484 Part V: On a 3D Spree

the UCS will be aligned with. Click over the face to align the UCS with the face, and then finish the command. The circle is aligned with the face of the 3D solid, and the previous UCS is then restored.

Figure 21-4: Dynamically create a UCS on 3D solids.

Navigating the 3D Waters

If you’re brand new to 3D in AutoCAD, you may be wondering how to look at whatever it is you’re modeling from whatever angle you desire.

One way to change viewpoints that works in both AutoCAD and AutoCAD LT is to use the preset views from the Ribbon by selecting the View tab, and

then clicking the down arrow at the bottom of the 3D preset views drop-down menu on the Views panel (shown in Figure 21-5) to switch to one of the standard orthographic 3D views or an isometric view.

The six standard orthographic (straight-on) views are Top, Bottom, Left, Right, Front, Back.

The four standard isometric views are SW (left-front), SE (right-front), NE (right-back), and NW (left-back). (An isometric view is one in which you see the object from above — or below, but AutoCAD’s standard views don’t do below.)

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Figure 21-5: 3D Preset Views drop-down menu.

An isometric view is an unrealistic 3D view because all lines that are parallel in reality are also parallel in the view. In a perspective view, lines that are parallel in reality appear to meet at a vanishing point, as parallel lines appear to do in reality (think of railroad tracks heading across the prairie). The isometric views we mention previously are parallel projection views, and you can view 3D models by using those views in either the full version of AutoCAD or AutoCAD LT. Perspective views are more commonly used in architectural applications, and parallel views are more commonly used in mechanical design.

This warning is for AutoCAD LT users (users of the full version of AutoCAD don’t need to worry). As we point out earlier, AutoCAD LT can open drawings created in the full version. Those AutoCAD drawings can be saved with perspective projection current, and if you open such a drawing in AutoCAD LT, you can see it in perspective. You can switch from perspective to parallel projection by changing the PERSPECTIVE system variable’s value from 1 to 0. Be very careful if you do this because you can’t change it back again! You could use UNDO to reverse the change, but if you save and close the drawing, you’re sunk.

The six orthographic and four isometric views work well for showing 3D models of common objects such as mechanical components and buildings. You can also change to plan view, which is a top-down view of either the world coordinate system or a user coordinate system.

AutoCAD LT has limited 3D viewing capabilities. The same preset views are in both AutoCAD and AutoCAD LT, and you can also use the Viewpoint Presets dialog box (DDVPOINT command) in which you set a viewing position by specifying angles in and from the XY plane. Finally, there’s the really ugly command line only VPOINT command. (Man, that one’s soooo ugly, it’s not even on the Ribbon!)

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486 Part V: On a 3D Spree

Orbit à go-go

Preset views are fine for many 3D modeling tasks, but if you really want to have fun, 3DOrbit (not in AutoCAD LT) is your ticket to it. (Orbiting a 3D model in AutoCAD is similar in concept to orbiting around the Earth in a satellite — only a lot cheaper.) The two orbiter modes are Constrained and Free. Constrained mode is pretty much like the Free mode with training wheels.

Free Orbit displays an arcball on the screen — a circle representing a sphere around your object. You click various places inside, outside, and on the arcball and then drag to change the 3D view. The idea is that you’re spinning an imaginary sphere containing your model. As you drag the cursor, AutoCAD updates the screen dynamically.

You can start Constrained orbiting, the 3DOrbit command, by holding the SHIFT key down and then holding the middle mouse button while dragging the mouse.

You might also want to experiment with different projection modes:

Parallel projection is the default AutoCAD projection. Lines that are parallel in the 3D object remain parallel in the projected view on the screen.

Perspective projection makes objects look more realistic (for example, train tracks appear to converge in the distance), but lines that are parallel in the model don’t appear parallel in perspective projection.

If you manage to 3DOrbit out of control and can no longer see your model, right-click to display the 3DOrbit shortcut menu and choose Zoom Extents. The Zoom, Pan, and Preset Views options offer other ways of getting your model back in your sights.

When you start orbiting with no objects selected, AutoCAD tries to update the display of everything in your model, and this can take some time. To speed things up or to simply regain your bearings, try selecting some objects before you start orbiting. Then AutoCAD updates the display of the selected objects only. When you exit orbit mode, the entire model redisplays based on the new viewpoint.

Taking a spin around the cube

With so much talk about hybrid devices these days, Autodesk decided to create its own in the form of the ViewCube (not in AutoCAD LT). The

ViewCube (see Figure 21-6) is an interactive tool that provides visual feedback about the current viewpoint, allows you to set a preset view current or orbit the model, restore a named UCS, and define and restore the Home view of a model.

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Chapter 21: It’s a 3D World After All 487

Compass

ViewCube menu

UCS menu

Adjustment facet triangle

Figure 21-6: ViewCube, the multifunctional viewing device.

To change the view of the model by using the ViewCube, you can do one of the following:

Click a corner, an edge, or a face to align the current view with the same viewpoint represented on the ViewCube.

Click and drag the main area of the ViewCube to orbit the view of the model.

Click one of the roll arrows to rotate the current view 90 degrees.

Click one of the adjacent face triangles to switch to the adjacent orthographic view that is indicated by the triangle.

Click one of the letters (N, E, S, W), or click and drag the compass to rotate the view around the center of the drawing.

Home view is a special view that you can define so you have a known reference view in the model. That way, if you lose your bearings, you always have a way to get back home. You can restore the Home view from the ViewCube or the SteeringWheels right-click menus.

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