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AutoCAD

Pick an Object, Any Object

14 Feb, 2005 By: Bill Fane Cadalyst

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Harness the power of AutoCAD’s selection sets and change is easy

It was 7 a.m. on a dark and stormy morning, the day before Christmas Eve. Captain LearnCurve and his beautifully tanned wife stepped off the airplane after a week basking in the sunshine of the Dominican Republic. The eight-hour overnight flight home to Vancouver had almost negated the benefits of the vacation. Their spirits soared, however, when their son Trevor announced that he had made his selection, and Jenn had accepted his engagement ring.

Speaking of selections, I haven't written about AutoCAD selection sets since 1993. "Your segues are getting worse with each issue!"

Always trying to establish new standards?.

It's Time for a New Look
Even though the selection set mechanism has not changed significantly for many years, I felt it would be timely to review it. Often, many beginners are not fully conversant with this feature, and even many experienced users do not take full advantage of it.

Let's start with the basic principles.

The real power of AutoCAD comes not just from its ability to draw things; it is when we begin to edit and change that we really make money. This is so true that unlike when creating a pencil-and-paper drawing, in AutoCAD it is often faster to do it wrong and edit than it is to do it right the first time. We can draw wherever it is convenient without regard to how things fit on the sheet and then move as required; we can copy sections and then erase, trim, extend and so on.

AutoCAD makes the change processes even more powerful and versatile by providing selection sets. To speed things up, AutoCAD lets us work on a whole set of objects at the same time, rather than having to change everything individually.

Whenever we issue an editing command, AutoCAD prompts, "Select objects," to indicate that we can select an object or for a bunch of objects on which to operate. We can now build a set of selected objects by using several subcommands to add and delete objects until we are happy with the set. When we are done, the editing function will then be performed simultaneously on all objects in the set.

Let's start with the selection of a single object. All you need to do is to click on it and it will be selected. Selected objects are highlighted by being displayed with a dotted appearance. This seems simple enough, and it is, but there are two features that can make it even simpler.

Normally, AutoCAD displays a crosshair cursor with a small square surrounding the intersection point. When you are in Select Object mode, the crosshair goes away and only the box is displayed. This box is called the pickbox. To select an object, it must touch or fall within the pickbox.

Pick the Size
By default, the size of the pickbox is three screen pixels square. This seems to work most of the time for a typical 1024X768 screen resolution on an average monitor. On the other hand, you may want to increase its size if you are running at a higher resolution on a larger screen to avoid having to be too precise in your picking. The PickBox command will change it. This value is stored in the system registry, so it remains the same for subsequent editing sessions.

As implied in the previous paragraph, a larger pickbox can make it quicker and easier to select objects. The downside is that it can become difficult to select a small object in a crowded drawing.

Now here is a little trick that can help solve the latter problem. The real issue is that if two or more objects fall within the pickbox then AutoCAD selects the younger one. This may not be the one you want. No problem. All you need to do is hold down the Control key while you pick a multiobject point. AutoCAD will highlight the youngest object, and will then display a Cycle On prompt.

Each succeeding click of the mouse will display the next available object. When you get the one you want, just press Enter to select it and to return to the normal selection mode, or you can press Escape to return to the normal selection mode without selecting anything.

This technique is particularly effective when you have two or more lines stacked on each other and you want to delete the shorter ones, but they are also older.

Having selected an object, AutoCAD then continues to invite you to select objects to add to the selection set. This simple prompt does not begin to hint at the power that is available, however.

In and Out the Windows?
Most users know that you can add multiple objects to a selection set with just two picks. If the first point is in thin air (not on an object), then AutoCAD displays a rectangular window or crossing box depending on what you do next. If the next pick is to the right of the previous one you are in Window mode, but if the next pick is to the left you are in Crossing mode.

In Window mode, any object that falls entirely within the window will be added to the set and will change appearance. Anything that crosses the window boundary will be ignored. The selection window frame displays as a solid line.

In Crossing mode, anything that falls within or crosses the window boundary will be added to the selection set. The selection window frame displays as a dotted line on most displays to differentiate it from the Window mode.

Having picked an object, or two window points, AutoCAD tells us what it found and then comes back to ask for more things to add to the set. When we have selected everything we want, we just press Enter and the editing command operates on every object within the selection set.

We now come to a few details about selection sets that often slip by even experienced users.

What if we inadvertently select something that we don't want? No need to start over. We can remove objects from the set by entering an R in response to the prompt. We can click on objects, and we can window or cross them as desired. Anything we select will be removed from the set and its appearance will revert to normal until we change mode. Entering an A at the prompt will turn off the Remove mode and get us back to where we are adding items to the set.

Current releases have an even easier mode-switch mechanism. All we need to do is press the Shift key while picking or windowing. Anything that was already selected will be unselected.

There are several other selection modes we can invoke by typing a letter or three or two during the selection process:

L: The Last object drawn that is currently visible on the screen will be added to the set.

P: Use any editing command except Erase. Now start a different editing command, or the same one over again. When it asks you to select objects, simply reply with the single letter P and press Enter. Surprise! The entire previous selection set will automatically be reselected. You can then continue adding or removing as desired.

The Previous mode can be used at any time during selection. You might pick a few objects, then add the previous set, then add a few more, then remove a few and so on.

ALL: As it implies, this selects all the objects in the drawing that are not on frozen or locked layers, even if they are not currently visible on screen. Objects on all other layers will be selected. Note that this includes objects on off Off layers, even though they are not visible.

This can be used to select everything except certain objects; select All, then Remove the undesired areas.

We now come to three modes that are often overlooked by many users.

WP: Window Polygon selects objects just like a window, but with one major difference: It does not just ask for two points to define a horizontal/vertical rectangular window. Instead, when you pick the first point it anchors an elastic line. Move the cursor and pick a point. Move the cursor again and you have two elastic lines; one from the last point and one from the initial one.

You can keep going as desired. Each pick point becomes the new anchor for one of the elastic lines while the initial point continues to anchor the other one. You can thus draw an irregular polygon around a set of objects.

When you press Enter instead of picking a point, AutoCAD will select all the objects that fall completely within your irregular boundary and will add or remove them to or from the selection set.

CP: Crossing Polygon behaves almost exactly like Window Polygon except it displays a dotted elastic boundary and ends up selecting everything that is totally within or crosses the boundary.

F: Fence is a pure crossing mode. You get to trace out a series of line segments until you press Enter. When you do, it selects all objects that actually cross the fence. It does not close back on itself like WP or CP and so does not surround an area.

Fence is particularly useful when used with Trim. You can select your trimming edges and then use Fence to select a bunch of objects to be trimmed. All will be trimmed at once, not like very early releases in which objects to be trimmed had to be selected one at a time.

Fence is not a window, so it won't work with Stretch.

AutoCAD also supports noun/verb selection. You can select objects right from the Command line prompt even though an editing command is not active. Simply pick objects, or use window and crossing boxes. Selected objects will be highlighted. Once again, you can remove objects from the selection by reselecting them while pressing the Shift key.

If you then start an editing command, it will automatically operate on the objects that you have already selected.

Undocumented Features?
?are not always bugs. OK, I lied. This one is not really undocumented, but it was in earlier releases and just gets a quick passing reference in the Help facility of current releases.

Start the Trim or Extend command. When asked, "Select boundary edges," don't select anything. Instead, just press Enter. AutoCAD will not highlight anything, but will simply drop through to the "Select object to trim?" prompt. In fact, what it has done is select every object in the drawing as a potential trim or extend boundary.

And Now For Something Completely Different?
Be careful when renting a car. Rental agencies will often try to get you to upgrade to a more expensive model for "just a few extra dollars." Resist. The more they try, the more important it is that you stand firm. The truth is that they often do not have "your" selected car available. I once had the shuttle bus driver for a major chain admit that the company did not actually own any of its advertised lowest-priced cars. In that case, if they can't sell you the upgraded model, they have to give it to you anyway.