542 Part III Working with Data

11.Choose External Reference from the Reference toolbar. In the Xref Manager dialog box, select ab19-04.dwg and click Detach. (Detaching xrefs is covered later in this chapter.) Click OK. You can now see that ab19-03.dwg includes the North symbol and letter because they were removed from the xref.

12.To see the results of the editing on the xref, open ab19-04.dwg. You can see the changes in the title block text and that the North symbol and letter are gone.

13.Close both drawings, saving changes to ab19-03.dwg.

Controlling Xref Display

You can control the display of xref layers so that you see only those layers you need. Several features let you control the process of displaying xrefs, making it easier to see only part of an xref and speeding up the display of very large xrefs.

Xrefs and dependent symbols

Dependent symbols are named items in a drawing, such as layers, text styles, dimension styles, and so on. When you attach an xref, these symbols are listed in your current drawing. For example, the Layer Control drop-down list displays the layers of the xref. Xref symbols have the format xref_name|symbol_name. This system distinguishes xref symbols from those of your current drawing and ensures that there are no duplicate symbols.

Xrefs and layers

You can turn on and off, or freeze and thaw, xref layers. You can also change an xref layer’s properties in the Layer Properties Manager dialog box. By default, these changes are retained. However, you can set the VISRETAIN system variable to 0 to discard these changes. The next time you open the drawing or reload the xref, the original settings are restored.

Objects created on layer 0 do not take on the typical xref layer name format, but stay on layer 0. If objects in the xref are on layer 0 with the color and linetype set to ByLayer, they take on the color and linetype properties of the current layer in the current drawing. If color and linetype are set to ByBlock, objects assume the current properties when the xref is attached. If you explicitly set color and linetype, objects retain those settings.

The XBIND command

You can use the XBIND command to import only the symbols you want from the external reference into the current drawing. This makes it easy to work with a consistent set of symbols in the current drawing and the xrefs. For example, you can choose to import the titleblk layer and the dec dimension style. Type xbind on the command line. The Xbind dialog box opens, listing each xref in the drawing and its symbols in a Windows Explorer–like display, as shown in Figure 19-8.

Click the plus sign next to any symbol type to open a list of symbols. Click the one you want and choose Add to add it to the Definitions to Bind list. Click OK when you’re done.

Later in this chapter, I explain how you can use the DesignCenter to move xrefs and other dependent symbols from one drawing to another.

Chapter 19 Referencing Other Drawings 543

Figure 19-8: Using the XBIND command to import symbols, such as layers, text styles, and so on.

Circular references

If drawing a includes drawing b as an xref and drawing b includes drawing a as an xref, you have a circular reference. Circular references can exist among three or more xrefs when you have nested xrefs. The program detects circular references and loads as much as it can. If you try to load an xref in such a situation, you see the following message:

Warning: Circular reference from XREF to current	drawing.
Circular reference(s) have been found. Continue?	<N> Type y to continue
to load the xref.
Breaking circular reference from XREF to current	drawing.

Clipping xrefs

You may want to see only part of an xref. This option is especially important when you’re using very large xref drawings. The XCLIP command enables you to create a border in an xref and hides any part of the xref outside the border.

AutoCAD LT doesn’t include the xref clipping feature.

To clip an xref, choose Modify Clip Xref or choose External Reference Clip from the Reference toolbar. At the prompt to select objects, pick the xref you want to clip. Note

that any nested xrefs are clipped with the main xref you select.

Table 19-1 explains the options of this command.

544 Part III Working with Data

		Table 19-1: XCLIP Options
	Option	How to Use It
	ON	Turns the clipping boundary on, displaying only the portion of the xref inside
		the clipping boundary. By default, the clipping boundary is on. Use this after
		you’ve turned it off to see only the clipped portion again.
	OFF	Turns the clipping boundary off, displaying the entire xref. The clipping
		boundary is still retained. This is somewhat like turning off a layer. You may
		want to see the entire xref for a while (for example, while redefining the
		boundary). Then you can turn the boundary back on (using the ON option)
		when you need only the clipped portion again.
	Clipdepth	This is used for 3D drawings only. After you set a clipping boundary, you can set
		front and back planes parallel to the boundary to display only the portion of the
		xref within that three-dimensional space. You create the front and back planes
		by specifying a distance from the clipping boundary. The Remove suboption
		removes the clipping planes.
	Delete	Deletes the clipping boundary. The boundary is no longer retained in the
		drawing.
	Generate polyline	Creates a polyline from the clipping boundary, using the current layer, color, and
		linetype. If you want to change the clipping boundary, you can edit the polyline
		by using PEDIT and redefine the boundary with the new polyline.
	New boundary	This is the default option. Press Enter to get the suboptions.
	Select polyline	Enables you to specify the clipping boundary by selecting an existing polyline.
		This option decurves fit-curved or arc portions of the polyline when creating the
		boundary.
	Polygonal	Enables you to specify a polygonal area — such as a polyline with straight edges.
		This option creates a rubber-band line as you pick points, keeping the polygon
		closed. You can use this option to create an irregularly shaped area that includes
		only the portion of the xref you wish to see.
	Rectangular	Enables you to pick two points on diagonally opposite corners of a rectangle,
		such as creating a selection window.
Tip	You can also clip blocks.
	To see the clipping boundary (if you haven’t used an existing polyline to define it), change
	the value of the XCLIPFRAME system variable to 1.

Figure 19-9 shows an xref clipped with a polygonal boundary. Compare this to Figure 19-4, which shows the entire xref.

Chapter 19 Referencing Other Drawings 545

Figure 19-9: An xref clipped with a polygonal boundary.

Speeding up the display of large xrefs

In order to reduce the time needed to display large xrefs, such as those used in GIS or 3D drawings, you can use demand loading, which enables you to load only the objects necessary to display the xref in your drawing. Demand loading works together with spatial and layer indexes.

The spatial index is created when you save a drawing. This index is used when you have enabled demand loading and attach a clipped xref that was saved with a spatial index. The index determines how much of the xref needs to be read to display it.

The layer index is also created when you save a drawing. This index is used when you’ve enabled demand loading and attach an xref that was saved with a layer index and has frozen or turned off layers. The index determines how much of the xref needs to be read to display it.

To make it perfectly clear, you need all the following to use this demand loading:

Demand loading must be enabled in the current drawing.

The xref must have been saved with a spatial or layer index.

The xref must either be clipped (for a spatial index) or have layers that are frozen or turned off (for a layer index).

Cross-	Demand loading is similar to partial opening and loading of drawings, explained in Chapter 8.
Reference

Demand loading

You turn on demand loading in your current drawing. To turn on demand loading, choose Tools Options, and then click the Open and Save tab. In the Demand Load Xrefs drop-down list, choose Enabled. Others on a networked system cannot then edit the original drawing while you’re referencing it. To let others edit the original drawing, choose Enabled with Copy. This option uses a copy of the referenced drawing for your xref. Click OK. You can turn on demand loading just before you attach an xref. You don’t need to keep demand loading on all the time.

546 Part III Working with Data

Spatial indexes

You save a spatial index for a drawing that you expect to use as an xref. The saving process takes a little longer, but you save time at the other end when you load a clipped xref or clip an xref for the first time. To create a spatial index in AutoCAD, choose File Save As to open the Save Drawing As dialog box. From the Tools menu at the top-right of the dialog box, choose Options to open the Saveas Options dialog box, shown in Figure 19-10 with the DWG Options tab on top.

	Figure 19-10: The DWG Options tab of
	the Saveas Options dialog box.
	From the Index Type drop-down list, choose Spatial or Layer & Spatial. Click OK. Then click
	Save in the Save Drawing As dialog box. In AutoCAD LT, use the INDEXCTL system variable
	on the command line and set its value to 2 for just a spatial index or to 3 for both spatial and
	layer indexes.
Tip	If you want to create an index for an existing drawing, click OK once to return to the Save
	Drawing As dialog box. Click Cancel. In other words, you don’t have to actually save the draw-
	ing to set up the index, which is controlled by the INDEXCTL system variable.
	After you create a spatial index, each time you save the drawing you see the following
	message:
	Updating Indexes for block *MODEL_SPACE
	To stop saving the index each time you save, choose File Save As. Choose Tools Options.
	In the Saveas Options dialog box, choose None from the Index Type drop-down list. Click OK
	and then click Cancel.
	Layer indexes
	You save a layer index for a drawing that you expect to use as an xref to create an index of all
	the layers in the drawing. As with a spatial index, the saving process takes a little longer, but
	you save time at the other end when you load an xref with frozen or turned off layers. To cre-
	ate a layer index in AutoCAD, choose File Save As to open the Save Drawing As dialog box.
	From the Tools menu at the upper-right corner of the dialog box, choose Options to open the
	Saveas Options dialog box, shown in Figure 19-10.

Chapter 19 Referencing Other Drawings 547

From the Index Type drop-down list, choose Layer or Layer & Spatial. Click OK, and then click Cancel. In AutoCAD LT, set the INDEXCTL system variable to 1 for just a layer index or 3 for both layer and spatial indexes.

After you create a layer index, each time you save the drawing you see the following message:

Updating Indexes for block *MODEL_SPACE

To stop saving the index, choose File Save As. Choose Options. In the Saveas Options dialog box, choose None from the Index Type drop-down list. Click OK once and then click Cancel.

On the	The drawings used in the following Step-by-Step exercise on controlling xref display — ab19-a.
CD-ROM	dwg, ab19-b.dwg, ab19-01.dwg, and ab19-02.dwg — are in the Drawings and Results
	folders on the CD-ROM.

STEP-BY-STEP: Controlling Xref Display

1.Open ab19-01.dwg from your AutoCAD Bible folder if you did the first exercise in this chapter.

If you didn’t do the first exercise in this chapter, use Windows Explorer to find ab19-b. dwg in the Drawings folder on the CD-ROM and ab19-01.dwg and ab19-02.dwg in the

Results folder on the CD-ROM. Copy all three files to your AutoCAD Bible folder. In Windows Explorer, right-click each file and choose Properties. Uncheck the Read-Only option (if checked) and click OK. Then open ab19-01.dwg from your AutoCAD Bible folder. If you get a message that the drawing cannot find the xrefs, choose Tools Options and click Project Files Search Path on the Files tab. Click Add and add your AutoCAD Bible folder. You can click Browse to choose the folder from a dialog box.

2.To create layer and spatial indexes, do one of the following:

•If you have AutoCAD: Choose File Save As. From the Tools menu of the dialog box, choose Options. In the Index Type drop-down list of the Saveas Options dialog box, choose Layer & Spatial. Click OK. (It may already be set for these indexes.) Click Cancel.

•If you have AutoCAD LT: On the command line, enter indexctl . Then enter 3 .

3.Choose Save from the Standard toolbar. Note the message on the command line that the indexes are being updated.

4.Close ab19-01.dwg.

5.Open ab19-02.dwg from your AutoCAD Bible folder. This drawing has an attached xref of a house plan and a nested xref of a title block, as shown in Figure 19-11.

6.Save it as ab19-05.dwg in your AutoCAD Bible folder.

7.Choose Tools Options and click the Open and Save tab. In the Demand Load Xrefs drop-down list, choose Enabled. (It may already be set to Enabled.) Click OK.

8.Click the Layer Control drop-down list. Click the On/Off icon next to the Ab19-01|notes layer to turn the layer back on. Click the top of the drop-down list box to close it. The notes layer displays.

548 Part III Working with Data

1

	2
0	3	4

9

6 5

8 7

Figure 19-11: Picking a polygonal boundary to clip an xref.

9.In AutoCAD only, choose External Reference Clip from the Reference toolbar or choose Modify Clip Xref to start the XCLIP command. (If you have AutoCAD LT,

skip this step.) Follow the prompts:

Select objects: Pick anywhere on the xref in Figure 19-11.

Select objects: Enter clipping option

[ON/OFF/Clipdepth/Delete/generate Polyline/New boundary] <New>: Specify clipping boundary:

[Select polyline/Polygonal/Rectangular] <Rectangular>: Right-click and choose Polygonal.

Specify first point: Pick 1 in Figure 19-11. It might help to turn

off OSNAP if it is on.
Specify next point or	[Undo]: Pick 2.
Specify next point or	[Undo]: Pick 3.
Specify next point or	[Undo]: Pick 4.
Specify next point or	[Undo]: Pick 5.
Specify next point or	[Undo]: Pick 6.
Specify next point or	[Undo]: Pick 7.
Specify next point or	[Undo]: Pick 8.
Specify next point or	[Undo]: Pick 9.
Specify next point or	[Undo]: Pick 0.
Specify next point or	[Undo]:
This action clips the xref.

10.Turn off the Ab19-01|notes layer again.

11.Save your drawing. It should look like Figure 19-12. (If you have AutoCAD LT, you still see the entire xref.) Keep the drawing open if you’re continuing on to the next exercise.