LT On-line: Lesson 11
31 Jan, 2001 By: Mark MiddlebrookXrefs:
layers, loading, and logs
by Mark Middlebrook
Page
1: Xref layer visibility
My previous two LT Online lessons covered xrefs and how to attach and
manage them with the Xref Manager dialog box. This lesson rounds out xref
coverage with a trio of topics: how to control xref layer settings with
the VISRETAIN system variable, when to use demand loading, and how to
troubleshoot xref problems with text window messages and the log file.
The procedures described
here work with AutoCAD LT 98 and 2000 and AutoCAD Release 14 and 2000.
Xref
layer visibility
As described in the "Layer-palooza"
section of lesson 9, when you attach one file to another as an xref, AutoCAD
adds new layers to the parent drawing that correspond to the layers in
the xrefed DWG. The xrefed layers have the form Xrefname|Layername, as
shown in figure 1. You might wonder what happens
when you make changes (on/off thaw/freeze, color, linetype, etc.) to these
xref layers or to the corresponding layers in the xref itself. 
Figure 1.
In particular:
• If you change the settings of an xref layer in the parent drawing,
do the changes persist after you close and reopen the parent drawing?
• If you
change the settings of a layer in the child drawing, do the changes appear
the next time you open the parent drawing?
The answer to both of these questions depends on the setting of the VISRETAIN
system variable in the parent drawing. VISRETAIN is short for visibility
retain, and this system variable determines whether layer settings in
the parent drawing or the child drawing control the appearance of xref
layers in the parent.
The default VISRETAIN setting is 1 (= on), which means that the parent
drawing's settings for xref layers control over the settings for the corresponding
layers in the xref drawing itself. For example, assume that you have attached
MYSCREW.DWG
(the child drawing into MYWIDGET.DWG
(the parent drawing). If you turn off the xref layer Myscrew|Text in the
parent drawing MYWIDGET.DWG,
that layer will still be off the next time that you open MYWIDGET.DWG.
Furthermore, if you later open MYSCREW.DWG
and change its Text layer from red to green, the Myscrew|Text layer ignores
this change. The Myscrew|Text will still be red when you open MYWIDGET.DWG.
If you change VISRETAIN to 0 (= off) in a parent drawing, you tell AutoCAD
that the child drawing's layer settings control over the parent drawing's
settings for the corresponding layers. Each time you open the parent drawing,
AutoCAD applies the current layer settings from the child drawing. For
example, if you turn off the xref layer Myscrew|Text in the parent drawing
MYWIDGET.DWG,
it goes off for the current session. The next time that you open MYWIDGET.DWG,
though, the Myscrew|Text layer is back on, because that's its current
setting in MYSCREW.DWG.
Furthermore, if you later open MYSCREW.DWG and change its
Text layer from red to green, the Myscrew|Text layer reflects this change
when you open MYWIDGET.DWG
(i.e., the layer Myscrew|Text layer is green).
Note that all of this
VISRETAIN discussion applies only to how the layers appear in the parent
drawing. No matter what you set VISRETAIN to in any drawing, the child
drawing's layers don't change when you change xref layer settings in a
parent drawing. In other words, changes to the Myscrew|Text layer in MYWIDGET.DWG
don't affect the Text layer in MYSCREW.DWG,
regardless of what VISRETAIN is set to.
There are ferocious arguments about whether to turn VISRETAIN on or off.
I prefer when possible to turn off VISRETAIN and thereby control the visibility
and properties of xrefed layers from the child drawings. But you may want
to or need to set VISRETAIN to 1 and control xref layer visibility and
properties from the parent drawings (in order to be able to assign different
properties to the same layer in different parent drawings, for example).
The most important thing is that you choose one method and stick with
it. If you aren't consistent, you can't predict whether changing a layer
in a parent or child drawing will persist.
In order to change the VISRETAIN setting in AutoCAD or LT 2000, open the
Options dialog box (Tools|Options), and choose the Open and Save tab,
as shown in the figure below. Click the Retain Changes to Xref Layers
check box to turn VISRETAIN off or on. In LT 98 or AutoCAD Release 14,
you must type the system variable name at the Command line. Type VISRETAIN,
press ENTER, type 0 (off) or 1 (on), and press Enter again. Whether you
change the system variable from the Command line or the Options dialog
box, you're changing it only in the current drawing file. The setting
is stored in the DWG file, so that different drawings can have different
VISRETAIN settings shown in figure 2.
Figure 2.
If all of these abstract file and layer names have your head spinning,
the best thing to do is to test both settings:
1. Create a sample parent and child drawings (e.g., PARENT.DWG
and CHILD.DWG).
Draw some objects on different layers in CHILD.DWG.
2. Open PARENT.DWG
and xref attach CHILD.DWG
into PARENT.DWG.
3. Set VISRETAIN to 0.
4. Use the Layer Properties Manager dialog box to change settings for
some of the xref layers in PARENT.DWG.
5. Save and reopen PARENT.DWG.
Note that the layer changes have disappeared.
6. Open Child drawing, make some changes to the layer settings, and save
the drawing.
7. Reopen PARENT.DWG.
Note that the layer changes from CHILD.DWG
appear in PARENT.DWG.
8. Repeat steps 3 through 7, but set VISRETAIN to 1 instead. Note that
in step 5 the layer changes now persist and in step 7 the layer changes
from CHILD.DW
do not appear inGPARENT.DWG.
A tutorial on my Web site demonstrates these concepts.
Xrefs: layers, loading, and logs
Page 1: Xref layer visibility
Page 2: Demand loading
Page 3: Xref messages and the xref log file
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