2. Learning Viewing Options in MGED

In this lesson, you will:

Create a model radio.
Locate viewing information in the Command Window.
Identify elements of the MGED viewing system.
View your radio from different angles.
Work with Shift Grips.

Models in BRL-CAD are constructed in a single xyz coordinate system, which we sometimes refer to as model space. The Graphics Window of MGED displays a portion of this space. The xyz coordinate system is used for specifying both the geometry and the view of the geometry that is presented in the Graphics Window.

MGED offers a default view and a variety of optional views. You can switch back and forth between these views during and after model creation. This lesson is designed to help you understand the viewing process and options.

1. Creating a Radio

To gain practice viewing actual geometry, let’s build a simple geometric object-a "walkie-talkie" radio. Note that the commands we use to do this are not discussed here because our current concern is on applying the principles of viewing. Later lessons on creating geometry address these commands in detail. Begin by launching MGED and creating a new database named radio.g. Remember that one way to do this is to type the following command in a Terminal Window:

$ mged radio.g

Type the following in the Command Window, carefully checking each line before pressing ENTER. If you make a mistake, use BACKSPACE or the left/ right arrow keys to make corrections before pressing ENTER (see Appendix B for the editing command list).

Be especially careful to note the difference between the numeral one [1] and the letter l [l] in ell1 on the third line.

       in body.s rpp 0 16 0 32 0 48 ENTER
       in btn.s rec 8 30 36 0 3 0 4 0 0 0 0 2 ENTER
       in btn2.s ell1 8 33 36 4 0 0 2 ENTER
       in spkr.s tor 16 16 16 1 0 0 12 1 ENTER
       in ant.s rcc 2 2 46 0 0 48 1 ENTER
       in knob.s rcc 4 4 40 8 0 0 5 ENTER

Note that in the preceding Command Line expressions, btn is an abbreviation for button, ant is an abbreviation for antenna, and spkr is an abbreviation for speaker. Also note that the numbers could have been separated by single spaces. The extra spaces were inserted simply to improve readability. For some usages (e.g., the r and comb commands, which are discussed later), the number of spaces has to be exact.

An image similar to the following should now appear in the Graphics Window.

Figure 1. Default View of a Radio

2. Locating Viewing Information in the Command Window

Now take a minute to look at the Command Window. Even if nothing is in the window, enclosed in the bottom border is a string of information about the Graphics Window. An example string might read: cent=(8.000 16.000 24.000) sz=96.000 mm az=35.00 el=25.00 tw=-0.00 ang=(0.00 0.00 0.00)

As detailed in the following table, this information contains four groups of viewing data about the Graphics Window.

Table 1. Viewing Data at the Bottom of the Command Window
Screen Designation	Viewing Information	Location of Variables	Default Units	Default Values
cent=	Center of View	First 3 numbers	Millimeters	0.000 0.000 0.000
sz=	Size of View	4th number	Millimeters	Dependent upon size selected
az=	Viewing Angle	5th and 6th numbers	Degrees	35.00
el=	Viewing Angle	5th and 6th numbers	Degrees	25.00
tw=	Twist	7th-10th numbers	Degrees	0.00
ang=	Angle of View	7th-10th numbers	Degrees	0.00 0.00 0.00

3. Identifying Elements of the `MGED` Viewing System

3.1. Center of View

The first set of information tells you the center of what you are viewing. You can change the center of where you are looking through both the GUI and the Command Window.

To change the center of your view of the radio using the GUI, press the SHIFT key and any mouse button while dragging the mouse. (This is an example of a Shift Grip, which is described later in this chapter.) You can also change the center of view by placing the mouse pointer where you want the center to be and clicking the middle mouse button.

To change the center of view using the Command Window, simply type at the prompt the word center followed by three values for x, y, and z (which is the 3-D coordinate system mentioned previously). For example:

center 0 15 325.735 Enter

As you change your view of the geometry, notice that the numbers in the brackets after the cent= title will change to reflect the new center of the view.

3.2. Size of View

The size of the view is the amount of model space that is shown in the Graphics Window. For example, consider using a camera with a zoom lens to photograph a rose. As shown in the following figures, if you zoom in on the rose, it will appear large in relation to your viewing field. If you zoom out, it will appear smaller. In actuality, the view size for the rose image on the left might represent only 15 mm across while the view size for the image on the right might represent 100 mm across. In both cases, however, the actual size of the rose is the same.

Figure 2. Zoom In to View Details (small size of view)

Figure 3. Zoom Out to View Object in Relation to Environment (large size of view)

To change the view size of your radio through the GUI, click the right mouse button to zoom in and the left mouse button to zoom out. Each time you click the left or right mouse button, the view of the design will increase or decrease in size by a factor of 2 (i.e., two times larger or two times smaller than the previous size).

You can also zoom in or out on your design by going to the View menu and selecting Zoom In or Zoom Out. A drawback to this method is that you can only zoom in or out one time because the drop-down menu closes once you make a selection.

If you get lost at any point while creating a model, you can use the zap (Z) command to clear the geometry from the Graphics Window and then recall the shape with the draw command. When drawing in an empty Graphics Window, MGED automatically sizes the view to fit what you draw into the window.

You can control the view size of your radio more accurately with the Command Window. To set the size to 100 (of whichever units you have selected), type at the prompt:

size 100 Enter

You can also zoom in or out on a design by typing zoom on the Command Line. To make your radio appear 50% smaller, you would type:

zoom 0.5 Enter

To make your radio appear twice as large, you would type:

zoom 2 Enter

Remember that changing the view size does NOT affect the size of the object. You will change the size of an object in Lesson 6.

3.3. Angle of View

Azimuth, elevation, and twist (all measured in degrees) determine where you are in relation to the object you are viewing. Azimuth determines where you are around the sides of it (i.e., to the front, left, right, behind, or somewhere in between), elevation determines where you are above or below it, and twist determines the angle you are rotated about the viewing direction.

To better understand azimuth, imagine walking around a truck with a camera to photograph it. As shown in the following illustrations, you would be at 0'0 azimuth if you stood directly in front of the truck to take its picture. If you circled around slightly to your right, you would be at 35'0 azimuth. If you moved further around until you were looking directly at the driver’s side (in U.S. trucks), you would be at 90'0 azimuth. Standing behind it would put you at 180'0 azimuth. If you were facing the passenger’s side, you would be at 270'0 azimuth.

The terms azimuth, elevation, and twist are similar to the terms yaw, pitch, and roll, respectively, which are common terms in the aerospace industry.

Figure 4. Front (az=0, el=0)

Figure 5. az=35, el=0

Figure 6. Left (az=90, el=0)

Figure 7. Rear (az=180, el=0)

Figure 8. Right (az=270, el=0)

Elevation, on the other hand, involves the viewer’s position above or below an object. In the preceding example, you circled around a truck without changing your relative height. You had an elevation of 0'0, which means you were level with it. As the following figures illustrate, however, imagine stopping at the 35'0 azimuth position and then climbing a ladder to photograph the truck from 25'0 elevation. Climbing higher, you would be at 60'0 elevation. If you were directly above it with the camera facing down, you would be at 90'0 elevation. If you crawled under the truck and looked directly up at it, you would be at -90'0 elevation.

Figure 9. az=35, el=0

Figure 10. az=35, el=25

Figure 11. az=35, el=60

Figure 12. az=35, el=90

Figure 13. Top (az=270, el=90)

Figure 14. Bottom (az=270, el=-90)

Finally, twist (which is an optional setting in MGED) specifies a rotation about the viewing direction. This rotation is applied to the view after azimuth and elevation have been designated. So, returning to our truck example, imagine standing in front of the vehicle (az=0, el=0) and then tilting your camera counterclockwise 14'0. This would give your view a 14'0 twist angle, as shown in the following figure (on the left). Note again that it is not the truck that is tipped up, but simply your view of it. For more information on specifying twist, see the ae command in Appendix A.

Figure 15. Front (az=0, el=0, tw=14)

4. Summing up on Azimuth and Elevation and the xyz Coordinate System

As mentioned at the start of this lesson, MGED operates in a three-dimensional coordinate system (determined by the x, y, and z axes). Azimuth is measured in the xy plane with the positive x direction corresponding to an azimuth of 0'0. Positive azimuth angles are measured from the positive x axis toward and past the positive y axis. Negative azimuth angles are measured in the other direction.

Figure 16. Azimuth, Elevation, and the xyz Coordinate System

If the azimuth angle is 0, then elevation is measured in the xz plane with +90'0 corresponding to the positive z direction and -90'0 corresponding to the negative z direction. However, if azimuth is not 0, these angles are in a plane aligned with the azimuth direction.

5. Viewing Your Radio from Different Angles

Let’s now experiment with different views of your radio. MGED has several standard default views, which you’ve already seen in the preceding truck example. They include Top (az270, el90); Bottom (az270, el-90); Right (az270, el0); Left (az90, el0); Front (az0, el0); Rear (az180, el0); az35, el25; and az45, el45.

Go to the View menu and try viewing your radio from different angles.

Figure 17. Top

Figure 18. az35,el25

Figure 19. Right

Figure 20. Front

You can also select any azimuth-elevation combination from the Command Line. For example, at the prompt type

ae 128 17 Enter

As with many of the Command Line options, this method of selecting views provides a finer degree of control/precision when you need it.

MGED can also display multiple views simultaneously. Go to the Modes menu and select Multipane. Four small panes with different views should appear in your Graphics Window, as shown in the following illustration.

Figure 21. Multipane View of the Radio

6. Working with Shift Grips

The Shift Grip options of MGED are handy hot-key and mouse button combinations that can be used in two different ways. With regard to our present discussion on viewing, the Shift Grips can, in effect, "drag" the world around in front of the viewer (but without actually changing the coordinates of the viewed objects). The same Shift Grips can also be used in Edit mode to actually move or alter the geometry of your objects. In both cases, the Shift Grips appear to do the same thing, so it is important always to know the mode in which you are operating.

In general, the SHIFT key translates (moves), the CTRL key rotates, and the ALT key constrains (or limits) translation or rotation to a particular axis (x, y, or z). These axes correspond to the three mouse buttons as follows: the left button represents the x axis, the middle button represents the y axis, and the right button represents the z axis. In addition, the SHIFT and CTRL keys can be used in conjunction with any mouse button to scale an object (although the ALT key will not constrain this action). The following table lists all of the key bindings and their functions.

Table 2. Shift Grip Keys and Effects
Function	Key Combination	Effect in Normal Viewing	Effect in Edit Mode
Translate (Move)	SHIFT + any mouse button + mouse drag	Moves view in any direction	Translates object in any direction
Rotate	CTRL + any mouse button + mouse drag	Rotates view in any direction	Rotates object in any direction
Constrain Translation	SHIFT + ALT + left mouse button + mouse drag	Moves view in the x direction	Translates object in the x direction
	SHIFT + ALT + middle mouse button + mouse drag	Moves view in the y direction	Translates object in the y direction
	SHIFT + ALT + right mouse button + mouse drag	Moves view in the z direction	Translates object in the z direction
Constrain Rotation	CTRL + ALT + left mouse button + mouse drag	Rotates view about the x axis	Rotates object about the x axis
	CTRL + ALT + middle mouse button + mouse drag	Rotates view about the y axis	Rotates object only about the y axis
	CTRL + ALT + right mouse button + mouse drag	Rotates view about the z axis	Rotates object about the z axis
Scale	SHIFT + CTRL + any mouse button + mouse drag	Scales view larger or smaller	Scales object larger or smaller

Depending on your window manager or desktop environment settings, some key combinations may already be designated to perform other tasks (e.g., resizing or moving a window). If so, you may need to adjust settings to allow the Shift Grip options to function. Furthermore, left-handed users may have switched the behavior of the left and right mouse buttons in their system configurations. In such instances, the terms left mouse button and right mouse button should be switched throughout this document.

Probably the easiest way to familiarize yourself with the Shift Grip options is to try them out on your radio. Using the preceding table as a guide, experiment with translating, rotating, constraining translation and rotation to particular axes, and sizing your radio view.

Remember, although the Shift Grip options may appear to be manipulating objects, unless you are in Edit mode they are only manipulating your view of the objects.

7. Review

In this lesson, you:

Created a model radio.
Located viewing information in the Command Window.
Identified elements of the MGED viewing system.
Viewed your radio from different angles.
Worked with Shift Grips.