AutoCAD has many advanced features that enable detailed drawing at higher resolution, with many different colors, and in three dimensions. These features can help you as an architect, interior designer, or landscape architect, as you develop not only drawing skills, but also develop your proficiency at presenting ideas to clients. That is, AutoCAD has many options that can help you make attractive presentations of your ideas and concepts for buildings, interiors, and site plans.
In this section, we discuss the capabilities of layered drawings, graphics with surface and volume rendering, and more advanced topics of three-dimensional drawing. The section is structured as follows:
- 5.1. Layered Structure of AutoCAD Drawings
5.2. Concepts of 3-D Modelling and Rendering
5.3. Surface and Solids Modelling
When one has many objects in a drawing and wants to group them logically, rendering each group of objects in different colors, textures, or linewidths, the layering commands discussed in Section 5.1 are useful. Section 5.2 discusses general concepts and implementation of three-dimensional (3-D) drawing and image production, called rendering. Often in architectural drawing, one needs to produce 3-D visual effects such as walls, towers, etc. The 3-D drawing commands required to produce these effects are discussed in Section 4.3.
5.1. Layered Structure of AutoCAD Drawings.
AutoCAD's LAYER command and associated options provide the user with a convenient method of logically grouping graphics objects such that each group is displayed in a visually unique way. For example, in drawing a house plan, the studs and sheathing could be drawn in black as Layer #1, the electrical services in red as Layer #2, and the plumbing in blue as Layer #3. With the LAYER command and its associated dialogue box, one can turn various layers on or off (displayed or not displayed), change layer colors, linetype, and display modes, as well as lock layers to prevent inadvertent or mistaken changes.
5.1.1. Layers -- Structure and Function.
A layer in AutoCAD is like a clear piece of plastic that you can lay directly over a drawing and then draw upon. Since the layer is transparent, you can see through it to the original drawing (sometimes called the base layer). Layers can be made visible or invisible, and can be named.
The attributes of layers can be set and viewed in the Layer Control dialogue box, which groups layer information into categories that are easy to read and understand.
5.1.2. LAYER Commmand and Options.
In the MS-Windows version of AutoCAD, the LAYER icon is located at the upper left, just below the Toolbar that is itself located beneath the Top Bar of menu names. The LAYER icon looks like a stack of pages or sheets of paper.
To the right of the LAYER icon is a graphics window that contains associated icons, which are status indicators for the LAYER command options. For example, an open padlock indicates that a given layer is not locked. To activate the Layer Control dialogue box, click on the LAYER icon.
Figure 5.1. Schematic diagram of AutoCAD Layer Control dialogue box.
Creating New Layers -- On the Layer Control dialog box, illustrated in Figure 5.1, type the name of the new layer in the text entry box below the New button. Then, click on the New button, which is located beneath the large display area. The layer name will appear in the display box.
Changing Layer Color -- Move your pointing device cursor to the display area and click on the name of the layer you have just created. Select the Set Color box, and select the color by clicking on it. Click on the OK button to set the new color for the selected layer. The name of the color will appear in the Layer Control dialogue box display area, on the same line as the layer name you selected.
Changing Layer Linetype -- Move your pointing device cursor to the display area and click on the name of the layer you have just created. Select the Set Linetype box, and select the linetype from the displayed list by clicking on it. Click on the OK button to set the new linetype. The name of the linetype will appear in the Layer Control dialogue box display area, on the same line as the layer name you selected. Note: Some AutoCAD implementations have only two linetypes -- continuous and dashed. Check with your systems administrator to determine if additional linetypes are available.
Locking a Layer -- In the Layer Control dialogue box, first select a layer by clicking on its name in the display area. Then, click on the button labelled Lock or Unlock (to the right of the dialogue box) to inhibit/allow drawing in a given layer.
Renaming a Layer -- To change the name of a layer that you have already created and/or specified attributes for, first select a layer by clicking on its name in the display area of the Layer Control dialogue box. Then, enter the new name of that layer in the text box below the New button. When you have entered the layer's new name, click on the Rename button, and the new name will appear in the appropriate place in the display area.
Hiding/Displaying a Layer -- In the Layer Control dialogue box, first select a layer by clicking on its name in the display area. Then, click on the button labelled On to display the layer, or Off to inhibit display and thus hide the layer. These buttons are located at the top right hand corner of the dialogue box.
Drawing within a Layer -- In the Layer Control dialogue box, first select a layer by clicking on its name in the display area. Then, click on the button labelled Current to select the layer for drawing. If the layer is locked (see above), it will not be possible to draw in the layer. After you have selected the layer, click on the OK button and go to the AutoCAD Drawing Window, and commence drawing within the specified layer.
5.1.3. Additional Features of the Layer Command.
Thaw and Freeze are features that are used with AutoCAD's database interface, and will be discussed in class. We recommend that you keep the status of all layers used in this class as Thaw.
This concludes our overview of basic layer command options. We next overview three-dimensional modelling.
5.2. Concepts of 3-D Modelling and Rendering.
It is often useful to visualize a drawing of a three-dimensional object in terms of a picture of the object itself. For example, if you are designing an interior for a home, it would be useful to be able to simulate a "walk-through" of the home on your computer. It would likewise be useful for you to display such a tour to the client(s) for which the home was designed. This would be a handy sales tool, but would also enable your clients to spot things they didn't like about your design. With sufficient proficiency in 3-D design and rendering, it would be relatively easy for you to make changes in the house plan or interior design, then display the results to your client(s).
In this section, we discuss the basic concepts of computer graphics, solids modelling, and rendering (image production from a specification of a 3-D drawing). In Section 5.3, we overview the basic AutoCAD commands that make solids and surface modelling possible.
5.2.1. Components of a 3-D Model.
Descriptions of three-dimensional objects arranged in a scene, which are processed by computer to produce an image, have the following salient components:
Object Model -- A scene is comprised of many different objects, each of which must be specified in detail. For example, an interior scene such as a living room would have tables, chairs, lamps, a carpet, perhaps some wall hangings or pictures, etc. The wall would have windows that might be tinted to protect furniture from solar radiation, and there might be one or more skylights or spotlights mounted in or on the ceiling. Each of these objects is described in terms of its attributes, such as size and shape, color, composition from other objects (e.g., spheres, cylinders, rectangular boxes), texture (e.g., fabric, woodgrain), and surface reflectivity (e.g., matte or shiny covering).
Lighting Model -- The vast majority of rooms and buildings have lighting effects that cannot be modelled with a simple diffuse sky model, or a point source such as an idealized solar illuminant. Architectural light sources vary from point sources (e.g., spotlights or miniature track lighting) to extended sources (e.g., fluorescent tubes), to diffuse lighting (e.g., skylights in a ceiling or stained-glass windows in a church). The color, field-of-view, angular emission pattern, and intensity of each light source can be specified in detail, given appropriate software support.
Scene Model -- Objects in a scene have various positions relative to each other and relative to the scene coordinate system origin. The scene model encodes these object positions, as well as positions and aiming direction of each light source. Thus, it is the scene model that groups the individual object descriptions into a composite object which can be projected onto the camera through the viewport.
Viewport Model -- The scene that is comprised of graphics objects and lighting models is imaged through a viewport, which restricts the portion of the scene that can be rendered. There are various methods and formats for describing viewports. For example, a viewport may be thought of as a rectangular aperture in space through which one views a scene. Alternatively, the viewport may specify a region of the scene's coordinate space where a scene is displayed (with display suppressed elsewhere).
Camera Model (Optional) -- In order to form an image of the scene model as seen through a viewport, one must have an imaging device. The camera model portrays such a device in terms of focal length, aperture, depth-of-field, field-of-view, and (occasionally) spectral responsivity. The optical parameters of the camera model are important because they establish the type and amount of perspective transformation that must be applied to produce a realistic-looking image of the scene.
5.2.2. Overview of Rendering Processes.
There are many different types of modelling, projection, and rendering processes that form the "camera image" from the object, lighting, scene, and viewport models. We will discuss two of these, namely, the surface- and ray-based techniques. A surface-based rendering approach has the following steps:
Step 1 - Surface and Solids Modelling -- The scene model is used to locate objects within the scene. The objects are individually described in the object model, which may include a catalog of objects. The result of this process is a geometric model of the scene.
Step 2 - Lighting and Shading -- The lighting model is applied to the geometric model, whose surfaces have various colors, textures, and reflectivities preassigned. The geometric and trigonometric equations used in this projection are quite complex, and have been published extensively in the literature. The result of this step is a model of light intensity on various surfaces of the object.
Step 3 - Geometric Projection -- Using the viewport as a limiting aperture, the camera model determines which regions of the light intensity model are projected onto a given camera pixel. (A pixel is an image element, similar in concept to the individual dots that comprise a photograph in a newspaper.) This process is called ray casting, as opposed to ray tracing, which we discuss below. The result is a two-dimensional image, which is then projected onto the computer monitor.
Disadvantages of the surface rendering approach are due in part to its computational complexity, as well as the large storage (memory or disk) requirement for retaining the lightfield projection image. Also, lightfield directionality may not be completely specified, leading to errors in simulating specular reflection (e.g., light reflected in mirrored surfaces) as well as diffusion from textured surfaces, such as upholstery or wall hangings.
AutoCAD provides several capabilities for basic
surface and solids modelling, including coordinate system selection
(Section 5.3.1), surface modelling (Section 5.3.2), and solids
modelling (Section 5.3.3).
Three-dimensional modelling begins with the selection
of a coordinate system. AutoCAD's absolute coordinate system is called
the World Coordinate System (WCS). The default settings for
the WCS are such that you are looking perpendicular to the XY plane.
This is the mode in which one draws 2-D line drawings in AutoCAD.
The displayed orientation of the WCS may be changed
by selecting a different drawing viewpoint. There are two ways to do
this -- (a) use the View menu, or (b) click on one of the icons
on the View toolbar. The View pulldown menu is selected by clicking
on the word View on the Top Bar menu of AutoCAD's main
interface. From the View menu, one selects the item 3D Viewpoint
Presets, which pops up a menu of different viewpoints. Select
SE Isometric from this menu to get a standard isometric
perspective view. We will examine the purpose and utility of other
viewpoints in class and in the laboratory sessions. To return to the
original (XY-plane) view, select the View menu and the 3D Viewpoint
Presets menu, then click on Plan View and World.
Another type of coordinate system is called the
User Coordinate System (UCS). User coordinate systems are
defined relative to the WCS. Drawings often contain several UCSs,
which can be saved and recalled. For example, one could draw a track
light (in the WCS called SE Isometric Perspective), then rotate
or scale the light housing to conform to its proper placement in a
simulated room. This light housing could then be saved as a UCS, for
further elaboration.
Creation of a UCS involves the following procedure:
Step 1.Select the View pulldown menu,
as discussed above.
Step 2. Select the Set UCS option on
the View menu. The UCS toolbar will appear on-screen. For
ease of direction, we continue narration of how to set the UCS
from the pulldown menu stack.
Step 3. Select the option labelled
3-Point, and select the default answers to the Command
Line prompts by depressing the
Step 4. Select the lower right corner of the
object you want to use to define the UCS. This establishes the
origin (lower right corner) of the UCS XY-plane.
Step 5. Using the OBJECT SNAP command with the
Endpoint option, select the upper left corner of the
object, which establishes the positive y-axis portion
of the UCS XY-plane.
Step 6. Save the UCS so that it will not need
to be redefined, by entering the SAVE UCS command, then
answering the prompt with the name of the UCS (e.g.,
spotlight, as in the preceding example).
Step 7. Return to the WCS by selecting the
World option on the Set UCS menu that was
discussed above.
Important Note: One must use the OBJECT SNAP
command with the Endpoint option to locate the point in Step 5. Merely
clicking with the pointing device is not sufficient.
Drawing with AutoCAD in 3D is limited to the UCS
orientation. You can only draw in the current UCS. To select a
different UCS (e.g., one that allows you to draw on a cylinder or
slanted surface), execute the following steps:
Step 1. Select the Named UCS menu
entry, and the UCS Control dialog box will appear.
Step 2. In the dialog box, select the name of
the UCS you wish to draw in, then click the button labelled
Current. This establishes the current UCS for
drawing.
Step 3. Draw the desired figure using standard
graphics primitives.
Step 4. Return to the WCS by selecting the
World option on the Set UCS menu that was
discussed above.
UCSs are very useful for drawing circles, boxes,
cones, etc. that is based on (or intersect with) the surface of a
curved object. AutoCAD can compute the surface intersections
automatically. In a complex drawing, this slows the speed of drawing
refresh if the REGEN option is used. Hence, we prefer to use REDRAW
for 3D drawing refresh, since REDRAW does not completely render the
graphics objects and is therefore much faster than REGEN, which draws
objects point-by-point.
Surface modelling creates 3D objects by joining
surfaces together. The surface commands are found in the MS-Windows
Surface Toolbar, which can be accessed from the Toolbar
entry on the Tools menu (located on the Top Bar of AutoCAD's
main interface).
AutoCAD's surface commands generate faceted surfaces
using a mesh or wireframe (looks like a Tinkertoy model or an object
amde of coat hangers), where each cell of the mesh encloses a
polygonal region of space. This means that AutoCAD generates only
approximations to curved surfaces, not the surfaces themselves. On
most computer displays, however, the approximations are visually
attractive and present minimal distortion.
Surface modelling differs from solid modelling in
that individual surfaces only are constructed, whereas solid modelling
renders solid shapes in perspective, and with a present lighting
model. For example, a box can be defined as (a) an object comprised
of six surfaces that are mutually perpendicular, or (b) a solid object
that has six mutually perpendicular sides. The key difference is that
surface models cannot be unioned, joined, or subtracted as solid
models can.
Surface modelling is well suited for drawing complex
wireframe meshes, or for depicting sharp transitions, such as the
abrupt change in elevation and orientation as one proceeds from a
sidewalk to a vertical retaining wall.
Example. Consider the procedures for drawing a
box. Other surface commands will be reviewed in class.
Step 1. Select the Box icon from
the Surface toolbar.
Step 2. Select (with your pointing device) a
corner location for the box.
Step 3. Move the cursor until the cursor
display reads (for example) "3,4", which locates the opposite
corner point 3 units in the x direction and 4 units in
the y direction from the WCS origin.
Step 4. When prompted for the length of the
box, enter a positive number (e.g., 10 in this example).
Step 5. You will then be prompted for the
width and height, which could be 6 and 8, respectively. You
can also set these values by moving your pointing device
(e.g., the AutoCAD crosshair cursor).
Step 6. When AutoCAD prompts you for the
rotation angle about the Z-axis, it is in dynamic mode,
which means that you can rotate the box whichever way you
desire using only the pointing device. After you select the
final position, the box will be drawn completely.
Other surface structures, such as WEDGE, TETRAHEDRON,
CONE, DOME, DISH or BOWL, are similarly constructed.
Extruded Surfaces are a special class of
AutoCAD surfaces that are in concept like a picket fence. That is,
one draws (in the XY plane of the WCS) a curved or straight line using
the freehand drawing option, or a polyline generation command.
This line then forms the base of the surface, which you "extrude"
upward using the EXTRUDE command to form a fence-like structure. This
will be demonstrated in class.
Ruled Surfaces can be similar in appearance to
extruded surfaces, and join two existing curves, or a point and a
curve. In the former case, corresponding lines are drawn between
points that divide the length of each curve into m-tiles (e.g.,
tenths, fifths, or quarters). In the case of a point and a curve, a
fan-like effect is produced, with lines radiating outward from the
point to equally spaced lines on the surface.
Edge Surfaces are constructed by specifying
the boundaries of a region in space. AutoCAD then interpolates
an approximation to a surface that is so bounded, based on the
curvature of the boundary objects.
In any of the above cases, the surface is rendered as
a transparent wireframe model, i.e., looks like it was made out
of coat hangers. As one changes the viewpoint by changing the UCS
(per Section 5.3.1), one can see different aspects of the wireframe
model, some of which are hidden behind other features of the same
model. We will encounter this concept again when we construct solid
models.
Solid modelling allows the creation of graphic
objects as though they were actual solid objects. Solids models
differ from surface models in that solid models have density and are
not simply constructed from surfaces that are joined together. Solid
models are created similar to surfaces, i.e., by unioning or joining
various primitive solid shapes. This unioning process can be reversed
to implement solids subtraction, where one may use a given type
of solid (e.g., a cylinder) to create a hole of a desired shape in
another solid.
AutoCAD's solids modelling capability can be accessed
through the SOLIDS toolbar that is invoked from the Tools menu
on the Top Bar of AutoCAD's main interface, by selecting the
Toolbars option, then choosing Solids.
Primitive solid objects that are currently available
in AutoCAD Release 13 are Box, Sphere, Cylinder, Cone, Wedge, and
Torus. (A torus is better known as a doughnut-like shape, very
similar to a large inner tube.) Other icons on the Solids toolbar
generated Extruded solids, objects of revolution (e.g., a curve
revolved about a line), slices or sections of solids, and
intersections of solids.
We begin with a simple example of creating a torus by
executing the following steps:
Step 1. Select the TORUS icon from the
Solids Toolbar (looks like a donut).
Step 2. In answer to the prompt for the torus
origin, depress the
Step 3. When prompted for the diameter or tube
diameter of the torus, make sure that the number you enter for
the torus diameter is larger than the tube diameter. The
torus diameter refers to the width of the entire torus
(like the diameter of a circle). The toroidal tube
diameter refers to how thick the donut is, i.e., the
diameter of the torus' cross-section.
Step 4. The torus will appear on your screen,
and you can relocate it with the MOVE command.
Extruded Solids. The EXTRUDE command can be
applied only to a polyline. To generate a solid from a polyline,
first draw the polyline in the XY-plane of the WCS, then select the
EXTRUDE command. You will be prompted to select the polyline by
clicking on it. Then, specify the height of the object and its taper
angle (if you want to make the extruded solid look like a cone). If
you don't want a tapered solid, enter zero as the taper angle.
The REVOLVE, SLICE, and SECTION commands will be
reviewed in class.
Intersecting Solids. The INTERFER command
defines a volume common to two solids, which is delimited by (a) the
boundaries of the respective solids and (b) by the intersection line
between those boundaries. The procedure for intersecting solids
follows:
Step 1. Select the INTERFER command from the
Solids toolbar.
Step 2. Select the two solids to be
intersected, in answer to the respective prompts displayed on
the Command Line.
Step 3. Enter No in response to the
prompt about creating interference solids.
Step 4. Use the ERASE command to remove the
two original solids. The common volume is easily seen on the
screen.
Unioning Solids. One may want to combine
various solids to make a composite object (e.g., making an overstuffed
chair from a box and cylinders). Unioning is a simple process, as
follows:
Step 1. One clicks on the lower-left-hand icon
of the MODIFY toolbar, which pops up a selection strip with
three icons.
Step 2. Then, select the icon signified by the
two joined white circles (leftmost icon in the popout strip).
Step 3. At that point, one selects the two
solids you want to join (by clicking on each one). The solids
must be in sufficiently close proximity to each other that
they have points in common.
Step 4. After a short time, AutoCAD computes
the new solid and displays it on the computer monitor.
Further applications of solids modelling will be
reviewed in class, with demonstrations given in the laboratory
session.
This concludes our discussion of advanced AutoCAD
features.
5.3. Surface and Solids Modelling.
5.3.1. Coordinate System Selection.
Enter key twice.
5.3.2. Surface Modelling.
5.3.3. Solids Modelling.
Enter key, or move the
pointing device to establish the origin point with the
crosshairs (then, click to set the origin).