systems is blurring distinctions among them even more. This taxonomy is illuminates, however, the relative strengths and weaknesses of the different paradigms for producing illustrations.

This course is concerned with documentation, so pictures are only interesting here if they can be included in a document. The minimum requirement for including the output of a picture making system in a document is that the image must be expressible in a stand alone manner. Page description languages provide the ideal representation for this purpose. When creating images with an interactive computer system a key consideration is whether it is possible to aesthetically translate the screen image to the print media. The key word here is aesthetically. Once on a printed page we adapt the quality definitions of the print media, which has very different characteristics than display media. Text and shapes printed at screen resolution on a printed page look unacceptably jaggy. Textures suitable for displays will look coarse on paper. Printers typically have a more limited number of gray levels and colors. Many vivid monitor colors cannot be reproduced on reflective media at all and approximating these colors may dramatically change the impact of the picture.

The minimum page layout system applies a simple paste-up model to positioning figures. The figures need only be reproducible, though it is helpful if each includes its own size (bounding area). To meet layout requirements it may be necessary to scale or crop the image, which translates in computer graphics terms to translation, scaling and clipping. The next step up in layout flexibility is to be able to replace or remove elements. For example, commercial layout systems that accept CAD produced figures often include a way to replace the plotter-oriented text produced by the CAD system with typeset labels. The most flexible layout system would allow full editing of illustrations in place on the page. Commercially available systems do combine formatted text and graphics7 but with limitations on the complexity of the final page image.

The combined illustrations in a document should all be stylistically similar and coordinated with the text. Style elements include text and symbol fonts, line weights, color and texture. Text in pictures should coordinate with the text in the document. Line weights, dash styles and arrowheads should be the same from figure to figure and harmonize with the style of the document. Color coding should be uniform, color schemes should not clash and should be suitable for the final output device. It has been suggested2 that these style elements could be separated from the geometric or schematic definition of the figures to enforce consistent style and allow for uniform media dependent adjustments. Such a separation would make it possible to combine illustrations from disparate sources by adjusting only the style parameters. However, to our knowledge no commercially available systems support the notion of graphical style.

The definition of the ideal integrated text and graphics system for publication applications is still a research area because it must combine all the best features of graphics and text editors with a layout editor, a database for reference libraries, clip art and revision history, be extensible to include new features as required, and have a well designed user interface, suitable for the publication professional.

As stated in the introduction, these notes will focus on four general areas when describing illustration sources. First of these is the user model, the model the system presents for building up a picture. Systems typically treat the image as either an array of pixels or as a collection of geometric shapes. The user can manipulate this model with a set of interactive tools. The second area will be to list some of the computer graphics techniques that are part of implementing a particular type of system. The list of techniques will not be exhaustive but should include most of those specific to the type of system under discussion, particularly those that may not be obviously difficult. Third will be a general discussion of the hardware and system requirements for a particular application. This discussion is meant to be relative rather than absolute. For example, most geometric systems require more computing power than do painting systems. Finally, we will discuss how suitable the system is for meeting the graphic arts quality issues discussed in the introduction.

There are some basic issues that are common to all the systems analysed so they will be discussed here. These topics are resolution, gray-scale and color reproduction, and graphic arts quality typography.

The output devices commonly used in preparing documentation graphics are monitors and digital printers. Resolution is a measure of the number and/or density of picture elements in a raster device. A resolution independent representation of an illustration is one that makes it possible to render an image on at any resolution achieving the "best possible" result for that resolution. The easiest way to achieve resolution independence is to use a geometric representation as the basis for the illustration which can be smoothly scanconverted to an arbitrary resolution. Pixel oriented illustrations are difficult to translate to paper without visible aliasing effects or jaggies.

Typically an illustration will be designed on a monitor and ultimately output to a printer. A typical color monitor resolution is 640 by 480 pixels, and each pixel can have up to 256 intensity levels for each primary color. Black and white displays are often higher resolution, but still on the order of 1000 pixels across. The spacing of the pixels depends on the size of the monitor tube, but 72 spots to the inch (spi) is a typical resolution for a black/white display. Digital printers have a wide range of resolutions up to 2000 spots to the inch but they are bi-level devices. Typical resolutions for dot matrix printers are around 100 spi or less, for laser printers around 300 spi, for photographic image setters about 1000 spi and for prepress quality film plotters up to 2000 spi.

In summary, monitors are low resolution but may have intensity levels whereas graphic arts quality printers are high resolution but must simulate intensity levels with patterns modeled after the halftoning techniques developed for the offset printing industry. Line art must be anti-aliased to appear smooth on a monitor, but will appear smooth at resolutions of 300 spi and greater when printed. Text must be specially crafted to be readable at monitor resolutions but can be rendered from outline representations (with care) at 300 spi and easily at 600 spi and up. In general, higher resolution devices produce higher quality images and cost more. Illustration systems must balance the desire for high resolution with the overall cost of the system.

Intensity variations on a color monitor are achieved by exciting the monitor phosphor to produce different amounts of light. Intensity variation on a digital printer is achieved by the use of patterns called halftones, as in conventional offset printing15. The technique of halftoning trades spatial resolution for intensity levels as shown in Figure 1. The number of gray levels obtained by this technique is a function of the number of dots simulating the halftone pattern. Since the printer dot size is fixed, there is a tradeoff between edge sharpness and the number of gray levels—a higher resolution dot is bigger. Commercial printing typically uses halftone frequencies of 60 to 150 lines per inch. To simulate this on a digital printer and maintain at approximately 100 gray levels we need printer resolutions of approximately 600 to 2000 lines per inch.

One common problem with printing computer generated continuous tone images is contour lines in smoothly shaded areas caused by quantization in the printed rendering of shades of gray. This is caused by the limits of the resolution available to simulate the halftone sceen. For example, using a 4 by 4 array of pixels to model a halftone dot produces a maximum of 17 gray levels. This is not adequate to produce the appearance of continuously changing shades of gray. This dramatic a limitation would cause artifacts in any example, but the problem exists even at higher resolutions. The problem is worse for computer generated images than for scanned images because scanned images have background noise level as an artifact of the scanning process which tends to mask the contours whereas computer generated images are noise free. One solution is to add random noise to the image to break up the contour patterns.

A color monitor produces colors when the electron beam stimulates red, green and blue phosphor dots to produce a pattern of red, green and blue luminous spots. The brightness of the dots is a function of the power supplied by the electron beam. The color system described is additive and three primaries are independent. A color for a monitor, therefore, is specified as a red, green, blue triple, hereafter called an RGB value.

A color printer produces color by overlaying spots of magenta, cyan, yellow and (optionally) black inks on paper. Patterning inks on paper is an additive system, with the white light from the uncovered part of the paper adding to the colored light reflecting from the inked portion of the paper. Overlaying inks on paper is a subtractive color system, that is, the inks act as a set of filters so the color is determined by multiplying the transmission values of the inks together. Halftoning is a combination of these two color models. Mapping between the additive color model of a color monitor and the complex, non-linear color model of color printing is a difficult task. Some recent work in this area is has been published recently12, 14. A more general discussion of color in the digital graphic arts environment is available in a recent article13, which has been reproduced elsewhere in these notes.

The problems of producing graphic arts quality text is discussed in detail elsewhere in these notes. It is sufficient to note here that fonts designed as rasters for displays will look terrible on paper. Furthermore, any small shape or symbol, such as an arrowhead, has the same rendering and representation problems as fonts.

Popular painting systems let the user change the color of each pixel on the display independently, using the model of an electronic paintbrush and paint. The output is an array of pixels, either black/white or color, and is typically limited to display resolution. Often scanned images can be combined with freehand drawing to quickly produce complex pictures. Many full color painting systems are combined with video editing systems and are intended to produce video animation. Painting systems provide the most flexible way of producing images on a computer, but they are limited by resolution of their representation. They are easy to learn and use. The majority of computer art and computer illustration systems are painting systems.

The use of computers to aid mechanical design is well established. Drafting systems provide tools for making accurate orthogonal or isomorphic projections of three dimensional objects. Many design systems will also produce a simulated three dimensional view of an object. The principal goal of a CAD system, however, is to produce a mechanically accurate model represented either by its surface geometry or by its solid geometry. The user model is one of building up an object description, and the user interface contains many features that are directed towards the accuracy of the model rather than its appearance. However, it is often desirable to use these drawings as illustrations, especially in technical publishing environments where the drawing is needed as part of the documentation. The problem is to extract a description of the model in strictly rendering terms such as a page description language. Typical CAD systems take many months to learn to use effectively.

Geometric illustration systems are those which use a geometric model in an illustration tool. The user constructs outlines with control points and menus or keyboard commands. The outlines can represent lines of different widths or can bound areas which are filled. The imaging model is 2 1/2 D, that is, flat with overlap. Geometric illustration systems are similar to 2D CAD systems in their approach to representing images but, the goal of the system is to produce a good looking illustration rather than building an accurate model. The intended user is a possibly computer naive graphic designer. However, even if the user interface is well presented, significant experience with system is usually required to produce effective illustrations.

While the majority of business graphics systems are simple chart producing programs, we include here a description of more sophisticated data driven graphics applications. The user selects a template from a set of available formats, often called a chartbook. Data is entered either by hand or directly from another application. The final illustration is generated automatically. Facilities may be available to modify the finished image with either a painting system or a geometric editing system. While the emphasis in these systems is to allow a user with little drawing skills to quickly produce produce graphical representations of their data, some training in design aesthetics is necessary before effective illustrations are produced.

Digital prepress systems allow the user to digitally paste-up full color images with typeset text. The output of these systems is halftoned color separations suitable for offset printing. These systems have very little structure in the representation. Instead, they operate on very high resolution raster data. Much of the emphasis is on replacing the traditional darkroom techniques with digital ones. These include color balance, cropping, scaling and matting. The user operates on full color scanned images. Regions of images can be selected, either by outlining or by color discrimination. The colors can be adjusted, the edges blurred, and regions can copied and blended into the existing image. Some of these techniques overlap those of full color painting systems. Others, such as color correction, are specific to the prepress industry. Commercial systems are complex and require many weeks of training before a user can operate them effectively.

In summary, there the most significant factor in analyzing illustration systems is the underlying representation presented by the system. CAD systems and geometric illustration systems work on a geometric model of the image. Painting systems and digital prepress systems use a raster representation of the image. Business graphic systems may use either, but are more likely geometric in the design of the initial diagram.

Systems with similar representations tend to share user interface concepts and will tend to merge in the future. For example, prepress systems are importing many of the techniques common to painting systems. Painting systems are using low cost video cameras to add scanned images to painted illustrations. CAD systems are being implemented on low cost personal computers and promoted as technical illustration systems. Research in geometric design systems is exploring adopting precision placement techniques common to CAD systems3 for illustration design. There is a tend in business graphics systems to allow the user to customize the illustration using a painting system. This operation destroys whatever structure was present in the illustration. Hopefully, as the market develops a demand for higher quality business graphics, this trend will be redirected towards merging business graphics with geometric design systems.

Raster oriented systems are more flexible and usually are more attractive to users whereas geometric systems produce higher quality output and have potentially more power. In the long term, it should be possible to combine painting techniques with structured techniques in a single system to produce a signicantly enhanced set of functions for producing illustrations.