IllustrationSources.tioga
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STONE
ELECTRONIC SOURCES OF ILLUSTRATIONS
SIGGRAPH '87 TUTORIAL COURSE NOTES
DOCUMENTATION GRAPHICS
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Electronic Sources of Illustrations
Electronic Sources of Illustrations
Maureen Stone
Xerox Palo Alto Research Center
1. Introduction
In this paper we will discuss the common sources of electronically produced illustration material. This is not 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. This analysis will focus on four areas: the user model, computer graphics techniques, hardware and system requirements, and graphic 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 not having unintended jaggies, having sophisticated, well-produced fonts, smooth lines in a variety of weights with smooth or mitered corners as appropriate, elegant arrowheads and symbols, a 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. Our analysis will relate the semantics of the image description presented by the illustration system to that provided by a page description language.
Our analysis groups commercially available systems 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 dis-tinctions even more. Our taxonomy illuminates, however, the relative strengths and weaknesses of the different paradigms for producing illustrations.
2. Including Illustrations in a Document
This course is concerned with documentation, so the only pictures that are interesting here are those that 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 primary consideration is whether it is possible to aesthetically translate the screen image to the print media. The keyword here is aesthetically. Once on a printed page, we adapt the quality definitions of the print media, whose characteristics are very different from the display media's. Text and shapes rendered on a printed page at screen resolution look unacceptably jaggy. Textures suitable for displays 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 (in computer graphics terms: 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 graphics [9] 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. Line weights, dash styles, and arrowheads should be the same from figure to figure and should 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 suggested [2] 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. 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.
3. Areas of Comparison
As stated in the introduction, these notes will focus on four general areas when describing illustration sources. The first of these will be 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. Our list will not be exhaustive but should include most of the techniques specific to the type of system under discussion, particularly those that may not obviously be difficult to implement. The third area 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.
4. Common Issues
There are some basic issues that are common to all the systems analysed so we will discuss them here. These topics are resolution, gray-scale and color reproduction, and graphic arts quality typography.
Resolution
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 at any resolution, achieving the best possible result for that resolution. The easiest way to achieve resolution independence is to use a geometric representation which can be smoothly scan converted to an arbitrary resolution as the basis for the illustration. Pixel oriented illustrations are difficult to translate to paper without visible aliasing effects or jaggies.
Normally, 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 and 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.
Gray-scale and Color Reproduction
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 printing [8,17]. 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. Commercial printing typically uses halftone frequencies of 60 to 150 lines per inch. To simulate this on a digital printer and maintain 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 the appearance of contour lines in smoothly shaded areas. These contour lines are 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 screen. 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.
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[Artwork node; type 'ArtworkInterpress on' to command tool]
Figure 1: Digitally produced halftone patterns
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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 the 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 work in this area has been published recently [14,16]. A more general discussion of color in the digital graphic arts environment is available in a recent article [15], which has been reproduced elsewhere in these notes.
Graphic Arts Quality Typography
The problem of producing graphic arts quality text is discussed in detail elsewhere in these notes [3]. 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.
5. Bitmap/Pixel Painting
User Model
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 and 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 the resolution of their representation. They are easy to learn and use. The majority of computer art and computer illustration systems are painting systems.
Computer Graphics Techniques
Simple painting systems are easy to implement on any system with a raster display and a pointing device. The basic paint operation follows the motion of the pointing device, drawing a rectangle or other simple brush shape at every new position. Some care must be taken, however, to provide smooth motion. The input from the pointing device should be filtered to remove duplicate points and jitter. If a continuous line is desired, it may be necessary to interpolate the stroke between input positions. The brush image is typically stored as a bit pattern which is used to mask the application of the paint. A fast, two-dimensional bit operation such as RasterOp [10] is very useful for the time-critical inner loop of the painting operation.
Bitmap painting systems use textures instead of paint colors. Display textures must be carefully designed to avoid flicker. When painting or filling an area the phase of the texture must be defined relative to some fixed grid such as the absolute position of the pointing device on the screen, so that overlaid areas of the same texture blend together correctly. Textures can either be opaque black and white or transparent, where only the black is written into the image (Figure 2).
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[Artwork node; type 'ArtworkInterpress on' to command tool]
Figure 2: Opaque and transparent texture patterns.
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Color painting systems usually provide a way for users to mix custom colors. These colors can be opaque or translucent. Translucent colors add their value into the existing image and so follow the rules for additive color systems. To achieve soft edges and other subtle effects, a variety of spatial weighting functions can be used to mix the paint with the underlying image, simulating a mechanical painting technique called airbrushing.
Most painting systems include features beyond the basic painting paradigm. Area fill by ``flooding'' a region of a single color with another is a common addition. Flood-fill algorithms are discussed in standard graphics textbooks [6]. A system aspect of using flood-fill algorithms is the importance of helping the user guarantee that the region specified is contained, or that its borders have no hole where the paint can ``leak out.'' Another common addition is structured brushes that produce straight lines, boxes, and circles by specifying a few design points. Once the shape has been displayed, the structure is lost. A special case of structured brushes is text. Most painting systems have some way to type or paste text labels into the picture.
Hardware and System Requirements
The minimum system requirements are an all-points-addressable raster display, a pointing device such as a mouse, and a processor with sufficient power to provide smooth interaction during the painting operation. The deluxe system has a full color monitor, image processing capabilities, and significant storage capacity since each image will take a quarter of a megabyte or more to store. Most personal computer systems support some form of painting program. Output is typically intended to be viewed on the monitor, although a video tape recorder would be required for video animation systems.
Graphic Arts Quality Issues
The principal limitation with painting systems is the unstructured, display-oriented output. Black and white images are unacceptably jaggy. The illustration can only be manipulated as an array of bits, independent of the visual structure. Colors and textures are monitor-oriented and may not reproduce well on the printed page. Furthermore, the overall appearance of an illuminated display is qualitatively different than that of a reflective print. The artist using a display to design for print media must be aware of the effect of the differences in brightness, contrast, and resolution.
The resolution limitations can be relieved somewhat by providing subpixel zooming; that is, the ability to paint at a higher resolution than that of the display. This does not provide any additional structure, however. Some research has been done to define shapes from their raster representation [11,12] but these algorithms are typically expensive and not 100% reliable.
6. Drafting/CAD Systems
User Model
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. It is often desirable, however, 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 strictly in rendering terms such as a page description language. It typically takes many months to learn to use CAD systems effectively.
Computer Graphics Techniques
Most mechanical designs are designed and displayed as line drawings, so most CAD systems need fast line drawing techniques. Originally, these systems used vector displays. For systems with raster displays, there is often an accelerator for lines in microcode or hardware. Lines need to be displayed in different weights, colors, and styles (such as solid, dashed and dotted). Straight lines and circular arcs are the principal geometric forms, although certain industries require the use of spline curves. A real mechanical design is large and detailed, so fast pan and zoom are common features.
The user interface must provide techniques for precise construction, such as gravity to snap the cursor to lines, points, and intersections; the ability to add numeric information; or compass and protractor equivalents for common geometric constructions such as parallel or perpendicular lines. A general purpose shape intersection algorithm is needed to support this user interface.
The appearance of mechanical drawings is very standardized. Typical CAD systems help the user maintain these standards by providing special routines for dimensioning, crosshatching, symbols, and patterns. The appearance of dimensions is particularly well controlled. Labeling, too, is standardized, controlling the size and orientation of text. While not completely freeform, text must be displayed at arbitrary angles and in a range of sizes. Fonts are often vector defined to simplify this transformation and to accommodate the use of plotters as output devices.
The user of a drafting system is accustomed to producing three dimensional designs by operating on orthographic and oblique views. A good design system will support this process by providing simultaneous construc-tion of orthographic views and automatic construction of oblique views from orthographic projections. Many systems can produce a nicely shaded rendering of a 3D view of a design.
Hardware and System Requirements
A drafting system should have a high-resolution display and a digitizing device such as a tablet. A digitizing device differs from a pointing device in that it produces absolute numeric information rather than relative positions. While there are several personal computer based drafting systems, production CAD systems are often large (both storage and processor intensive). Floating point support for geometric operations is essential. Most CAD systems include a hardcopy output device capable of supporting large format drawings, usually either a pen plotter or a wide format electrostatic plotter.
Graphic Arts Quality Issues
CAD systems produce a geometric description of the image that is resolution independent. It is easy to apply geometric transformations such as scaling and clipping which are useful in page layout. Many CAD systems assume a pen plotter as the final output medium. This, along with the industry standards, produces a particular style: lines in multiple colors and styles, stroke defined text, and cross hatching or textures rather than filled areas. Line widths are usually restricted to ones so narrow that joint and end conditions are not an issue. The most common improvement desired for CAD drawings is to replace the labels with typeset quality text.
The three dimensional renderings from CAD systems are treated as images by page description languages. As mentioned above, the smooth, uncluttered images produced by a CAD system are very susceptible to contouring artifacts unless added noise or some other image processing technique is used to mask this problem.
7. Geometric Illustration Systems
User Model
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 pleasing illustration rather than building an accurate model. The intended user is a graphic designer who is possibly computer naive. However, even if the user interface is well presented, significant experience with the system is usually required to produce effective illustrations.
Computer Graphics Techniques
Geometric illustration systems share many techniques with CAD systems. However, special rendering and user interface problems arise because the goal of the designer is to control the visual aspects of the image. It is important to provide the most accurate rendering possible of lines, shapes, colors, and textures. The user interface must provide accurate control but should not interfere with the design process. The hidden structure produced by the construction methods should harmonize with the visual structure.
Shapes in a geometric illustration system can be bounded by lines and curves of various forms—arcs, conics, and cubics—which can be filled or outlined. An efficient and accurate scan conversion routine that renders all these shapes is a basic requirement for these systems. Wide line shapes provide a particularly challenging problem. While it is easy to compute lines of arbitrary width when they are straight or circular arcs, the algorithms for higher order curves such as conics and parametric cubics are much more difficult. Additional care must be taken to correctly model the joints and end conditions on lines wide enough for these aspects to be visible. Producing an outline representation of the curve bounding a wide curved line is the topic of a recent Ph.D. thesis [7].
The display should always reflect the current state of the design. For graphic design systems, this can present a significant performance problem because of the complexity of the shapes to be rendered. A useful technique to minimize this problem is to localize the refresh, or repaint only the part of the display that has been affected by user action. The design of an efficient and effective localized refresh algorithm provides an interesting set of problems for the system designer [1].
If localized refresh is used, care must be taken that the scan conversion algorithms for shapes have well defined edge conditions to avoid artifacts along seams. Errors can also occur when bitmap operations are used to repaint part of the image. When a page is scrolled, for example, then the remaining part of the image is drawn from the original representation. Careful design of scan conversion and clipping algorithms will avoid these problems. Another type of solution is to provide a special representation for each object; one that is already bound to the screen resolution. All display updates are driven from this representation, which is kept consistent by avoiding incremental changes.
Scan conversion precision at the pixel level is even more critical if the system uses an XOR (exclusive OR) function to erase and paint parts of the scene. Pixels that are inadvertently written more than once may end up in the wrong state at the end of the refresh. This phenomenon will leave little speckles across the screen, whimsically called ``pixel dust.'' The use of XOR is tempting, especially for interactive display techniques such as rubber-banding or dragging. However, we have found the difficulties outweigh the advantages for most graphic arts applications. Not only are there the scan conversion problems mentioned above, but the visual presentation of inverted textures is often very distracting. Furthermore, XOR does not generalize to color displays.
Selective operations on shapes require the target shapes to be selected in some manner. The preferred approach is to position the cursor on the desired object and click a button. This requires a fast hit-testing algorithm that will work on all shapes in the picture. A good approach [1] is to provide a suitable encoding that can be quickly intersected with a point. This same encoding can be used to provide efficient and accurate incremental refresh. Selection feedback is a difficult problem in a system where reserving any color or shape to indicate selection will eliminate the possibility of using that technique in an image. This problem becomes progressively more difficult as the amount of structure in the system increases. For example, it is important to distinguish between selected objects, selected parts of objects, and selected groups of objects.
The object definition in the page description language combines lines, arcs, conics, and cubics to define the outline of shapes and the holes in shapes. Construction of such complex shapes requires a sophisticated user interface for manipulating lines combined with free-form curves. Additional complexity is added by the use of holes in shapes. For example, if a closed curve is defined as the boundary of a hole, how does it behave when it overlaps the edge of its enclosing shape? Figure 3 shows two possible interpretations.
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[Artwork node; type 'ArtworkInterpress on' to command tool]
Figure 3. (a) Hole A overlaps shape B (b) Result of conventional scan conversion. (c) Alternative interpretation of users intention.
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When shapes are overlaid they visually combine. This combination may suggest a different structure than the one used to construct the shape. Figure 4 shows three possible structures for a simple shape. This structure becomes important for rendering when outlines are added and is always of interest when the user wants to modify the shape. Ideally, the user would be allowed to easily redefine the structure as desired.
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[Artwork node; type 'ArtworkInterpress on' to command tool]
Figure 4. Three possible constructions for a shape.
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Hardware and System Requirements
A geometric graphic design system must have a raster display and a pointing device. While such systems are available on personal computers, the complexity of the basic shapes and finished illustrations will be limited by the power of the processor. Many of the operations on curves require floating point arithmetic support. Since it is impossible to precisely duplicate the appearance of a design intended for paper on a display, ready access to a laser printer or similar printing device is desirable.
Graphic Arts Quality Issues
Since the model of an image used in a geometric design system is similar to that used in page description languages, the graphic arts quality issues of such a system are inherently addressed. The ideal geometric design system would use precisely the semantics of the page description language for the ultimate output device. Such complete control, however, is only available in a programmable system. An interactive system will typically have some stylistic limitations which may need to be overcome when the illustration is added to a document. The use of graphical style would minimize the difficulty of this integration.
8. Business Graphics
User Model
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 skill to quickly produce graphical representations of their data, some training in design aesthetics is necessary before effective illustrations are produced.
Computer Graphics Techniques
The most frequently used representations in business graphics are bar and pie charts, so a business graphics system must be able to render rectangles, circles, and circular arcs. The bars and circles may be filled with a distinctive color or texture. The regions must be labeled, often with an associated arrow. For plotting scientific data, it is useful to add dashed lines in a variety of styles. The precise rendering of boxes and circles is not difficult, but as many personal computer chart programs have shown, it is possible to do it poorly. Bars should cleanly meet their axis. Circular segments in a pie charts should smoothly align to form a circle. High quality text, symbols, and arrowheads do present problems in rendering because typical display devices have inadequate resolution for traditional designs. Careful design of fonts and symbols to accommodate the low-resolution devices typically used in this application would be the most powerful solution here.
The principal algorithmic issues have to do with formatting an image from data and a template. A bar chart has a variable number of bars, the number and height of which are defined by the data. Given a template for style, rendering the chart is generally straightforward. Difficulties arise when the data will not fit correctly in the template or if the style is inadequately defined. For example, what should the system do if a label is too long to fit in the available space? Many of these problems must be resolved by interaction with the user, which is an interesting user interface problem.
Included elsewhere in these notes is a discussion by Bill Bowman on the topic of idiomatic illustration, or the process of defining visual idioms that can be modified to give a specific illustration. It describes not only the simple charts and graphs common to commercially available systems, but looks to the future to include diagrams, maps, plans, and pictorials.
Hardware and System Requirements
Simple charting programs can be implemented on almost any computer system that has access to a printer. It is useful, but not strictly necessary, to have a graphics display to preview the chart before it is printed. It is very important to have the program accept data from other business applications. The value of these programs is in their ability to produce graphical visualizations from real application data. Most business systems are small computers with low-resolution dot matrix printers or pen plotters. As computer systems in businesses become more powerful, the output of these systems should become more sophisticated.
It is interesting to note that there is a trend in business presentations towards projecting the monitor image rather than making printed slides. This eliminates the problem of transferring the image from the monitor to paper and allows the designer to include animation as well as static images in the presentation.
Graphic Arts Quality Issues
A recent issue of PC World reviewed 36 chart making programs, and that was only the subset they chose to review. Given a personal computer and a printer, it seems anyone can write a charting program. Visually speaking, however, the output of most of these programs was terrible. It is possible to make simple graphics, even at low resolutions, aesthetically pleasing by choosing appropriate fonts, textures, and colors. And, as the sophistication of the output device is increased, the appearance of the output should improve. Some programs do keep a geometric rather than a raster representation of the design so that it is possible to produce presentation quality slides from the output.
Automatically generated images will tend to have an inflexible style which may be an issue when including them with other images. To achieve the best quality, it will generally be desirable to move the automatically produced illustration into a general purpose editing system to finish it.
9. Full Color Prepress
User Model
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 be 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.
Computer Graphics Techniques
The basic page makeup task involves selecting sections of scanned images and positioning them on a page. Text is typically typeset separately, then treated like the images for paste-up. That is, the operator can cut out blocks of text and position them, but not edit them. The minimum set of operations is rectangular cropping and simple transformations. Scaling and rotation are also useful and enable the user to produce more complex layouts. Page makeup is often performed on low-resolution versions of the final images to improve interaction time. The result of the page makeup task is a set of commands which can be repeated on the original data as a batch program.
Each image in the page may require modifications. The most basic are tone adjustment or redistributing the brightness values to match those of the target output device. This operation improves overall brightness and contrast. Portions of the image may be ``cut out'' for further modification. The region of interest is indicated either by tracing along the boundary with the pointing device or by indicating the background color to an algorithm that automatically generates the border between this color and the desired image. Such algorithms should have an option to operate only on the selected hue, ignoring the changes in lightness, to facilitate the masking of images on a shaded background.
Two images may be combined by overlaying or abutting them. This will require some technique for blending the two images so that no artificial edge is visible. One general solution to this problem has been published [5]. Another approach is to define soft or translucent edges on an image while it is being cut out of the original image. Experimental techniques have been developed to compute the alpha channel (transparency) component of the edge from the color definition and user hints. This information can be used to smoothly blend the edges into a new background [13].
A major component of digital prepress systems is color correction. The colors in an image will be modified when scanned and again when printed. Manipulating these colors to get the desired result on the printed page currently requires significant experience and expertise. Traditional techniques involve manipulating the three or four color separations independently. Recently, some systems have been developed to allow the operator to manipulate colors on a color monitor, treating it as a proof copy of the printed page. These systems provide a quick way of visualizing color relationships and minimize the amount of adjustment performed at the separation stage of the production process. Effectively mapping colors from device to device is still an inadequately understood problem. Commercial systems that address this problem are carefully controlled and specialized to match their specific configuration. The details of these systems are often kept proprietary.
Hardware and System Requirements
Graphic arts quality images may be as large as 4000 by 4000 by 24 bits, and several may be combined to form a page. For production systems, therefore, the most significant requirement is adequate storage space. System performance will be largely a function of the amount of local memory and the disk swapping speed. The usual source of input for these systems is a graphic arts quality scanner. These scanners operate at resolutions up to 2000 lines per inch and can scan either reflective prints or transparencies.
If the system is used for interactive page makeup, a high-resolution, good quality color display plus a pointing device is required. The display may be specially constructed and mounted to provide maximum control over the color quality, especially if the system is intended to simulate the color of the printed page. The principal output device for these systems is an output scanner that produces halftoned films. These plotters scan at resolutions between 1500 and 2000 spots per inch and typically contain the algorithms necessary to convert the intensity values in the image to halftone patterns.
Graphic Arts Quality Issues
Prepress systems maintain the highest possible quality while controlling the manipulation and reproduction of images. The principal limitation is the lack of structure in images that could be structured. All art, whether line or continuous tone, is scanned and manipulated uniformly. Text and line art may be kept at higher resolutions than the continuous tone images, but they are still rasters. Some commercial systems are beginning to include simple structured graphics for rules and borders. Future systems may well combine general purpose structured graphics with scanned images.
10. Conclusion
In summary, 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 to be 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 how to adopt precision placement techniques common to CAD systems [4] for illustration design. There is a trend 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 potentially have more power. In the long term, it should be possible to combine painting techniques with structured techniques in a single system to produce a significantly enhanced set of functions for producing illustrations.
11. References
[1] Baudelaire, P. and Stone, M.C., ``Techniques for Interactive Raster Graphics,'' Computer Graphics, 14, 3, July 1980, 314-320.
[2] Beach, R.J. and Stone, M.C., ``Graphical Style—Towards High Quality Illustrations,'' Computer Graphics, 17, 3, July 1983, 127-135.
[3] Bigelow, C., "Notes on Marking." these notes, 11-14.
[4] Bier, E.A. and Stone, M.C., ``Snap-Dragging,'' Computer Graphics, 20, 4, August 1986, 233-240.
[5] Burt, P.J. and Adeleson, E.H., ``A Multiresolution Spline with Application to Image Mosaics,'' ACM Transactions on Graphics, 2, 4, October 1983, 217-236.
[6] Foley, J.D. and Van Dam, A., Fundamentals of Interactive Computer Graphics, Addison-Wesley Publishing Company, Reading, Massachusetts, 1983.
[7] Hobby, J.D., ``Digitized Brush Trajectories,'' Stanford University Technical Report number STAN-SC-85-1070, August 1985.
[8] Holladay, T.M., ``An Optimum Algorithm for Halftone Generation for Displays and Hard Copies,'' Proceedings of the Society for Information Display, 21, 2, 1980, 185-192.
[9] Lipke, D.E., Evans, S.R., Newlin, J.K., and Weissman, R.L., ``Star Graphics: An Object-Oriented Implementation,'' Computer Graphics, 16, 3, July 1982, 115-124.
[10] Newman, W.M. and Sproull, R.F., Principles of Interactive Computer Graphics, second edition, McGraw-Hill Book Co., New York, 1979.
[11] Pavlidis, T., ``Curve Fitting with Conic Splines,'' ACM Transactions on Graphics, 2, 1, January 1983, 1-31.
[12] Plass, M. and Stone, M.C., ``Curve-fitting with Piecewise Parametric Cubics,'' Computer Graphics, 17, 3, July 1983, 229-239.
[13] Porter, T. and Duff, T., ``Composing Digital Images,'' Computer Graphics, 18, 3, July 1984, 253-259.
[14] Starkweather, G.K., ``A Color Correction Scheme for Color Electronic Printers,'' Color Research and Application, 11, (Supplement), June 1986, 67-72.
[15] Stone, M.C., ``Color, Graphic Design, and Computer Systems,'' Color Research and Application, 11, (Supplement), June 1986, 75-82.
[16] Stone, M.C., Cowan, W.B., and Beatty, J.C., ``A Description of the Reproduction Methods Used for Colour Pictures in this Issue of Color Research and Application,'' Color Research and Application, 11, (Supplement), June 1986, 83-88.
[17] Yule, J.A.C., Principles of Color Reproduction, John Wiley & Sons, Inc, New York, 1967.