The following chapter accompanies the book AutoCAD 2006 For Dummies by Mark Middlebrook and David Byrnes.
Chapter A
On a 3D Spree
In This Chapter
* Understanding 3D pros and cons
* Looking at 3D models in multiple viewports from different viewpoints
* Orbiting around 3D models
* Using user coordinate systems and specifying coordinates in 3D
* Drawing 3D wireframes, surfaces, and solids
* Getting the rendered look
For millennia, people have documented the design and construction of three-dimensional objects by drawing two-dimensional views of them. Most people have continued to use these “classical” methods with CAD because the methods are well understood and work reasonably well. After all, if 2D drawing was good enough for guys like Leonardo da Vinci and Andrea Palladio, it should be good enough for us, right?
Nonetheless, there’s a growing trend of using CAD programs to create 3D models and letting the CAD program generate the 2D views more-or-less automatically. This approach seems more logical, especially if the documentation requires numerous, complex views of the same object. 3D modeling also is a virtual necessity (pun intended) when you want to create rendered (shaded) views for presentation purposes.
The 3D construction and visualization tools in AutoCAD have improved dramatically over the years (trust us – you should have seen what they were like ten years ago!) to support this 3D modeling approach to CAD, but it’s still a complex process that requires sophistication on the part of the CAD user. Although 3D modeling requires only one more dimension than 2D drafting, developing 3D CAD models is considerably more complicated. Users must master new techniques and contend with the 2D limitations of most display screens and input devices.
This chapter introduces you to the concepts, tools, and techniques for AutoCAD 3D modeling and visualization. Be prepared to spend some additional time wading around in the AutoCAD online help system and experimenting if you want to become proficient at 3D modeling. And if 3D CAD makes you feel a little wobbly at first, Figure A-1 shows that you’re not alone.
Figure A-1: Leaning toward 3D.
AutoCAD LT:
Full 3D support is one of the main differentiators between full AutoCAD and AutoCAD LT. If you’re using AutoCAD LT, you can look at and plot 3D models created in AutoCAD, but you can’t do much 3D object creation or editing yourself. Also, viewing 3D models is less flexible in AutoCAD LT because it lacks AutoCAD’s 3DOrbit command.
The concept of 3D hardly seems to need introduction. We live in a three-dimensional world, and most of the objects that you represent in your 2D AutoCAD drawings are three-dimensional.
Traditional 2D drawings provide clues to help the viewer’s mind construct a 3D model from the 2D image on paper. Multiple views from different viewpoints in 3D space give experienced designers, drafters, and builders the information they need to make 3D sense of 2D drawings. Design and drafting have succeeded pretty well by using 2D representations as the guide to creating 3D objects. But at some point, nothing can replace a true 3D model, such as in helping someone understand how a building will look when constructed or how two parts fit together.
3D thumbs up
Using 3D takes time to master, creates additional work, and slows down your computer. Why bother using it? Here are four key reasons why anyone in his or her right mind would bother with 3D:
* It’s the wave of the future. As CAD pursues greater realism and production efficiency, 3D is becoming important for more tasks in more professions. Drafters and designers who want to keep up with how CAD is likely to be used in the future should become familiar with 3D now.
* Sometimes it’s nice. Drawing in 3D is useful for several tasks, including creating shaded renderings to help sell a design to a client, and fit-and-finish testing to find potential problems before a design is put into construction or manufacturing.
* Sometimes it’s needed. Drawing in 3D is required for a small but growing number of tasks. Many mechanical designs are done in 3D or converted into 3D at some point in the design process. 3D perspective views make drawings easier to understand. And the shaded renderings used for both designing and selling are becoming a practical necessity in some fields.
* Sometimes it’s faster. The fastest way to create a single view of something usually is to draw that view in 2D. If you need multiple views, it may be faster to create a 3D model and then slice and render it as needed for the views you want to create.
3D thumbs down
Consider these issues before you decide how much to use 3D on a particular project:
* 2D input and editing: The mouse, keyboard, and drawing tablet are all 2D devices; the more complex the 3D object you’re trying to model, the more complicated it is to construct the object with these devices. In addition, the AutoCAD 3D editing tools are fairly limited in scope -- especially for solids. For most heavy-duty 3D work, you need a third-party application, a discipline-specific version of AutoCAD (such as Architectural Desktop), or a special-purpose solid modeling program.
* 2D output: Most of the currently available output methods, notably paper and the computer screen, are 2D. Presenting your 3D model in its full, three-dimensional glory requires extra work by you or the viewer.
* Performance: Today’s personal computers are adequate for the task of storing fairly complex 2D models and displaying them on-screen and on plots; if the model is 3D, the difficulty increases geometrically, and performance seems to slow geometrically as well. You may need a faster computer and more memory to meet these demands. You may also find yourself taking longer coffee breaks as you wait for your computer to load or render complex 3D models.
What does using 3D in CAD mean? Fundamentally, it means creating models instead of views. Rather than generate cross sections of an object or individual views of it from certain perspectives, you create a three-dimensional model of the object. This 3D depiction of each object includes all the necessary information for AutoCAD to create a view from any point of view. With a properly constructed 3D model, AutoCAD can output commands to machines to create actual 3D objects, whether plastic prototypes carved from a tank of jelly by lasers or an actual bolt, valve, or piston created by computer-controlled machine tools.
AutoCAD, like most 3D CAD programs, enables you to create three different kinds of 3D models of objects:
* Wireframes: A wireframe model is like a skeleton of a 3D object; it shows the edges of the object, not any of its surfaces. You create a set of 2D objects that represent an outline of each part of the object and then connect them three-dimensionally to make the wireframe. It’s like building a model from wire coat hangers. One of the biggest limitations of wireframe models is that you can’t shade them; there aren’t any surfaces “inside” the wire edges to catch the light -- imagine shining a flashlight on a coat hanger.
* Surfaces: A surface model represents the “skin” of an object but not the solid mass inside. AutoCAD uses objects called meshes to create surfaces. A mesh is a faceted surface that represents the edges and surfaces of a 3D object. You also can create a surface mesh by sweeping a 2D object such as a polyline around an axis. Creating a surface model is like building a physical model out of thin sheets of balsa wood. A surface model is one step up from a wireframe model because you can apply material properties and shading to its surfaces.
Warning:
Surface meshes are just the thing for some 3D modeling tasks, but they have limitations:
* Some 3D objects are awkward to build by pasting surfaces together.
* You can’t check mass properties or interferences of a surface model. (AutoCAD doesn’t recognize that there’s any solid mass inside the surfaces.)
* Solids: A solid model is as close to true 3D as you can get without whipping out some Play-Doh and building a real-world model yourself. You build solid models in AutoCAD by constructing basic 3D shapes and then combining them -- adding, subtracting, or finding their intersections -- and modifying them. It’s like using lots of fancy saws, drills, and glue to build a model made out of wooden blocks. You can render a solid model, as well as check mass properties and interferences.
In most practical applications of 3D, you select one type of representation -- wireframe, surface, or solid -- for all or most of the objects in the drawing, based on ease of construction and intended use of the model. However, AutoCAD doesn’t prevent you from mixing all three types of 3D objects in the same drawing.
After you determine the type of 3D representation to use, you decide on the appropriate level of detail and construct the model, using the commands and techniques introduced in this chapter. Finally, you create the required 2D and/or rendered views for plotting or viewing on the screen.
You can do some experimentation with 3D on any computer system that can run AutoCAD. If you want to pursue serious work in 3D AutoCAD, pay attention to the following prerequisites:
* Know AutoCAD well. You need to be pretty comfortable using AutoCAD for 2D work before doing much with 3D. You should be able to control object properties, use precision techniques, draw and edit 2D objects, and zoom and pan -- in other words, all the stuff covered in Chapters 5 through 8 of AutoCAD 2006 for Dummies. If you aren’t comfortable with these techniques in 2D drafting, you’re likely to find 3D modeling in AutoCAD a real struggle.
* Get a fast computer. For beginning 3D work, any AutoCAD-adequate system will do the job. For serious work with 3D models, you need a fast computer, lots of memory, and lots of disk space.
* Get and master additional software. In addition to AutoCAD, you may need other programs -- either AutoCAD add-ons or separate packages -- to do work that AutoCAD isn’t as good at. Specialized 3D modeling and rendering programs are among the tools of the trade of most people who do a lot of 3D work. Illustration packages can help you jazz up the appearance of your drawing.
Tip:
Many of Autodesk’s newer software products -- AutoCAD-based ones like Architectural Desktop and Mechanical Desktop and non-AutoCAD ones like Revit and Inventor (see Chapter 1 for more information) -- use 3D modeling as their fundamental approach to CAD. If you intend to use 3D on real projects, you’ll probably use 3D-centric programs such as these, not plain AutoCAD. The information in this chapter will get you started on the right track, especially if you eventually use one of the AutoCAD-based applications.
* Do a real project. Real work is the best motivation for discovering 3D. If you don’t have an actual work assignment, create a task. Something as “simple” as creating a 3D model of your chair will make the difference between really finding out something useful about 3D and just reading about it in the manuals.
The first challenge in 3D modeling is being able to see your three-dimensional model on a two-dimensional computer screen. The normal model space view on the Model tab in the drawing area shows a single, projected 2D view of your model -- the top-down, “plan” view by default.
AutoCAD provides two model space capabilities that enable you to escape this visual flatland:
* With viewports, you can carve the model space drawing area into smaller rectangular areas, each of which shows a different view of the model.
* With viewpoints, you can change the point in 3D space from which you look at the model. By setting a different viewpoint in each viewport, you can look at several sides of your model at the same time. It’s like looking at one of Picasso’s cubist paintings, only more orderly.
<Tip>
If you want to experiment with viewports and viewpoints in an existing 3D drawing, try one of the AutoCAD 2006 samples, such as \Program Files\AutoCAD 2005\Sample\Welding Fixture Model.dwg. To remove shading and make the model faster to work with, choose View>Shade>2D Wireframe.
Chapter 4 discusses viewports in paper space, which are useful for creating layouts for use in plots and presentations in both 2D and 3D. Model space viewports, cousins of paper viewports, are less powerful but simpler and are intended to help you construct 3D models.
Model space viewports divide the screen into separate rectangles with no gaps between them. Unlike with paper space viewports, you can’t move, stretch, or overlap them. You can’t plot multiple model space viewports; that’s what paper space is for. And, unlike in layouts, a layer that’s visible in one model space viewport always is visible in all of them.
Technical Stuff:
You may hear or read references to tiled viewports, which is just another name for model space viewports. Tiled refers to the way in which model space viewports always fill the drawing area, with no gaps and no overlapping allowed. Conversely, paper space viewports are sometimes called floating viewports because you can move them around, leave gaps between them, and overlap them.
Model space viewports enable you to see several views of your model at one time, each from a different viewpoint. For this reason, model space viewports are especially useful when you’re creating and editing objects in 3D. As you draw and edit, the different views help ensure that you’re picking points that are located correctly in 3D space.
To set up model space viewports, use the Viewports dialog box:
1. Choose View>Viewports>New Viewports.
The Viewports dialog box appears.
2. Choose one of the Standard Viewport arrangements and choose 3D in the Setup drop-down list.
3. Click OK.
Figure A-2 shows the dialog box and the model space result of choosing the Four Right Standard Viewport arrangement and the 3D Setup. This arrangement, along with the Four Equal, and Four Left standard viewport arrangements, works well for creating viewports that show top, front, right, and isometric views of a 3D model.
Figure A-2: The Viewport dialog box makes setting up model space viewports and different viewpoints easy.
To return to a single viewport later, click in the one whose view you want to use, then open the Viewports dialog box and choose Single in the Standard Viewports list.
When you choose 3D in the Viewports dialog box’s Setup drop-down list, you direct AutoCAD to change the viewpoint in each viewport. The default viewpoints when you choose a four viewport arrangement are top, front, right, and SE (“southeast”) isometric. These viewpoints work well for viewing and constructing simple models, but eventually, you’ll probably want to specify your own, custom viewpoint in a particular viewport.
The easiest way to change viewpoints is to use the View>3D Views submenu (shown in Figure A-3) 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, but not too high above -- as though you were hovering in a low-flying helicopter.
These ten views are called standard because they’re often used in manual drafting and rendering work. They 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. We describe coordinate systems in “A Cartesian Orientation,” later in this chapter.)
Figure A-3: The 3D Views submenu.
You can specify nonstandard viewpoints by choosing View>3D Views>Viewpoint Presets. In the Viewpoint Presets dialog box that appears, specify the following settings:
* A viewing angle in the XY plane (imagine moving a camera on a dolly around an object, while keeping the camera at the same elevation)
* An angle from the XY plane (imagine using a boom to swoop the camera up to a different height so that you’re looking at the object from increasingly steep angles)
Tip:
By default, AutoCAD shows 3D models in 2D wireframe mode, even if you’ve created surface or solid objects. If you want to better visualize which objects are in front of which other objects, especially in an isometric or other non-orthogonal view, you have a couple of options:
* Choose View>Shade and then choose Hidden or any of the Shaded options.
* Render the model, as described later in this chapter.
Standard views and the Viewpoint Presets dialog box are fine for many 3D construction tasks, but if you really want to have fun with a model, 3DOrbit is your ticket to it.
AutoCAD LT:
AutoCAD LT doesn’t include the 3DOrbit command. In LT, you can use the Viewpoint Presets dialog box or the DView command to look at a model from different points of view.
The 3DOrbit command 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 the imaginary sphere containing your model. As you drag the cursor, AutoCAD updates the screen dynamically.
3DOrbit provides many other options through its right-click shortcut menu. You can change the shading mode and projection type, and you can turn on several visual aids that help you understand where you are in 3D space. Additional shortcut menu options enable you to pan, zoom, and restore standard or named views.
The following steps show some of the things that you can do with 3DOrbit:
1. If you’ve divided model space into two or more viewports, click in the viewport in which you want to change the viewpoint.
2. Choose View>3D Orbit.
The 3DOrbit arcball appears, as shown in Figure A-4.
Figure A-4: The 3DOrbit arcball and right-click menu.
3. Move the cursor inside the arcball.
The 3DOrbit full orbit cursor appears: two oval arrows circling a sphere.
4. Click and drag, keeping the cursor inside the arcball.
You can rotate the model in all directions. Imagine that the cursor is your finger pushing on a globe that rotates freely in all directions.
Tip:
Pay attention to the shaded UCS (user coordinate system) icon at the lower-left corner of the drawing area as you change the view. The UCS icon helps you visualize how each orbiting operation works. (The next section of this chapter tells more about UCSs.)
5. Release the mouse button and move the cursor outside the arcball.
The 3DOrbit roll cursor appears: a circular arrow circling a sphere.
6. Click and drag, keeping the cursor outside the arcball.
You can rotate the model around an axis at the center of the circle, coming out of the screen. Imagine that you’re turning the steering wheel on a car, with the steering column pointing into the screen.
7. Release the mouse button and move the cursor over one of the small circles at the quadrant points (that is, 12 o’clock, three o’clock, six o’clock, and nine o’clock) of the arcball.
The 3DOrbit horizontal or vertical rotation cursor appears: an elliptical arrow circling a sphere.
8. Click and drag away from the small circle.
You can rotate the model around a horizontal axis (if you clicked the circle at 3 o’clock or 9 o’clock) or a vertical axis (if you clicked the circle at 12 o’clock or 6 o’clock) passing through the little circle. Imagine that you’re turning a piece of meat on a spit, with the spit located horizontally or vertically in the plane of the screen.
9. Release the mouse button and right-click in the drawing area.
The 3DOrbit shortcut menu, shown in Figure A-4, appears.
10. Experiment with the additional options, such as Shading Modes and Projection.
Here’s the lowdown on some of these options:
* Gouraud Shaded usually looks more realistic than Flat Shaded.
* Hidden removes hidden lines but doesn’t shade.
* Wireframe is the default wireframe view, without hidden lines removed.
* 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 remain parallel in perspective projection.
If you manage to 3DOrbit out of control so that you no longer see your model, right-click to display the 3DOrbit shortcut menu and choose More>Zoom Extents. The Zoom, Pan, and Preset Views options offer other ways of getting your model back in your sights.
11. When you’re finished orbiting, right-click in the drawing area and choose Exit.
Tip:
Choose More>Adjust Clipping Planes on the 3DOrbit right-click menu (or the 3DCLIP command outside of 3DOrbit) to slice away part of your model temporarily to view what’s inside the surfaces or solid boundaries. See “clipping, 3D objects” in the AutoCAD online help for more information.
Technical Stuff:
You can control the camera point and target point that 3DOrbit uses (the point that you’re looking from and the point that you’re looking towards). Before you start the 3DOrbit command, type CAMERA at the command prompt and press Enter. Click or type the coordinates of the desired camera point target points.
Viewing your model from different viewpoints in multiple viewports is all well and good, but unless your 3D spree is limited to looking at other people’s models, you need to know how to specify points and distances in 3D. To do that, you need to understand AutoCAD’s coordinate systems and conventions for entering 3D points and distances.
AutoCAD stores the locations of all the objects that you draw as 3D Cartesian coordinates (X, Y, and Z) in the world coordinate system (or WCS). The WCS defines the origin (0,0,0) point for the drawing and the orientation of the X, Y, and Z axes. A particular point in the Cartesian coordinate system is defined by its X, Y, and Z coordinates, where the coordinate indicates the distance from the origin (0,0,0) point along the X, Y, or Z axis.
The WCS is the default XYZ coordinate system when you start a new drawing. X is horizontal on the screen, Y is vertical on the screen, and Z extends out of the screen, towards your face.
The X-Y plane (the set of points where Z = 0) is the construction plane in which you create 2D objects. It’s also important for creating 3D objects, because many commands operate with respect to the X-Y plane.
When you’re drawing and editing 3D objects, it’s often useful to be able to move or rotate the construction plane. You do so in AutoCAD by defining a user coordinate system (UCS). A UCS can have a different origin (0,0,0) point and can have its X, Y, and Z axes point in different directions than they do in the WCS.
Choose Tools>Orthographic UCS>Preset to run the UCSMAN command, which opens the UCS dialog box, shown in Figure A-5. This dialog box simplifies the job of changing to common UCSs, such as Top, Front, and Right. The Tools>New UCS submenu runs the UCS command, which provides additional options. Look up “UCS command” in AutoCAD’s online help to find out more about UCSs and how to work with them.
Figure A-5: Get with the UCSMAN.
To keep you informed about which coordinate system is current in each viewport, AutoCAD displays a UCS (user coordinate system) icon in each viewport by default. The icon looks like two or three little arrows (depending on the viewpoint), each of which represents the direction of the positive X, Y, or Z axis. This icon shows the working construction plane -- the plane in which AutoCAD places 2D objects when you draw them. Use the View>Display>UCS Icon submenu to turn the icon off or on or to change its properties.
Warning:
Don’t confuse viewpoints and coordinate systems:
* The viewpoint controls what you see in a particular viewport, but doesn’t control what you do to objects -- that is, the viewpoint doesn’t dictate the results of drawing and editing commands.
* The current coordinate system defines the construction plane in a particular viewport -- that is, the UCS (or WCS) dictates how the points that you specify during drawing and editing commands influence the specific locations and orientations of objects. The UCS icon shows the current coordinate system, not the current viewpoint.
As we indicate in Chapter 5, you specify points in 2D drafting by typing a pair of numbers in the format X,Y for absolute coordinates (with respect to the point 0,0) or @X,Y for relative coordinates (with respect to the previous point that you specified). In both cases, you ignore the Z coordinate, but AutoCAD still stores it -- either as zero or the current elevation.
When you create 3D models, you no longer can ignore the Z axis. You need to worry about three coordinates for each point that you pick or type. AutoCAD provides several ways of specifying 3D points. These are the most common:
* Typed coordinates: When you type coordinates, you simply add a Z coordinate: X,Y,Z for absolute coordinates (with respect to the point 0,0,0) or @X,Y,Z for relative coordinates.
Technical Stuff:
There are 3D analogues to 2D polar coordinates (@distance<angle), too. See “polar coordinates, entering, cylindrical” and “polar coordinates, entering, spherical” in the AutoCAD online help.
* Object snaps: Happily, the object snap techniques described in Chapter 4 for picking precise locations on existing objects work equally well in 3D. You can object snap to endpoints, intersections, midpoints, and so on, without having to worry about the current construction plane as defined by the WCS or UCS.
Warning:
In practice, you’ll have to be more careful when using object snaps in 3D work. Remember that there are now three coordinates instead of two to be precise about. If you’re viewing a 3D model from just one viewpoint, it’s easy to osnap to a point that looks right in that view but in fact lies “deeper” or “shallower” in 3D space that you realize. Thus it’s a good idea to display your model from several different points of view, as described in the “Getting Your 3D Bearings” section, earlier in this chapter. The different points of view help you predict which location in 3D space a given object snap mode will grab and make any object snap mistakes more obvious.
New in 2006:
A new AutoCAD 2006 system variable called OSNAPZ can help you reduce undesired pick points. When OSNAPZ is set to 0, AutoCAD acquires and uses the actual Z-value of the point (the way it’s always worked). When OSNAPZ is set to 1, AutoCAD replaces the actual Z-value of the selected point with the current elevation. One caveat: AutoCAD gives you no visible indication that this system variable is active, and it can really mess you up if you’re not acutely aware of its setting. This is yet another reason to work with multiple viewports.
Technical Stuff:
An additional object snap mode, Apparent Intersection, is useful when you want to snap to the imaginary intersection of two objects that appear on the screen to intersect, but don’t in fact intersect because they lie in different planes.
* Coordinate filters: You’ll sometimes need to pick a point some of whose coordinates (for example, the X and Y coordinates) match one object snap point and some of whose coordinates (for example, the Z coordinate) match another object snap point. Coordinate filters -- also called point filters -- solve this problem handily. With coordinate filters, you use the coordinates of points on existing objects to help specify new points. You use coordinate filters together with object snaps to “build up” a point one or two coordinates at a time. When AutoCAD prompts you to pick a point, you activate a coordinate filter before you pick the point. The coordinate filter tells AutoCAD which coordinates of the subsequent point to use.
You activate a coordinate filter by clicking one of the six choices on the object snap cursor menu’s Point Filters submenu (shown in Figure A-6): .X, .Y, .Z, .XY, .YZ, or .XZ. (You display the object snap cursor menu by pressing Shift+right-click; see Chapter 5 for more information.) For example, if you select .XY and then pick a point, AutoCAD remembers only the X and Y coordinates of that point. AutoCAD then prompts you to pick another point and combines the Z coordinate of that second point with the X and Y coordinates of the first point to determine the 3D location of the new point. Look up “coordinate filters” in AutoCAD’s online help for more information and an example.
Figure A-6: The object snap cursor menu’s Point Filters submenu.
At this point, you may be feeling less then completely 3D-coordinated, despite the WCS, UCSs, and XYZs. All this stuff will become clearer after you create some viewports and draw some objects. The next section gives you some specific instructions, but don’t be afraid to experiment with typing Cartesian coordinates, object snapping in 3D, and working with different predefined UCSs. Or, you can console yourself with the following paragraph’s geometrical trivia and vile pun.
Technical Stuff:
In case you’re wondering, the Cartesian coordinate system is named after its inventor, the French mathematician René Descartes. (Yes, he of “I think therefore I am” and the Monty Python Philosophers’ Drinking Song fame.) He did not, however, invent the cart. The cart came earlier, and the horse came earlier still. It’s for that reason that we know not to put Descartes before de horse.
This section introduces four techniques for creating 3D objects: drawing 3D lines and polylines, extruding 2D objects, creating surface meshes, and creating solids. (In AutoCAD LT you can use the first two techniques only.)
Remember:
When you draw 3D objects, just like when you draw 2D objects, put them on appropriate layers and use precision techniques to specify each point and distance. See Chapter 5 for more information.
The most straightforward way to draw objects for a 3D wireframe model is to use the Line or 3DPOLY (3D Polyline) command and specify 3D coordinates. The 3DPOLY command is similar to the PLine (Polyline) command, which is described in Chapter 6. Both commands draw a series of connected line segments, but they have different capabilities:
* The 3DPOLY command accepts 3D points for the line segments’ vertices. The PLine command requires that all vertices be in the same plane.
* 3DPOLY is limited to straight line segments. PLine can draw arc segments and create segments with uniform or tapered width.
* Segments created with 3DPOLY can’t display dash-dot linetypes; 3D polyline segments always display as continuous lines.
The command sequence for drawing 3D segments with the Line or 3DPOLY command is the same as for drawing 2D segments with the Line command; see Chapter 6 for details. The only difference is that you specify 3D coordinates instead of 2D ones. Figure A-7 shows an example.
Figure A-7: Drawing 3D line segments.
Creating 2D representations in this way is straightforward, though tedious for all but the simplest objects. More important, a wireframe model becomes increasingly difficult to decipher as the complexity of the model increases. You see a mass of lines representing the edges, and you have difficulty telling which parts of which edges are in front of others. To reduce this visual confusion, you need to graduate to surface or solid modeling commands, as described earlier and in the subsequent sections of this chapter.
One way to create a surface model is to extrude, or thicken, 2D objects. For example, if you give thickness to a circle, it becomes a cylinder. If you give thickness to a 2D polyline or a series of lines segments, it looks like a fence.
AutoCAD calls this depth thickness, which is not to be confused with other characteristics that you may be tempted to call “thickness” -- linewidths that you add for plotting and uniform or tapered widths that you can add to polyline segments. Linewidths and polyline segment widths make objects fatter but leave them in the 2D plane. Thickness pushes objects out of the plane, giving them a new dimension.
AutoCAD LT:
Folks who do heavy-duty 3D modeling somewhat derisively refer to the result of adding thickness to 2D objects as 2-bf1/2D -- a bit more than 2D, but not quite 3D. That’s because extrusion is a limited 3D technique compared to creating real surface meshes and solids, as described in the following two sections. You can’t model most objects in a detailed way with extrusion alone. Imagine a set of wooden blocks in which every block has a uniform thickness, with no tapering allowed -- kind of dull, huh? Even AutoCAD LT, despite its advertised 3D limitations, can do extrusion (although only the simple kind of extrusion described in the following steps). Nonetheless, extrusion is a fun and fairly low-stress way to get started with 3D, and it’s adequate for some simple modeling tasks. It’s also a good warm-up exercise for full 3D solid extrusion, which we describe later in this chapter.
The following steps show the general procedure for creating “2-1/2D” extruded objects.
1. Define a suitable UCS (user coordinate system), as described earlier in “Coordinate systems: The WCS and UCSs.”
The extrusion direction will be perpendicular to the UCS, so think about the orientation of the 2D object and which way you want it to “pop out” into 3D space.
2. Draw a 2D object.
See Chapter 6 for more information.
3. Open the Properties palette.
See Chapter 7 for more information.
4. Press Esc to make sure that no objects are selected.
5. Click the object to select it.
6. In the Thickness field, type an extrusion thickness and press Enter.
AutoCAD extrudes the object perpendicular to the UCS in which you created it, as shown in Figure A-8.
Figure A-8: Just add thickness and voilà -- instant 3D! (Or 2-1/2D, anyway.)
Warning:
If you extrude closed polylines and then shade or render them, the extruded shapes appear hollow -- that is, without any tops or bottoms on them. You can work around this limitation by using the 3DFACE command to add three-sided or four-sided surfaces as “lids.” You can build up irregular shapes by using a combination of three-sided and four-sided faces. A better approach in most cases is to use the EXTrude command, which creates a true solid out of closed polylines and other closed shapes. (EXTrude requires AutoCAD -- it’s not included in AutoCAD LT.)
For surface models that are more complex than simple extrusions of 2D objects, you can use a variety of AutoCAD commands that draw 3D meshes. One set of commands draws surface model representations of 3D primitives: box, pyramid, wedge, dome, sphere, cone, torus, dish, and mesh. (In this case, the word primitive isn’t a pejorative. A 3D primitive is a relatively simple shape that can be used alone or as a subcomponent of a more complex 3D object.)
To create these objects, choose Draw>Surfaces>3D Surfaces, select a primitive shape from the image tile menu (shown in Figure A-9), and follow the command line prompts. It’s like constructing your own set of wooden blocks, except that they don’t take up any space in your closet after you’re finished playing with them!
Figure A-9: A torus mesh -- the perfect 3D surface snack.
Tip:
The Surfaces submenu of the Draw menu includes commands for creating complex 3D surfaces from 2D lines and curves. For example, with the Revolved Surface menu choice (the REVSURF command), you revolve a set of 2D line and arc segments around an axis of revolution to “sweep out” a 3D surface. But in most cases, you’re better off using solid modeling commands such as REVolve instead.
Solid modeling is in many ways the culmination of 3D CAD. Solids more accurately represent most real-world objects than do wireframes or surfaces. And even when representational accuracy isn’t the main issue, it’s easier to construct many kinds of models with solids.
Solid modeling also places more serious demands on computer software and hardware. It takes a sophisticated program and fast computer -- not to mention a capable human being running all this stuff -- to create useful solid models. That’s why solid modeling historically has lagged behind wireframe and surface modeling and only recently become common on ordinary PCs. AutoCAD’s solid modeling capabilities have improved steadily over the years, but most people who do real solid modeling use AutoCAD add-on applications or separate, stand-alone programs that have been created with this task in mind. Examples include Inventor from Autodesk and SolidWorks from SolidWorks Corporation.
Technical Stuff:
Many special-purpose solid modeling programs use a combination of solid and surface modeling techniques for maximum flexibility in constructing and editing 3D models. These kinds of programs -- and solid modeling in general -- are becoming especially popular in mechanical design.
Constructing the basic building blocks -- or solid primitives -- for a solid model in AutoCAD isn’t difficult. As with wireframe and surface objects, you follow these steps:
1. Define a suitable UCS (user coordinate system).
See “Coordinate systems: The WCS and UCSs,” earlier in this chapter. The UCS controls the construction plane and basic 3D orientation of the solid.
2. Choose Draw>Solids and then choose a solid from the top half of the submenu.
As shown in Figure A-10, your choices are Box, Sphere, Cylinder, Cone, Wedge, and Torus. (These are similar to the surface mesh choices shown in Figure A-9. In this case, though, you’re creating solid instead of surface versions.)
Figure A-10: Everything you need for a solid foundation.
Tip:
When you see a 3D object in a drawing, you can’t tell by looking whether it’s a 2D extruded object, surface mesh, or solid. If you want to find out, open the Properties palette and select the object. The drop-down list at the top of the palette shows the type of object that you selected.
Constructing solid primitives is pretty simple and immediately gratifying. For example, you can quickly put together a virtual cityscape by populating your drawing with boxes, cylinders, cones, and wedges representing buildings. For additional gratification, you’ll want to shade or render them (see the “Ending with Rendering” section, later in this chapter). If you have any questions about any of the 3D solid primitive commands, choose Contents>User’s Guide>Create and Modify Objects>Draw Geometric Objects>Create 3D Objects>Create 3D Solids in the AutoCAD online help.
After the solid primitives, the next pair of commands on the Draw>Solids submenu is Extrude and Revolve:
* Extruding is similar to adding thickness (see “Extruding from 2D to 3D,” earlier in this chapter) except that it creates true solids rather than “2-1/2D” objects. In addition, you can specify tapers with the EXTrude command.
* Use the REVolve command to create a solid by rotating a closed object around an axis.
Warning:
The commands described in this section are the building blocks of solid modeling. Developing useful 3D models takes a lot more than just knowing the basics. You can model simple objects by using solid primitives, extrusion and revolution, and the editing techniques discussed in the next section. But for most real-world solid modeling work, you’ll want additional software and learning resources.
When you’re ready to modify wireframes or surfaces, you can use most of the editing commands described in Chapter 7. Grip editing also works well on most wireframe and surface objects.
As with 3D drawing, you need to be a more careful when editing in 3D to make sure that the editing transformations you apply occur correctly in all three spatial dimensions. Display your model in multiple viewports from different viewpoints so that you can see what’s going on and catch mistakes right away.
Modifying 3D solids is trickier. Simple editing commands like Move, CoPy, and ROtate work fine, but most of the commands that alter an object’s shape (for example, Stretch, Trim, and Break) don’t work on solids. You can use grips to move and copy solids, but not to stretch them.
Instead, AutoCAD provides a set of specialized commands on the Modify>Solids Editing menu, shown in Figure A-11. The first three choices -- Union, Subtract, and Intersect -- perform the so-called Boolean operations, which create new 3D solids by combining and removing parts of existing ones. Look up “composite 3D solids” in the AutoCAD online help system for more information about these commands. Also see Contents>User’s Guide>Create and Modify Objects>Change Existing Objects>Modify 3D Solids for a description of the other options on the Modify>Solids Editing menu.
Figure A-11: Specialized solids editing commands.
If your family came from a farm in Iowa, rendering was what you did to fat to make it into lard. But in CAD, rendering is the process of illuminating a set of 3D objects with one or more imaginary lights and then creating a more-or-less realistic picture of the results. (No, computer programmers didn’t have lard on their minds when they borrowed the word rendering. Hand-drawn pictures of building facades were called renderings long before computers got into the act.)
A single example of this technique is called a still rendering. Multiple frames strung together produce computer animation. The objects that you see in movies such as Toy Story or A Bug’s Life are first created as 3D models and then rendered frame by frame -- a process that can take immense amounts of time even on ultrafast graphics workstations -- to produce the beautifully, well-rendered images you see.
Tip:
AutoCAD creates still renderings only. If you want to create animations, you need to use other software programs such as Discreet’s 3DS Max. Even if you use animation programs, AutoCAD can be useful for developing the initial 3D models.
Rendering has steadily improved in speed and usability as PCs have become faster, and programmers have improved their rendering algorithms. Rendering of simple scenes is now practical on a mainstream PC, and a fast personal computer can create some impressive images in a reasonable amount of time. Rendered images are useful for previewing how your models will work in real life and also can be powerful tools for sales and marketing communications for your company. A (rendered) picture can be worth quite a bit more than a thousand words.
To see how rendering works with the default options in AutoCAD, use the steps that follow. When you’re ready to get fancy, choose User’s Guide>Create Realistic Images and Graphics>Render 3D Objects for Realism in the AutoCAD online help system.
1. Create one or more 3D objects.
Use the steps in the previous section or use one of the sample 3D drawings in \Program Files\AutoCAD 2005\Sample.
Tip:
Unless you want a monochromatic rendering, create the objects on several different layers that you’ve assigned different colors.
2. Choose View>Render>Render.
The Render dialog box appears, as shown in Figure A-12.
3. Set a destination and click the Render button.
In the Destination area, set the drop-down list to Viewport. You can get okay results without changing the other options. When you click Render, AutoCAD renders the 3D objects. See Figure A-12 for an example.
Figure A-12: The Render dialog box and a rendered object.
To make rendered images look less cartoonish, you need to apply materials to 3D objects and define different light sources. For more information, navigate to the Render 3D Objects for Realism help page mentioned just before the preceding steps and then look at Create Rendered Images>Use Materials in Rendering and Create Rendered Images>Use Lights in Rendering.
Tip:
The SHADE command offers a simpler alternative to the RENDER command, without all the lighting, surface materials, and other options. To experiment with shading, choose any of the options on the View>Shade menu. (To return to an unshaded view, choose View>Shade>2D Wireframe.) As we mention earlier in this chapter, the 3DOrbit command’s right-click menu includes shaded modes that you can see even while navigating around your model in real time.
Technical Stuff:
In addition to viewing rendered and shaded views on the screen, you can plot them, whether they’re in a paper space layout or in model space. Use the Shaded Viewport Options area of the expanded Plot dialog box to control this feature. Chapter 12 contains more information about plotting.
New in 2006:
AutoCAD 2006 (but not AutoCAD LT) enables 3D DWF publishing through an unsupported command called (wait for it…) 3DDWFPUBLISH. This command is only installed automatically if you chose the the “Typical” option when you installed AutoCAD 2006.
If you want to plot multiple rendered views (or a rendered view plus other nonrendered views), create a paper space layout with multiple viewports, as described in Chapter 4. As we mention earlier in this chapter, tiled model space viewports are great for creating and viewing 3D models, but you can plot only one model space viewport at a time. Use paper space layouts to compose multiple viewports for plotting purposes.
People who do a lot of rendering and want higher quality, photorealistic results usually use programs other than AutoCAD to render their models. Discreet 3DS Max, Autodesk VIZ, and McNeel’s AccuRender are three popular photorealistic rendering programs. Most rendering programs can import 3D models from AutoCAD DWG files, but some people use specialized 3D modeling programs to do their modeling as well.
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