Up until Release 10, AutoCAD was essentially a two-dimensional drawing system. Following that release, it now supports a full three-dimensional database. This has had the effect that the features of AutoCAD that support two-dimensional drawing are fully self-contained and can still be used as a 2D system without being concerned with its 3D features. In this course we will focus on the 2D drafting aspects of AutoCAD since that better reflects the way in which this system is generally used in practice at the present time. Several sections in these notes explain how the 3D features have been incorporated into AutoCAD, but provide little detail of the 3D operations. 

In order to understand AutoCAD, we must focus on a few specific concepts.

The graphic primitives are defined geometrically in terms of the normal cartesian coordinate system (right-handed system with positive X-axis to the right, positive Y-axis up the screen and positive Z-axis coming out of the screen towards the user). Hence, for example, lines are defined by their end- point coordinates (x, y and z), while circles are defined by their centre coordinates and radius. Each entity also has certain attributes associated with it, such as line style, text font or colour. 

A block is a group of entities that can be manipulated as a single unit. Once created, a block may be moved, scaled, rotated, copied or deleted. A block can be created by collecting together a group of entities from the current drawing (the drawing being currently edited) and assigning a single name to that group. Alternatively, an existing drawing (from disk) can be inserted into the current drawing as a block. Equally, a block from the current drawing can be written out to a file as a new drawing. 

It is important to understand that a block, in AutoCAD, is unique to a specific drawing. That is, when an existing drawing is inserted into the current drawing as a block, AutoCAD simply copies the graphics from that drawing and includes it as a single object. The existing drawing is unaffected by the action. Indeed, if that existing drawing (from which the block was created) were subsequently edited then those changes would not affect the inserted block. It is possible, however, to update a block definition in a drawing by having AutoCAD redefine the block using the current version of the original drawing. 

As an alternative to inserting a drawing into another drawing as a block, AutoCAD now provides the facility to attach a drawing to the current drawing as an external reference. The external drawing is then treated like a block except that it is automatically updated each time the drawing to which it is attached is loaded. Naturally, if the external drawing is altered in the meantime, then the external reference will be updated to reflect those changes. 

Another important feature of a block (including external reference blocks) is that it can be duplicated many times within a drawing, with each copy pointing to the one graphic description, but having its own unique position, scale and rotation factor. This is known as instancing. There are two advantages to this approach:

• space saving - the graphics description is only stored once; and 
• ease of re-definition - the graphics description of a block can be altered and then re-defined in order to update each instance in the drawing. 

The final concept to be explained about blocks is the notion of exploding. Any instance of a block may be exploded in order to reduce it back to its separate graphic entities. Similarly, an external reference block can be unbound so that it becomes a local block and then, in turn, can be exploded. 

In order to understand this, it is best to picture model space as a very large piece of paper located somewhere in two-dimensional space. (AutoCAD can actually draw anywhere in 3D space, but let's ignore that for the moment!) Since, the size and location of that piece of paper is entirely up to you, it would make sense to choose something that makes drawing as simple as possible. Now, Australian architects typically use millimetres as their unit of measurement and would draw objects in their true size. This means that when drawing a plan of a building that is, say, 15 metres square, the "paper" size (in model space) would be set at something greater than 15,000 x 15,000 units (where "units" equals millimetres). Strictly speaking, it is not necessary to set the model space drawing size, but it is helpful to do so in order to establish a context for the drawing. 

When working on the drawing in model space, you should think of the graphics screen as a window through which you can view all or part of that drawing sheet. Thus, when viewing a drawing, a larger display scale will allow you to view only a part of the drawing on the screen. This is referred to as zooming in on the drawing: as you increase the scale, you can see less of your drawing, but what you can see will be in greater detail. If you zoom out, then you will be able to see more of the drawing, but in lesser detail. AutoCAD provides some fairly sophisticated tools for zooming in and out of your drawing and for panning back and forth across it. These are described later. 

Although it is possible to plot from model space, it is normal to set up a sheet in paper space, and create one or more viewports showing different views of the drawing in model space. Each view is displayed at a fixed scale, and the paper space drawing is then plotted at a scale of 1:1. In this case, text notes and other annotation on the drawing is best added in paper space, and plotted at its actual size. Any text that is entered in model space has to be drawn at an artificially large size so that when it is scaled down for plotting, it ends up at the size required. For example, text on a 1:100 drawing might be set at 250 millimetres high in model space to create plotted text that is 2.5mm high on the plot. 

Layering can also be used to advantage while editing a drawing. For example, suppose a plan drawing is substantially complete with only the brickwork cross-hatching to go. If all the brickwork is on one layer, each other layer could be turned off so that only the brickwork is visible and selectable. That makes it possible to cross-hatch the brickwork without inadvertently affecting something else. 

Layering in AutoCAD is manipulated through the properties that can be assigned to each layer. Once the interaction between layer properties is understood, then these can be manipulated to great advantage. The following notes summarise the properties of layers. 

Layer name. 

An unlimited number of layers may be created. Each is identified by a unique name assigned by the user. The name may be up to 31 characters long, made up from uppercase letters, digits and three special characters ($ - _). 

Current layer. 

AutoCAD maintains the notion of a current layer, being that layer upon which all newly created entities are placed. Any of the existing layers may be selected as current at any time. 

Visibility. 

Each layer can be either visible (ON) or invisible (OFF). Only visible layers are displayed or plotted. Layers can be turned on or off at will, in any combination. Entities can still be added to an invisible layer (indeed it is possible to have a current layer that is invisible), but entities that are not visible cannot be selected for editing. 

Colour and linestyle. 

Each layer has associated with it both a colour and a linestyle. As entities are drawn, they are assigned, by default, the colour and linestyle of the current layer. (This can be overridden by assigning each entity its own colour and linestyle.) By default, new layers are assigned colour 7 (white) and continuous linestyle. 

Freeze/Thaw state. 

At any time the user may freeze one or more layers of a drawing. A frozen layer is made invisible. Any entities on a frozen layer are completely ignored or forgotten by AutoCAD during the editing process. This has the effect of speeding up many editing operations. Naturally, a frozen layer can be thawed again at any time. 

Screen redraws are faster (but less thorough) than regens and AutoCAD will automatically offer to do the latter only when it is actually necessary. You will find that you will want to redraw the screen fairly regularly while you are using AutoCAD, while, on the other hand, you will very rarely need to force AutoCAD to do a regen. On those occasions when it does offer, you should generally allow the required regen to be carried out without interference. It is actually possible to disable regens entirely, but that should be done with caution, as it severely restricts the integrity of the displayed image. 

In the pursuit of speed versus integrity, there are three other facilities in AutoCAD that can be used to achieve a balance between interactive speed and the quality of the displayed image. These are things that you will not normally need to worry about unless your drawing is particularly complex. Text can be represented on the screen merely as rectangular boxes rather than the fully formed letters (this is referred to as quick text). Circles and arcs can be displayed as a series of straight line segments rather than smooth curves. Solid filled areas and thick lines can be shown hollow (drawn in outline only).

This facility allows prototype drawings to be set up which can be used as "templates" in the production of other drawings. This is a powerful concept as it permits firms to set up office standards which are accessible to all users of the system.

In order to support the 3D user, AutoCAD provides five basic ways to enter 3D information: 

• firstly, most of the basic graphic entities (lines, arcs and curves) can have a thickness associated with them which is effectively a height in the z-direction (this has always been a feature of AutoCAD and is sometimes referred to as 2.5D); 
• secondly, when specifying the position of any point during the drawing process, all three coordinate values can be entered by the user (again, if only two coordinates are specified, the point is assumed to lie in the base plane - note also that some entities must be perfectly planar, so if you attempt to enter a 3D point that would make such an object non-planar, then AutoCAD will display an error message); 
• thirdly, AutoCAD supports a number of specifically 3D graphic entities, including 3D polylines, faces and meshes, along with a set of commands that can be used to create 3D objects with them; 
• fourthly, and most significantly, AutoCAD allows the draftsman to set up a temporary user coordinate system (UCS) which can be positioned and orientated anywhere in space (relative to the WCS) - thus, you can picture the standard X, Y and Z axes of the WCS and then imagine an equivalent set of axes (the UCS) being moved away from the origin and then tilted and rotated to any orientation that you wish - once a UCS is established, all 2D drawing is done relative to those coordinates until a new UCS is defined or the user switches back to the WCS; 
• lastly, from release 11 onwards, AutoCAD has provided an extension package called AME (Advanced Modelling Extension) which provides 3D solid modelling capabilities - these allow you to form complex solid objects by adding, subtracting and intersecting a set of simple solid primitives (such as, boxes, cone, cylinders, spheres, etc) - for example, you can form a box with a circular hole by subtracting a cylinder from a box, and so on - we will not be concerned with these techniques in these notes. 

That now concludes the discussion of general concepts in AutoCAD. The remainder of this chapter is concerned with how to interact with AutoCAD in order to produce drawings.

If AutoCAD is running inside a windowing system (such as OpenLook on the SUN graphics workstation), then it may not occupy the whole screen and will include a window bar across the top of the window (as illustrated). On a DOS-based PC, AutoCAD occupies the whole screen. The major portion of the AutoCAD window is assigned to the drawing area. It will display the same portion of the drawing as was visible the last time the present drawing was edited. (If you are starting a new drawing then, naturally, the drawing area will be clear.) Above the drawing area is the status area where various items of status information are displayed (such as the current layer, current colour, coordinates of the last point entered, etc.). On a DOS-based PC, whenever the graphics cursor is moved into the status area, the status information is temporarily replaced by a row of pull-down menu names: the use of these will be explained later in the notes. On some platforms, such as the SUN graphics workstation running the Openlook graphics user interface (GUI), the pull-down menus are displayed permanently above the status line (not shown in Figure 5.1). To the right of the screen is displayed the permanent screen menu. Along the bottom of the screen is the command echo region, signified by the "Command:" prompt on the left side. 

• check that AutoCAD is in Command mode (signified by the prompt "Command:" on the left) - if not, hold down the Ctrl key on the keyboard and press C to cancel the current operation and go back to command; 
• enter a command - either by typing in the command word or selecting from a menu as explained later; 
• this activates Data Entry mode - AutoCAD will prompt for information needed to complete the command; 
• enter data required by AutoCAD; 
• AutoCAD will carry out the operation required by the command, update the displayed drawing and return to Command mode. 

The next section deals briefly with the characteristics of the graphics workstations that are used to run AutoCAD in this course. The two subsequent sections discuss the general principles of command entry and data entry respectively. The final two sections of this chapter are concerned with specific procedures to be followed for two types of data entry, namely object snapping and object selection. 

Thus, it is important to understand that while you are using AutoCAD, at least three programs are operating on the computer: in the background, UNIX is generally managing the overall operation of the machine; in front of that, OpenLook is managing the placement of windows on the screen and the functions that are occurring within those windows; and, in the foreground, one or more applications (such as AutoCAD) are operating. Generally, you will only be concerned with your foreground application, AutoCAD. 

The two windows that are opened for AutoCAD are used to display textual and graphics information respectively: predictably, these are known as the text window and the graphics window. When AutoCAD needs to display text information (such as prompting the user or providing a list of information), then it uses the text window. The drawing editor screen (described above) is displayed in the graphics window (it also includes a small 3-line text area at the base of the window that is used to display text information). You will become more familiar with these windows when you begin to use AutoCAD. 

The keyboard in AutoCAD is used primarily for typing in command words and data as described in the last section. Notice that across the top of the keyboard, is a row of function keys labelled F1 to F10 (F11 & F12 are not used). These are used by AutoCAD to allow you to perform specific operations. For example, the key labelled "F1" is used to switch between the graphics and text windows as discussed in the previous paragraph. This is important to remember because some AutoCAD commands automatically switch to the text window in order to display text data and you need to know how to get back to the graphics window in order to keep drawing. 

The mouse pointing device that is attached to the keyboard is used for pointing to things on the screen. It is used in both OpenLook and AutoCAD: you will notice that it behaves slightly differently in these two contexts, but you will quickly get the hang of it! As you move the mouse around on the silvered mouse pad, a pointer (known generally as a cursor) will be moved on the screen: you use that cursor to point to things. Note that it is important that the mouse pad is properly oriented: one of the long edges of the pad should "face" your forearm as you manipulate the mouse, otherwise you will find that the cursor does not seem to behave properly. 

Notice finally that the mouse has three buttons on it. These are used in a very specific way. The most commonly used button (in both AutoCAD and OpenLook) is the pick button: this corresponds to the left-most button. To pick something, simply move the cursor so that it points to the item and briefly press the left button (this process is often called "clicking on to something"). The middle mouse button is equivalent to pressing the Enter key on the keyboard. The right mouse button is used in two different ways: these will be explained in context later in the notes. 

To simplify command entry through the keyboard, AutoCAD permits the user to establish aliases, which are used most commonly as abbreviated forms of major commands. This is a customisation option which is set up outside AutoCAD and is therefore installation-specific. AutoCAD, as shipped from the vendor, has a number of aliases defined in this way in order to illustrate the concept. We make no attempt in these notes to identify the aliases, but examples will "crop up" in the tutorials section. 

To pick an item from the screen menu, simply move the mouse until the required item is highlighted (displayed in reverse video), and then press the pick (left) button. 

There are three types of items included in the screen menu (see Figure 5.1 again):

• pointers to sub-menus - these are displayed in all uppercase characters (not terminated by a colon ":") and cause a corresponding sub-menu to be called up; 
• command words - these are displayed in uppercase characters terminated by ":" and cause the corresponding command to be entered at the "Command:" prompt as if it had been typed (including the RETURN key); 
• command options - these are displayed in lower or mixed case and only appear when specific commands have been selected - they represent command options (relevant to that command) that can be picked rather than typed at the keyboard - command option menus are often displayed automatically when a command is issued from some other source. 

To activate pull-down menus on a DOS-based PC system, move the cursor to the status line at the top of the graphics window (noting that the pull-down menus then appear in place of the status line) and highlight the menu that you wish to pull down. When you press the pick button, that menu will appear on the screen. The menu items are either commands or pointers to further sub- menus. Move the pointing device again until the desired item is highlighted and again press the pick button. Once a command is selected, it is entered into the Command line and executed. To remove a pull-down menu (without selecting an item), move the cursor away from the menu and click the left mouse button. 

Under OpenLook, the pull-down menus are permanently displayed as a row of buttons above the status line in the AutoCAD graphics window. These are designed to function in exactly the same way as any other OpenLook menus, which may be confusing to first-time users because it involves using the mouse buttons in a different way to the rest of AutoCAD. To pull down an OpenLook menu, simply click it with the right mouse button. Items which are actually pointers to sub-menus are identified by a small arrow (triangle) pointing towards the right of the item. Clicking these items (again with the right mouse button) will open the sub-menu. Any command item in either the main menus or sub-menus can be selected by clicking (this time with either the left or right button, but normally the left). A nice feature of OpenLook menus is that they can be fixed semi-permanently to the screen and positioned anywhere that the user wishes. This is useful where you plan to re-use a menu frequently during some phase of a drawing task. To fix the menu in this way, first open it, then click (with the left button) on the "pin" displayed in the top left corner of the menu. The "pin" will change to appear as if it has been pinned into the screen. If you then position the cursor near the top of the pinned menu and hold down the left mouse button, you will be able to drag the menu to any part of the screen. Pinned menus can be removed by clicking (left button) on the pin. AutoCAD automatically removes pinned menus itself under various circumstances which can be a source of frustration to the user. 

A final comment about OpenLook menus which can cause some confusion: if you click on a menu or sub-menu name with the left mouse button, then a default item (usually the first) in that menu is automatically selected and executed. This can be very useful, but can also be very disconcerting and annoying. The only solution to this is to be aware of it. There is at least one case (which you will encounter in the tutorials) where this feature is very useful!!

Where workstations are equipped with digitising tablets, there will be a tablet overlay attached to the tablet on which are printed a wide range of menu items. Each of those items represent some kind of operation involving one or more AutoCAD commands. 

To select an item, simply move the hand-held puck until its cross-hairs (in its perspex viewing window) are positioned over the required item, and then press the pick button (on the puck). 

One final comment on command entry: the last command entered (by whatever means) can always be recalled very simply by pressing either the space bar or the RETURN key in response to the "Command:" prompt. 

AutoCAD will always prompt for the data that it requires. In general, your response will be either a piece of information (eg. coordinates of a point, a distance or a string of text to be inserted into the drawing), or one choice out of a range of options associated with the command selected. For example, in response to the GRID command, AutoCAD should display the prompt... 

    Grid spacing (X) or ON/OFF/Snap/Aspect <.25>:

All AutoCAD data entry prompts follow a similar pattern. Once that pattern becomes familiar, it is possible to almost guess your way through the system.

The following sub-sections explain briefly how to enter the most common types of data required by AutoCAD. 

The technique which involves locking onto positions relative to existing graphic entities is called object snapping and is often used in conjunction with what are known as X/Y/Z point filters to make them even more powerful. These techniques are discussed later in this Chapter. Snap grids and other devices that aid in accurate pointing are set up using commands that are described in Chapter 3 of these notes. 

As an alternative to pointing and to provide absolute accuracy, AutoCAD allows the user to type in the coordinates as numbers. There are three types of values that can be entered this way: 

• absolute coordinates - simply type two (or three) numbers separated by a comma (viz. 345.6,-234); 
• relative coordinates - these represent offset distances in X, Y and Z (if required) relative to the last point entered - simply type "@" followed by the numbers separated by a comma (viz. @25,-34.5); 
• polar coordinates - these represent an angular distance from the last point entered - type "@" followed a number specifying distance, followed by "<" and a number specifying an angle about the Z axis in degrees (viz. @234.7<45 - by default, zero degrees is taken as directly to the right of the starting point and angles increase in an anti-clockwise direction). 

Distances can also be specified by pointing with the cross-hairs. If the starting point (or anchor point) is obvious (such as when entering the radius of a circle once the centre is known), then one pointing operation is required. Otherwise, two pointing operations must be performed in order to specify the distance.

Alternatively, AutoCAD can be "shown" the angle by pointing with the screen cross-hairs. To do this, first identify an anchor point and then show the end of a vector that defines the required angle. The angle is then measured by AutoCAD in an anti-clockwise direction between an imaginary line pointing towards the right from the user's anchor point and the defined vector.

Object snap modes can also be selected from a menu by pressing the right mouse button and then selecting from the pull-down menu subsequently displayed. The right mouse button in AutoCAD is therefore known as the object snap button. Note that, under OpenLook, the pull-down menu displayed in this way is automatically "pinned" (refer to earlier discussion) to the screen and will remain until dismissed and can be positioned anywhere the user wishes. 

At the risk of confusing you, these snap modes can also be activated by selecting them from the Tools pull-down menu or by clicking on the row of asterisks (********) that always appears at the top of every screen menu. Generally however, it is easier to type the name of the snap mode or use the object snap button on the mouse. 

The available object snap modes are:

• NEArest - snaps to the nearest point on a line, arc or circle (unless that arc or circle is part of a block that has been unequally scaled in the X, Y or Z direction, as described earlier); 
• ENDpoint - snaps to the closest endpoint of a line or arc; 
• MIDpoint - snaps to the midpoint of a line or arc; 
• CENtre - snaps to the centre of an arc or circle (selected by pointing to its circumference); 
• NODe - snaps to the nearest point entity (created using the POINT command); 
• QUAdrant - snaps to nearest quadrant point on an arc or circle (selected by pointing to its circumference) - quadrant points are at "12, 3, 6 & 9 o'clock" on the circle as originally drawn; 
• INTersection - snaps to the intersection of two line, arc or circle entities (in any combination) or to the corners of solids (filled polygon areas); 
• INSert - snaps to the insertion point of a text entity or a block; 
• PERpendicular - snaps to the point on a line, circle or arc that would form a normal to that entity with respect to the last point entered (does not work for circles and arcs that are part of an unequally scaled block); 
• TANgent - snaps to the point on a circle or arc that, when connected to the last point entered, forms a tangent to that entity (does not work for circles and arcs that are part of an unequally scaled block); 
• QUIck - used in conjunction with other snaps modes to effect a faster, but less thorough, snap operation (I've never had occasion to use this one!!); 
• NONe - disables the effect of any snap mode set up by the OSNAP command for the current pointing operation. (The OSNAP command is described in Chapter 3 of these notes: essentially, it can be used to set any combination of the above snap modes to be automatically employed whenever a point is being entered ... this can be used to avoid the need to enter the snap mode every time.) 

The point filters can be used to enter any 2D or 3D point. The general form is a dot followed by any combination of 1 or 2 of the letters x, y or z (followed then by a SPACE or RETURN). For example, ".xz" tells AutoCAD to take x- and z-values from the next point selected and then prompt for a y-value. 

AutoCAD normally prompts you to create the selection set when it is actually required by a particular command. Thus, the normal process is to first issue the command and then select the objects to be effected by that command. However, under Release 12 of AutoCAD, you may choose to select the objects before issuing a command. The techniques for doing that will be explained later, but for the moment we will concentrate on the normal way of setting up selection sets after a command has been issued. 

Once a command has been issued and AutoCAD requires the user to specify a selection set, it issues the prompt "Select objects:" and replaces the cross- hairs with a small box called the object selection target. A number of techniques (explained below) can then be used to select objects. Once those objects have been identified and highlighted, the prompt is re-issued and the process repeated until a solitary space bar or RETURN is pressed (in response to the prompt) to indicate that no more selections are required. At that point AutoCAD continues with the command that initiated the selection process. 

The available techniques for selecting objects are as follows: 

• object pointing - merely point to an individual object (it should then change colour); 
• multiple object pointing - press m followed by space bar or RETURN - AutoCAD then allows the user to point to a number of objects until the space bar or RETURN is pressed again - at that point, AutoCAD searches for and highlights the selected objects before issuing the prompt again; 
• window selection - press w followed by space bar or RETURN - AutoCAD then requires that the opposite corners of a rectangle be specified and it selects all the objects that lie wholly within that window; 
• window polygon selection - press wp followed by space bar or RETURN - AutoCAD then requires that a series of points are entered that define a closed polygon and selects all the objects that lie wholly within that polygon; 
• crossing selection - press c followed by space bar or RETURN - AutoCAD again requires that a rectangle be defined - this time, all objects that lie partially or wholly within the rectangle are selected; 
• crossing polygon selection - press cp followed by space bar or RETURN - AutoCAD again requires that a polygon rectangle be defined - this time, all objects that lie partially or wholly within that polygon are selected; 
• fence selection - press f followed by space bar or RETURN - AutoCAD allows you to pick any number of points to create a fence and any objects that intersect the fence are selected; 
• last object - press l followed by space bar or RETURN to select the last object created (and still visible); 
• previous selection set - press p followed by space bar or RETURN to re-select the last selection set that was constructed; 
• undo - press u followed by space bar or RETURN to de-select the group of objects most recently added to the selection set - may be used repeatedly to step back through the selection process; 
• remove mode - press r followed by space bar or RETURN to switch to "remove mode" such that further selections are removed from the current selection set; 
• add mode - press a followed by space bar or RETURN to revert back to "add mode" after being in "remove mode"; 
• cancel - press Ctrl C to cancel the selection process (and the current command as well).