IntelliCAD and AutoCAD: Working Together

David Cohn

IntelliCAD, AutoCAD, differences, DWG, ACIS, unsupported entities, hatch patterns, True Type

[Ed: Since this article was first published, Autodesk has released AutoCAD 2000 with a DWG format that is not incompatible with previous versions of AutoCAD and IntelliCAD. Refer to the Visio website for updates on AutoCAD 2000 DWG compatibility.]

Although IntelliCAD 98 uses AutoCAD’s own DWG format as its own, and reads and writes AutoCAD drawing files in their native format without any loss of data, from AutoCAD Release 14 back to version 2.5, it’s not a 1-to-1 relationship. There are a few significant differences between AutoCAD, particularly Release 14, and IntelliCAD 98. If you regularly exchange drawings with AutoCAD users, understanding these differences can help you avoid potential problems.

A number of objects that may be present in an R14 drawing may not display at all in IntelliCAD. These objects include ACIS-based solid models, ARX proxy objects, and associative hatch patterns. Other objects, such as the text and shapes that make up AutoCAD’s complex linetypes, stacked fractions included as part of Mtext objects, and images included within blocks or external references also do not display, although the rest of the object, such as the dashed line components of complex linetypes, remain visible. In addition, IntelliCAD substitutes its own font display whenever it encounters a TrueType font in an AutoCAD drawing.

In all of these instances, no data is lost. The objects simply display differently, or not at all, when using IntelliCAD. If you save the drawing and re-open it in AutoCAD, all of the objects are once again visible. But, what happens if you modify these objects in IntelliCAD and then open the drawing in AutoCAD. In some circumstances, that’s where problems can arise. Let’s look at these differences in detail so we can avoid the problems.

Associative Hatch Patterns
The preview image displayed in IntelliCAD’s Open Drawing dialog box shows all these objects. The first indication you’ll have that something in the original AutoCAD drawing won’t display properly in IntelliCAD occurs as soon as you begin opening the drawing. When you open an AutoCAD drawing containing one of these objects, IntelliCAD displays a dialog box similar to the one shown below. Pay attention to this message so that you don’t inadvertently make destructive changes.

Upon loading a drawing, IntelliCAD displays a dialog box indicating that the AutoCAD drawing contains unsupported objects.

Hatch patterns are probably the first missing objects you’ll encounter. Associative hatch patterns—hatch patterns that automatically adjust when their boundaries are altered—were first introduced in AutoCAD Release 13. Since they’ve been around for several versions and are generally easier to use than the older non-associative hatch patterns, they are quite prevalent in AutoCAD drawings. These objects do not display at all, however, when viewed in IntelliCAD. Knowing that an area should be cross-hatched but not seeing the hatch pattern, users may have a tendency to recreate the hatch pattern in IntelliCAD. If you do so, however, when the drawing is once again opened in AutoCAD, the area will have two different hatch patterns applied, the associative hatch created in AutoCAD and a non-associative hatch pattern created in IntelliCAD. Of course, the new hatch pattern is easily erased.

Modifying the associative hatch boundary in IntelliCAD poses a different problem, but one which is also easily solved. Upon reopening the drawing in AutoCAD, the hatch pattern will no longer match its boundary. As soon as you modify either the boundary or the hatch pattern in AutoCAD, however, the hatch pattern readjusts so that it once again correctly fills the boundary.

The biggest problem occurs if you delete the hatch boundary in IntelliCAD. In that case, anything you do that modifies the hatch pattern will result in AutoCAD removing the associativity, since the boundary no longer exists. The hatch pattern reverts to the older, non-associative definition.

TrueType Fonts
While hatch patterns practically invite you to make changes, complex linetypes and TrueType fonts present few problems other than your having to deal with the visual differences. Complex linetypes simply display without their associated text or shapes, and can be modified without introducing problems when later opened in AutoCAD. If you length or trim a complex line, for example, the changes you make in IntelliCAD are carried back to AutoCAD, with the text and shapes once again appearing as part of the linetype.

Complex linetypes in AutoCAD can include text and shapes. But when viewed in IntelliCAD, complex linetypes lack their shape and text symbols.

Similarly, text created in AutoCAD using TrueType fonts simply appear in IntelliCAD as a simpler font based on the older SHX font definition files. You can edit the text as needed without altering the original style definition. When you later open the drawing in AutoCAD, the original style, including the TrueType font, is restored.

Although also not displayed, stacked fractions and some of the other formatting possible in AutoCAD pose no particular problem unless you modify the text in IntelliCAD. In that case, be aware that the text appearing in IntelliCAD’s Entity Properties dialog box contains formatting as visible text, in addition to the actual text strings. For example, the code \P appears at the ends of paragraphs and \S marks the start of stacked fractions. Be sure not to change these codes. You should also note that any new text you create using the TrueType font-based styles originally defined in AutoCAD will appear using the TrueType font when later displayed in AutoCAD.

TrueType fonts display as simpler SHX fonts in IntelliCAD. Mtext appears with formatting characters when viewed in IntelliCAD’ Edit Properties dialog box.

Invisible Objects
Objects that don’t appear at all—such as solids, ARX proxy objects (sometimes also referred to as zombies), and images—while disconcerting, pose no real problems. For example, if you load an AutoCAD drawing consisting entirely of solid objects and their dimensions, you’ll see the dimensions but the solids themselves will be completely invisible in IntelliCAD. Compare the appearance of the two drawings below. Similarly, ARX objects, even simple things such as Wipeout objects created with the R14-bonus tools, will not display in IntelliCAD. While the IntelliCAD rendition of these drawings will obviously be missing some of its components, since you can’t see them, you also can’t alter them in any way. Thus, you can’t make changes that would adversely affect the drawing when reopened in AutoCAD.

Solids created in AutoCAD as ACIS objects do not display at all in AutoCAD. But dimensions remain visible.

One other thing you may encounter in IntelliCAD are objects that display but can’t be modified. The most obvious such object is an MLine or multiline. While IntelliCAD 98 does not let you create one, this R14 object displays just fine in IntelliCAD. However, any attempt to alter an MLine results in IntelliCAD displaying the dialog box below.

Other than moving or deleting them, you can’t modify AutoCAD’s MLine objects in IntelliCAD, although they still display properly.

While few of the differences pose significant chances to introduce errors, being aware of the differences will help you avoid problems and understand what’s going on when drawings just don’t look quite right.

David Cohn

[Ed: Since this article was first published, Autodesk has released AutoCAD 2000 with a DWG format that is not incompatible with previous versions of AutoCAD and IntelliCAD. Refer to the Visio website for updates on AutoCAD 2000 DWG compatibility.]

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