More on External References in R14

31 Jan, 1998 By: Lynn Allen

Do you use external references in your drawings? Have you shied away from them because they were restricting or difficult to manage? External References are favorites of AutoCAD power users around the world. Last month, we discussed the Xref Manager that was added to AutoCAD R14. This new visual dialog makes it much easier to manage your external references than in previous releases. This month, we'll delve a little deeper into other improved changes to the world of external references.

Prior to AutoCAD R14, many users complained about the inability to clip external references. AutoCAD competitors could boast of their strong clipping capabilities when going toe-to-toe with AutoCAD. With AutoCAD R14, full xref clipping capabilities were added to please users and silence the competition.

Who needs to clip external references? Anyone who wants only a portion of an externally referenced drawing to display. You might XREF in an entire hospital building but only want AutoCAD to display one wing in your current drawing. Not only do you want just the single wing to display, but you don't want to be slowed down by the overhead of the entire hospital drawing. When AutoCAD R14 is set up properly, you'll find that it grants both wishes.

The XCLIP Command to the Rescue
XCLIP can be found in the Modify pulldown menu under Object => Clip. It can also be found in the Reference toolbar (third button across).

The XCLIP command lets you visually clip the display of a selected external reference. It does not affect the original drawing in any way. You can clip to a predefined boundary, or you can define a boundary on the fly. You can also clip many external references at one time. At any given time, you can choose to turn the clipping off and redisplay the entire xref. There's much to the XCLIP command, so we'll break it up into easily digestible doses. To follow along, open a drawing containing external references or attach a couple of external references to a new drawing for practice.

After entering the XCLIP command, AutoCAD will ask you to select the external references you want to include in the clip. All of the standard object selection methods are acceptable.

Command: XCLIP
Select objects: Other Corner:
4 Found
Select objects: [Enter]
ON/OFF/Clipdepth/Delete/generate Polyline/<New boundary>:[Enter]
Specify clipping boundary:
Select polyline/Polygonal/

After selecting the external references, you'll need to indicate the next step to take. For our first example, we'll jump straight to selecting a new boundary (which is the default). Hit [Enter] to specify a clipping boundary.

Three different types of boundaries are accepted in the XCLIP command. You can use an existing polyline as a boundary or you can create a polygonal (many-sided) or rectangular boundary on the fly. We'll choose the default again and select the two opposing corners of a rectangle. After doing this, you should find that the only xref area displayed on the screen is the one that lies within the rectangular area.

You can also choose to use an existing polyline as a boundary. To do so, indicate with an S for Select polyline, then pick the desired polyline. AutoCAD will also permit you to use open polylines. It internally closes the polyline by connecting the starting point with the closing point to determine the final boundary. If connecting the starting point with the closing point causes the polyline to cross itself, an error message will display.

I was able to apply a fit curve and a spline to the polyline without any problems. I noticed the spline accomplished more pleasing final results. The Help menu indicated that only straight pline segments were acceptable, but I found I had no problem using a curve-fitted polyline.

I even drew a square with the POLYGON command and splined it to get an image that was pretty darn close to a circle. Using this image as a boundary, I was able to create a circular clipped xref.

The XCLIP command also lets you draw a polygonal area on the fly to indicate a boundary. I use this option the most because it makes it easy to isolate irregularly shaped areas.

For even more clipping boundary options, try the new Extended Clip (CLIPIT) function located in the Bonus tools. [Editor's note: For a description of all the bonus routines shipped with R14, see Ralph Grabowski's "The Release 14 Bonus CAD Tools," CADENCE, August, 1997, pp. 42-55.] Here, you'll find that you can also use Circles, Arcs, Ellipses and Text as boundary edges.

The XCLIP command has many different options buried within. Let's review the remaining options found at the first prompt.

  • On. When XCLIP is set to On, AutoCAD displays the clipped external references.
  • Off. When XCLIP is set to Off, clipping boundaries are ignored, and the external references are displayed in their entirety.
  • Clipdepth. This option is used to set the front and back clipping planes. It can get very confusing if you're not familiar with the process (DVIEW anyone?). You will be asked to specify the distance from the front clipping plane and the boundary, as well as the distance from the back clipping plane to the boundary. You can also delete clipping planes within this option.
  • Delete. This option is used to delete a clipping boundary you don't intend to use again. If you just want to temporarily display the entire external reference, be sure to use the Off option. You can't use the standard ERASE command to delete a clipping boundary.
  • Generate Polyline. This option is used to turn a clipping boundary into a polyline. It's useful if you need to modify the boundary after the fact. AutoCAD places the polyline on the current layer using the current linetype and color settings. If you use grips, or perhaps PEDIT, on the new polyline, you will need to redefine the new boundary to ensure that the changes take place. Note: You cannot have more then one clipping boundary per xref.

You'll also find a nice addition to the DDMODIFY command when you select an xref for editing. In the lower left-hand corner, a new "Show clipped xref" toggle appears that makes it easy to turn on and off the clipping boundary.

Getting Top Performance Out of Your Xrefs
Now that you're only using the portion of the xref you need, how do you make sure AutoCAD is only using the displayed sections versus the entire drawing when it's performing such activities as zooming, selecting and regenerating? To make sure you're getting optimum performance, let's return to the Performance tab of the Preferences dialog.

The lower left-hand corner of the Performance tab contains an option entitled "External reference file demand load." Setting this to "enabled" ensures you're getting top performance out of your xrefs. Let's review the three options:

  • Enabled. AutoCAD's performance is improved by turning on Demand Loading. It should be noted that other users can access, but not edit, the file while it is being referenced.
  • Enabled with Copy. AutoCAD uses a copy of the referenced drawing in your drawing and turns On demand loading (thus improving performance). Other users may access and edit the original drawing.
  • Disabled. This option turns off demand loading, and means that you are no longer improving AutoCAD performance by displaying only a portion of an externally referenced drawing. Others may access and edit the referenced file.

This setting is saved under the system variable xloadctl.

One More Step
One last step to ensure you're maximizing your productivity with xrefs lies within the Saveas dialog box. As you're saving your externally referenced drawings, make sure they are saved with an index type of "Layer & Spatial." Ouch, that sounds painful, doesn't it? Let's go through this process one at a time.

In the Saveas dialog box, select the Options button from the lower right-hand corner. This action will take you to the Export Options dialog box's Drawing Options tab. (You might wonder why they are called different things when there is only one tab?)

Notice the Index types dropdown list. By default, this type is set to Layer and Spatial, which is exactly the way you want to leave it. I'm only pointing this out in case someone modified your setup. Layer and Spatial indicates that AutoCAD will experience optimum performance because it will load only those layers that are on and thawed and within a clipped boundary area. That's what we want!

Should you find yourself as a contestant on AutoCAD Jeopardy, I'll cover the remaining options:

  • None. This option means that neither layer nor spatial indexes will be created when you save your drawings (translation: you will go very slowly).
  • Layer. AutoCAD will load only those layers that are on and thawed.
  • Spatial. AutoCAD will load only the portion of the xref within a clipped boundary.

Do yourself a favor, check this once and forget about it.

Before I go, I'd like to point out that visretain has a new home in the new Layer dialog box under Details.

Notice the toggle "Retain changes to xref-dependent layers." When this box is selected, AutoCAD will remember the state of xref-dependent layers from drawing session to drawing session (so you don't find yourself constantly resetting them).

External references are key to many drawing disciplines. I think you'll find the changes made to them in R14 are a definite giant step in the right direction.

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