Designing Procedural Illustrations

IEEE TRANSACTIONS ON PROFESSIONAL COMMUNICATION, VOL. 47, NO. 1, MARCH 2004

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Designing Procedural Illustrations —ROBERT KRULL, SHREYAS J. D'SOUZA, DEBOPRIYO ROY, AND D. MICHAEL SHARP Abstract—Ideally, illustrations for procedural documents should show actions from the point of view of performers, especially if performers’ bodies need to be positioned a particular way to perform actions. At the same time, illustrations also should ameliorate the limits of two-dimensional displays (such as the printed page or electronic screen) by showing bodies, objects, and movements across the display plane. These two requirements may conflict, possibly necessitating use of compromise views.

Index Terms—Cognitive processing illustrations, performance support, procedural information, procedures.

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sychological research has shown that people develop mental models while they consume procedural information. Mental models of several kinds have been proposed: models of how electronic devices function [1], [2]; embodied language models that link text to the three-dimensional (3-D) physical world [3]; body schemata that integrate visual thinking with a person’s sense of the body [4]; and kinesthetic models based on people’s feeling for actions [5]. These models emphasize one or more of the verbal, visual, or kinesthetic information streams available to people. Technical communication can benefit from bringing such research findings into existing guidelines for procedural illustrations. Though the field of technical communication has plenty of graphics guidelines, the bulk of them treat layout of text blocks, typography, and charts (e.g., [6]–[8]). Comparatively few guidelines suggest how human bodies or physical movements should be illustrated in procedures. For example, Dondis systematically analyzed visuals in terms of fundamental graphic elements: dot, line, shape, direction, tone, color, texture, scale, dimension, and motion [9]. These form elements, while underlying complex visuals, do not relate specifically to illustrating human actions in procedures.

Closer to the latter is Horton’s analysis of how to show what 3-D objects look like. He suggested “a viewpoint above, in front of, and slightly to one side of the object [10, p. 94]. . . . Use angled views to make something recognizable and to help viewers identify parts of a solid object.” He went on to say that, while straight-on views lack depth, they are useful for illustrating physical dimensions. Illustrations of how to act on objects, he recommended, should show people how to orient themselves in space and how to exert forces by including arrows or a human hand. Hodgkinson and Manuscript received December 4, 2002; revised May 26, 2003. The authors are with Rensselaer Polytechnic Institute, Troy, NY 12180-3590 USA (email: [email protected]). IEEE DOI 10.1109/TPC.2004.824288 0361-1434/04$20.00 © 2004 IEEE

Hughes advocated similar techniques for improving wordless instructions [11]. This article links recent research to techniques such as those suggested by Horton [10] and Hodgkinson and Hughes [11]. To accomplish this, we review research about nonvisual and visual models of information processing. Then, we apply research to an example of a physical task, specifically, a woman operating a clutch in a car.

MENTAL MODELS OF INFORMATION Models Based on Declarative Information Several investigators have looked at the value of declarative information for people controlling equipment. For example, Kieras and Bovair found that test subjects who were given declarative information about electronic devices learned procedures faster, remembered them better, and executed them faster than did subjects not given a model [2]. Most important for the purposes of this paper, test subjects only benefited from declarative information that specified the relationship between equipment controls and subjects’ physical actions. General overview information did not help. In a similar study, Dixon and Gabrys found that people’s mental models are concretely grounded in the physical environment of equipment controls and their related action sequences [12]. Extending such studies to illustrations suggests that images should show equipment controls, their means of operation, and users’ actions, rather than providing just a general orientation to equipment. We explore this point further in the section on the illustration of actions performed on equipment. Models Built on Verbal Instructions People can build mental models of 3-D spaces from verbal instructions, as well as from illustrations [13]. For example, Glenberg and his colleagues tied together the 3-D world, people’s bodies, and language [3], [14]–[16]. Their findings suggest that people’s mental models link abstract spatial concepts to people’s role

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in the physical world. In similar research, Dixon found that providing people with a verbal overview of spatial configurations yielded faster performance when people perform steps [18]. One might infer that an overview graphic may perform a similar function when it precedes step-by-step procedures. Tversky and her co-workers conducted extensive studies of verbal descriptions of spatial concepts. For example, Bryant, Trevsky, and Franklin manipulated test subjects’ mental imagery by describing scenes from the points of view of observers, either within or outside of scenes [18]. Given descriptions from within scenes, test subjects mentally locate objects at speeds that depended on the distances relative to the main body axes. For example, subjects were quicker to determine which objects were in front of the body than behind it. Given descriptions based on an observer outside a scene, test subjects did not show the same pattern of responses. Tversky et al. argued that these data conform to people’s ordinary interactions with a physical world in which bodies are affected by gravity, people’s ability to see, etc. These findings from verbally-based studies of spatial mental models suggest that individuals apply their ordinary body experiences when thinking about 3-D space. To help people do that, scenes and objects may need to be depicted in different ways, depending on what is important to a task. If, as for some of Tversky’s tasks, the human body’s position in the situation is important, the scene may optimally be shown as well as described from that body position. If the body’s position is less important, an object-centered point of view may be more appropriate. These two alternatives turn out to be powerful variations in many of the studies of visual information reviewed in this article. Kinesthetic Models When performing physical tasks, people use different parts of the brain to plan, initiate and control actions [5], [19]. In addition to using visual information, people perceive some actions from a body-centered system of kinesthetic information telling them the position of limbs, and their heads, neck muscles, and eyes [20]–[25]. Such studies show that people add kinesthetic information about their bodies to the visual information they have about 3-D spaces. That combination of body-centered senses enables people to judge where objects are in their environment and how to act on them [4].

VISUAL MODELS OF INFORMATION PROCESSING Models Based on the Brain’s Processing of Spatial Information Milner and Goodale extensively reviewed research on the brain’s processing of visual information and concluded that there are two fundamentally different visual processing systems [26]. One system, which they called EGOCENTRIC

PROCESSING,

allows people to locate objects relative to their body center and their bodies in 3-D space. This body-centered system moves information from the primary visual processing areas at the back of the brain forward across the top-rear of the brain. The other system, ALLOCENTRIC PROCESSING, enables people to handle relationships centered among objects and identify objects. The allocentric system moves information from the back of the head, forward along its sides, and closer to parts of the brain that verbally categorize objects. The existence and effects of these two visual systems have been supported by many studies and their collective import for procedural illustrations is considerable. When a task demands body-centered information processing, illustrations probably should show scenes from the point of view of a person performing actions. When a task demands object-centered information processing, illustrations may be more effective when they clearly show how objects are positioned relative to each other. Parsons carried out research in this domain, asking people to perform real or imagined rotations of illustrations of objects and body parts [27]–[29]. Real and imagined rotations produced similar responses. Response times varied by the extent of rotation and by the axis of rotation, with the fastest rotations being for the horizontal and vertical axes of the page on which images were displayed and along the line of sight perpendicular to the page. Rotations along axes at diagonal angles to the page produced slower response times. Parsons concluded that people’s body schema (his term for people’s mental models) was based on information from many senses—visual, muscular tension, joint angle, and so on—that combine to form a body-centered frame [4]. Tversky and her co-workers extended Parsons’ research by showing that people apply either a body- or object-centered reference frame to illustrations, depending on the task they are given. Zacks and Tversky asked test subjects to compare line drawings of human bodies or objects that were placed at various angles on the page [30]. The body images were designed to induce test subjects to imagine their own bodies at the positions shown. Results suggested that, for the task studied, subjects consistently used a body-centered frame for body images and an object-centered frame for objects [31]. We can draw several interim conclusions from this set of visual processing studies. First, it is likely that, when illustrations show bodies and objects in ways that require people to perform additional mental operations, it increases their time and effort. The more mental manipulation required, the greater people’s time and effort. Second, people may perform differently depending on the content of illustrations

KRULL et al.: DESIGNING PROCEDURAL ILLUSTRATIONS

(objects or bodies) and the kinds of operations (e.g., such as mental rotation) they are asked to perform. Third, designers should consider whether they are trying to focus attention on objects or on how the human body needs to act. There are comparatively few studies of illustrations for specific procedures. Most of these studies concentrate on object-centered rather than body-centered procedures. Models of Spatial Processing and Procedures Bülthoff, Tarr, and their co-workers have researched images for object-oriented tasks. For example, Blanz, Tarr, Bülthoff, and Vetter reviewed the literature on “canonical” or highly-preferred views and found that illustrations scoring most highly for appeal, rapid response time, and other performance characteristics were three-quarter views in which many surfaces of an object are exposed to the viewer [32]. Their assessment of the literature matches Horton’s general guideline for illustrations of 3-D objects that we mentioned earlier in this article. However, this pattern in the experimental data is not universal. When viewers were asked to imagine unfamiliar objects or encountered objects with unusual shapes, they often chose views that placed distinguishing object features either directly into the display plane or across it [33]. That point of view is consistent with the head-on views Horton suggests for situations in which physical dimensions are important. In a related study, Gauthier, Hayward, Tarr, Anderson, Skudlarski, and Gore found that test subjects’ brains performed differently when asked to identify objects versus imagining body rotations [34]. As with other studies, subjects’ brain activity was different for object-centered tasks than for body-centered tasks. Using a physical task more like the kind of equipment manipulation seen in technical communication, Heiser and Tversky asked subjects to assemble a television stand from only an illustration of the finished product [35], [36]. After completing the assembly process, subjects were asked to generate visual and verbal instructional information they would like to have received when they started. Subsequently, other test subjects were asked to evaluate that instructional material. The vast majority of instructions generated by test subjects included both pictures and text, most of them in step-by-step order. Their drawings were integrated into the text, were accompanied by labels for parts, and included arrows or dotted lines to indicate motion. High-spatial-ability subjects drew illustrations as advocated by [10], [32] and [33] from three-quarter points of view. By comparison, low-spatial-ability subjects drew parts of the TV stand

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essentially head-on and without arrows to indicate motion. When other test subjects were asked to use these instructions, high-spatial-ability subjects were found to build the TV stand successfully with either set of instructions. Low-spatial-ability subjects performed considerably worse with the head-on, arrowless instructions. These findings could indicate greater support for the three-quarter views, at least for this kind of object-oriented task. Szlichcinski conducted a series of similar experiments on technical illustrations for electronic devices [37], [38]. Test subjects preferred illustrations that showed objects together with corresponding actions. Arrows represented the direction of actions effectively, though test subjects could not tell if arrows were intended either to draw attention to a machine control or to indicate the direction of a force to be exerted by a person. When Szlichcinski included body parts, such as hands in illustrations, he found that test subjects tended to interpret the illustrated body positions very literally. They tried to duplicate precisely the positions they saw. This finding suggests that, at least for some tasks, the body’s orientation to equipment and its actions need to be illustrated with care. Finally, Szlichcinski mentioned that people interpret perspective drawings well when the objects depicted have planar surfaces. People have trouble with true perspective drawings when the objects shown are rounded. Since the human body has both rounded limbs and joints with many interacting movement possibilities, we might expect peoples’ interpretations of body illustrations to be less certain than their interpretation of illustrations of equipment. The upshot of the studies reviewed in this section is that three-quarter views may be preferred for object-centered tasks such as building a TV stand. Body-centered tasks and rounded objects, such as the human body, may require a different point of view. For example, illustrating actions from a body-centered perspective may allow people to perform actions without having to mentally rotate what they see in the instructions. However, since fewer studies have posed body-centered tasks, the ideal point of view for body-centered illustrations still may be an open issue.

APPLICATION OF THE RESEARCH We demonstrate the differences between these two approaches, BODY-CENTERED versus OBJECT-CENTERED, through the example of driving an automobile. We omitted parts of the car to make the images simpler to interpret. Object-Centered Point of View If one were to use an object-centered approach to illustrate automobile driving, one would take a spectator’s view. A spectator

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could see the situation either from in front of the driver or to one side. Fig. 1 shows a frontal view

Fig. 1.

Fig. 2.

Automobile driver from the side.

Automobile driver from the front.

Here it is easy to see the horizontal distance between the feet and the driver’s spine because now this distance is across the display plane. On the other hand, the horizontal distance between the driver’s feet has become difficult to judge because it is along the line of sight into the display plane.

that is more directly head-on than one likely to be advocated by Heiser and Tversky [35], [36], Horton [10], or Szlichcinski [37], [38]. This view is in accord with some of the findings reviewed by Tarr, Bülthoff, and their co-workers [32], [33].

We have included two arrows in Fig. 1 to highlight the two dimensions (left-to-right and up-and-down) that are directly across the display plane of the page. For example, the distances between the right and left feet and from the hands to the eyes are easy to judge because these distances are directly across picture plane. (The arrows in the figure are intended to highlight distances rather than to indicate actions.)

When the action of pressing a clutch pedal is shown in a driving manual, it is sometimes done from this side view. The reason probably is that it clearly shows the movement across the display plane. In other words, the movement of the foot would be shown going from right to left across the page. Body-Centered Point of View Are either the front or side views the way a driver actually perceives pressing on a brake pedal? No, the driver, using a body-centered framework, would perceive pressing a brake pedal as being forward, away from the center of the body. To translate the frontal view or side-views shown in Figs. 1 and 2 to a driver’s point of view requires a 180- or 90- rotation of the images.

It is much more difficult to see how close the feet or hands are to the driver’s spine because those distances are into the surface of the display plane. In general, when the 3-D world is shown on a two-dimensional display plane, positions across the plane seem easier to judge than ones along the line of sight into the plane.

Despite the back view’s being closer to how a driver perceives the situation, there are still problems with the illustration. A driver actually sees the situation from further forward, nearer the vertical plane of the eyes. In other words, the driver would not see her own back. The position shown in Fig. 3 also prevents our seeing the left arm, something that does not happen to real drivers. A subtler problem is that, although this view shows the horizontal distance between the feet, it does not let us judge the front-to-back distance of the feet from the spine. When pressing on a clutch pedal, the driver will be pushing her feet away from the viewer and into the display plane, a distance that is difficult to show in this view.

Fig. 2 shows the same driving position but rotated 90 around the vertical axis of the display plane.

One could simply move the camera forward, but that would not show an overall configuration of the body position. Whether it is more important to show an exact duplication of the person’s view or a view of the overall configuration has not been thoroughly


Fig. 3.

Automobile driver from the back.

addressed by research. Another possibility is to move the camera overhead, thereby showing body positions and movement across the display plane and somewhat in the direction from the body center. The two images in Fig. 4 have the same viewpoint,

Fig. 4.

Compromise views across the display plane.

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Fig. 5. Two three-quarter views.

the canonical view advocated by much of the research literature we have reviewed. This view, while shown to be effective for some object-oriented tasks, has two disadvantages for the task being shown. First, a person using this image to learn to drive would have to rotate the image nearly 180 to picture herself in the same body position. Second, this position places the horizontal distances among limbs at an angle to the display plane. That makes them more difficult to judge than if they were directly across the display plane. The body shown on the right in Fig. 5 is an alternative that comes closer to the performer’s view of the task. While someone using this image to learn to drive would need to perform some mental rotation, the amount of rotation is smaller. However, this image shares one disadvantage of the canonical three-quarter view: horizontal distances among limbs are placed at an angle to the display plane. In addition, the driver’s body blocks our view of one arm, similarly to the view directly from the back. Krull, Roy, D’Souza, and Morgan asked test subjects to match upright images similar to those in Figs. 1–3 and 5, in order to ascertain the optimal image or point of view [39]. This matching task was designed to get test subjects to think about the image in a body-centered way. Krull et al., produced images of a woman driving and of a man holding a box. The body positions used in testing varied limb positions across and into the display plane.

a somewhat higher position than the driver’s real body sense. This viewpoint shows the direction of the movement of the left leg across the display plane without violating the driver’s sense of that movement relative to the spine. Supplying two illustrations offers viewers a before-after comparison of the foot positions.

Three-Quarter Views Fig. 5 shows two versions of three-quarter views. The body shown on the left is

Contrary to expectations, neither the three-quarter canonical view nor the three-quarter rear view produced the most accurate performance or highest confidence in choices on the part of test subjects. In general, the highest accuracy and confidence levels were generated by images that placed critical distances across the display plane. The front and, most surprisingly, the back view worked best. Perhaps the particular back views used in testing showed enough of the feet of the woman driving and the box being held by the man for test subjects to make the judgments required.

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The side view worked the least, even though it did place some distances directly across the display plane. Further analysis showed that this most likely was the result of a idiosyncratic interaction among test images. Additional testing is needed to confirm that interpretation.

The object-centered and body-centered three-quarter views may have tested poorly for two reasons. First, we asked test subjects to make fine comparisons among body positions. These comparisons required more subtle judgments of distances than seems typical of the research literature we reviewed. Second, rotations we examined in this study were around the vertical axis of the display plane. Most of the studies we examined rotated objects across the display plane, where it is easier to detect. Since objects illustrated in technical documentation are likely to be rotated around several axes and not only across the display plane, the issue of rotation axes may turn out to be an important one for technical communication.

WHERE DO WE STAND? Research suggests that human beings apply two different mental processes to visual information about the 3-D world. One process is object-centered; the other is body-centered. People appear to apply the process most suited to a specific task and environment. It is probable that procedural illustrations will be most effective when they represent the 3-D world in a way that matches object-centered or body-centered tasks. A complicating factor in choosing a specific point of view is that positions, distances, and movements shown across the display plane are easier to interpret than those shown into the display plane. So, while the bulk of research on object-centered tasks suggests that a three-quarter view of objects and bodies will be optimal in general, some research hints that this choice is not universally best. Finally, the optimal point of view for body-centered tasks is an open question. For these tasks, directly-facing views may be more effective than three-quarter views for some judgments.

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Robert Krull is Professor of Communication at Rensselaer Polytechnic Institute, Troy, NY. He has conducted research on layout and content for print and online media. His work on procedural illustrations is part of a line of research dealing with text and illustrations designed to support people’s learning physical skills. He has won best-article awards from the IEEE Professional Communication Society and the Society for Technical Communication.

Shreyas J. D’Souza is a master’s student at Rensselaer Polytechnic Institute, Troy, NY, and has worked as a technical communication professional in the computer industry. He has applied the principles reviewed in this paper both to computer documentation and instructions for competitive dancers.

Debopriyo Roy is a doctoral student of Technical Communication and Human-Computer Interaction at Rensselaer Polytechnic Institute, Troy, NY. He has master’s degrees in economics and human communication and has work experience in sales and management. Presently, his research is focused on graphical illustrations of human performance and complex systems functioning for technical manuals.

D. Michael Sharp is a doctoral student of Human-Computer Interaction at Rensselaer Polytechnic Institute, Troy, NY. He received his master’s degree from University of Colorado, Denver. He has worked as a technical writer in various organizations. He is research interests include HCI interface design and technical graphics for instructional purposes.