CHAPTER 4 Design Visualization

VISUALIZATION FOR DESIGN

4.1

Students will come to your class with a truly diverse abilities to mentally create and manipulate graphic imagery (visualization). Either through their life experiences or through innate ability, some students are simply better at visualization than others. This does not mean that those who don't come to your class with strong skills can't be taught many of the skills presented in the text. What it does mean is that it will be worth your while to try to informally assess your student's visualization abilities; either through exercises presented in this chapter, direct observation, or other methods. The ability level of your students may influence the level of instruction needed to get students to an appropriate level of proficiency.

It is important to emphasize the dynamic qualities of the visualization process. Not only can this dynamic process be taking place solely in one' head, but also between the mind, the eyes, and some physical stimulus such as a drawing or an object.

To apply these ideas in a more functional way, have your students experience this feedback loop. If you have already done some sketching exercises, then ask them to sketch a simple object in pictorial form. Now verbally describe changes you want them to make in their object (e.g. drill a hole through it, chamfer a corner, etc.). Ask them to first mentally imagine this operation and then sketch it. They can also do this completely on their own; have them start with a simple shape and then transform it into a common household object over a series of four or five sketches.

4.1.1

SOLID OBJECT FEATURES

4.2

Having physical objects of simple geometric shapes available is a great help in explaining the concepts and conducting exercises in this section. They can be quickly made from wood, foam, clay, paper, etc.

Make sure the students can 'see' the features which define the objects. Some of these features represented on drawings (e.g. edges and faces) have direct correspondence to physical attributes of the object. Other features depicted in a drawing (e.g. limiting elements and center lines) do not have a corresponding physical element. This does not mean that students cannot develop an ability to visualize these elements.

Make sure the students begin to develop both a beometric and topological understanding of the objects. That is, that if you ask them halve the area of the end face of a square prism, they understand its geometric effect on the side faces. To understand the geometric effect on the side faces, they must also understand the topological connectedness of the faces of the object. What happens when you halve the area of a face on a cube? Do you use the same process to gauge its effect on the faces? Are the same number of faces effected? What happens when you halve the end face of a cylinder?

GENERAL VISUALIZATION TECHNIQUES

4.3.1

Very soon into working with this chapter, you will probably want to start looking at combinations of simple primitives. Though introductory graphics classes have typically focused on working with single objects, these objects can become very complex by the end of the term. One way of helping students understand these more complex forms is to think of them as combinations of more simple geometries. As 3-D modeling becomes more prevalent, it will be that more important to teach visualization skills relating to the interaction of primitive objects. Solid modeling system largely create complex shapes through a series of operations with simple primitives.

Two important aspects to the interaction of solid objects are the geometries and topologies of the individual objects and the spatial relationship between them. The spatial relationships of solids requires a new set of visualization skills. When working on paper it is important to establish a coordinate system and its associated axes to work with. When visualizing, it is equally important to establish a frame of reference. Quite often when visualizing these objects, you are going to be doing it dynamically; for example, rotating one object relative to the other or moving one object along an axis towards the other.

When orienting objects relative to each other, features on the objects can be extremely useful in establishing local frames of reference. These features can be physical attributes such as a face or corner or elements such as the center of a whole. The ability to visualize these features on a single object is critical to use them in conjunction with other objects.

Once objects have been established relative to each other, students can begin to visualize interactions between them. One of the simplest is to join the objects together (additive) and then rotate or translate them as a pair. More difficult is to visualize subtractive operations between the two objects. Perform subtractive operations between a number of objects; look at the effects of passing one object through the other in a translating (linear) operation and in rotational (sweeping) operations. What happens when a cylinder and a sphere translate through another object? What is the shape of the holes they make? Does it matter what the orientation of the objects are relative to each other? What if they are swept instead?

With all of these manipulations between solids, always make sure that students are able to go back and evaluate the changes in geometry and topology in the object. What new faces and edges have been created? What ones have been removed? Learning to do there analyses mentally will help students troubleshoot their drawings and develop 3-D modeling strategies.

4.3.2

When thinking of cutting planes, you can imagine them passing into or through the object without disturbing it and you can use the plane to remove portions of the object. Being able to visualize both these operations are important.

Joining the plane with the object allows the students to develop new reference points on relative to the cutting plane. Though these reference points are most useful in making comparisons (such as for evaluating symmetry) internal to the object, they can also be used to make comparisons to external objects.

Using the cutting plane to remove material from the object gives the student another visualization tool to modify the geometry. This technique is very applicable to sectioning techniques and is also a common tool used in 3-D modeling systems to modify geometry. As was mentioned earlier, students should feel comfortable with rotating and translating the plane relative to the object and understanding how it will change the modification of the object.

Combining both the joining of a cutting plane to an object and its ability to 'remove' material is a useful method for teaching about normal, inclined, and oblique faces. With the cutting plane joined to the object, various existing faces on the object can be evaluated relative to their orientation to the cutting plane. Removing material on one side of the cutting plane generates a new face which can be visualized. Have students both translate and rotate the plane within the object. Which types of actions affect the type of new face (i.e. normal, inclined, oblique) produced? Which simply affect the shape of the new face? How does the geometry of the object affect the shape of the new face?

Symmetry can be a tough concept to teach. It does not have a direct correspondence to a physical attribute yet it is a critical feature of many objects; determining object/view orientation, number of views represented, dimensioning strategy, etc. Though symmetry is often easy to recognize in simple objects, it can become more difficult in more complex forms. Have students get comfortable using devices such as cutting planes to help them systematically analyze an object for symmetry.

4.3.4

VISUALIZATION TECHNIQUES FOR ENGINEERING DRAWINGS

4.4.1

Another important use of planes is as an image plane; a primary component of projection theory. This section can be used in conjunction with the introduction of the principles of projection in Chapters 4, 7, 8, and 9. There are many ways of teaching the concept of projecting an image on a plane though often the most effective method is a direct experiential one. This section presents a number of Practice Exercises incorporating a physical object and an image plane made from clear plastic. If there is time and materials, the best option is to allow the students to do these themselves.

You can choose whether you want to, at this time, emphasize the more formal nomenclature of projection theory such as line of sight, parallel versus perspective projection, etc. Probably more important to avoid introducing informal terms for some of these elements that later have to be ignored when more formal terms are introduced.

4.4.2

4.4.3

4.4.4

GRAPHICAL ANALYSIS OF ENGINEERING DATA

4.5

DATA VISUALIZATION ELEMENTS

4.5.1

This section begins the development of the taxonomy of visualization methods. Figure 2.66 lays out the structure of the taxonomy and the structure of Sections 4.5.2 and 4.5.3.

Though engineering students have been working with numbers intensively for years they may, in fact, have had little or no exposure to the terms used to describe data types. Many of these terms are more likely to show up in a statistics text than an engineering text. One of the goals is merging techniques which have been used in the social sciences — and to some degree in the hard sciences — with engineering analysis. Of interest is less the methodology than the data manipulation and visualization techniques.

Marks are at the heart of the perceptual basis of data visualization. What largely distinguishes one type of visualization from another is the type of marks it uses. Equally important is the subtle variations that can be applied to the marks by the user. This section and the next two show examples of how marks can be manipulated to more effectively communicate the information in the visualization. Good and bad usage of marks is a great way to demonstrate the power of these perceptual cues.

Though independent and dependent variables were alluded to in the first section of the chapter, they are formally introduced here. The design analysis problem in Figure 2.2 is also formalized with a visualization showing the relationship on a independent and dependent variable. The visualization types presented in Section 2.3 is broken down by the number of dependent and independent variables. This was done to assist students in matching visualization techniques with the type of data they are/will be working with.

VISUALIZATIONS FOR ONE INDEPENDENT VARIABLE

4.5.2

It is noted that graphs, charts, and plots are all common terms for types of visualizations. There is little consistency in how these terms are applied and the chapter tries to stay with the most common usage.

This section covers the most common types of visualizations: line and bar graphs. This section also introduces the student to a design theme that goes throughout the chapter: less is more. Test will be — just as with any engineering graphic — what information is being conveyed and are there graphic elements to support each of these pieces of information. There are numerous parallels between the impact of CAD in engineering drawings and the effect of visualization packages on graphs and plots.

The section on line graphs introduces the concept of regression lines and error bars. Though a full appreciation of these techniques requires some background in statistics, students can still be introduced to them so they know what they are seeing when they run across examples of them.

Quite a few examples of bar graphs are given, in part, because of their popularity and flexibility in presenting different types of data. It is probably worth spending a few minutes making sure the students have a basic understanding of the different types of bar graphs and their appropriate usage.

VISUALIZATIONS FOR TWO INDEPENDENT VARIABLES

4.5.3

Students are also likely to encounter analytic data with two independent variables. They are also much less likely to attempt visualizing this type of data, not so much because of a lack of tools to do so as a lack of understanding as to how to do so.

Multiple line graphs are a very powerful technique and allow the integration of a much higher density of data than a single line graph. One of the biggest pitfalls of this technique is choosing an appropriate encoding technique for the second independent variable representing the different lines on the graph. Remind students that they must keep in mind the different medium in which the visualization will be displayed. Will there be color? How big will the graphic be in relation to the distance it will be seen at? What is the resolution of the medium; will fine detail be lost?

As mentioned in the section, time is a popular second independent variable and animation techniques are a powerful method of displaying it. Even if the students do not have the opportunity to explore animation techniques as part of a lab, try developing a demo which compares the static presentation of sequential visualizations and the same data presented as an animation. It is also worth looking at the difference in presenting the sequential visualizations in parallel (i.e. all in a single graphic image) or serially (i.e. as a slide show). Each technique has its own advantages and disadvantages which is worth imparting on the students.

3-D graphs and plots represent the first use of the third dimension in the chapter. This and other perceptual cues such as color and hidden line removal (occlusion) are all important considerations in designing such visualizations. This is an example of a visualization that was rarely used before the advent of computer-based visualization tools. It is now a standard tool in many packages and is very effective when used properly.

Area rendering is not very common in engineering but has become a very popular technique in other areas such as medicine and the earth sciences. Images created by medical professionals, such as X-rays and CAT scans, lend themselves to encoding as digital renderings as do the satellite images used by geologists and meteorologists. Much more in-depth information on 2-D image generation and manipulation can be found in many of the references at the end of the chapter.

Vector and flow representations are less common than some of the other areas but have considerable potential for application in areas such as aerodynamics and fluid dynamics.

FUTURE DIRECTIONS

4.6