This topic offers some tips for choosing File Linking options and avoiding common pitfalls.
Linked Data and Face-Normal Conventions
Face normals can be a source of confusion when linking to AutoCAD, AutoCAD Architecture, or Revit drawing files. In 3ds Max, every face has a front and a back, corresponding to the inside or outside surface of a solid object. In a cube, for example, there is seldom the need to view the inside surface of any of the six squares that make up the cube. So for many viewing and rendering operations, 3ds Max ignores a face if it's facing away (that is, if its face normal is directed away) from a point of view.
When you create objects in AutoCAD, AutoCAD Architecture, or Revit, 3ds Max generally understands which way faces should be oriented and manages face normals accordingly. However, occasionally you may encounter linked drawing geometry that displays correctly in AutoCAD, AutoCAD Architecture, or Revit, but doesn't strictly respect face-normal conventions. This can make it appear as though elements visible in the drawing file are missing or appear “inside-out” in 3ds Max.
If this happens, try one of these four options:
Force 2-Sided switch. Also, to display the faces correctly in the viewports, turn on Viewport Configuration dialog Rendering Method panel Force 2-Sided.
If you experience a high volume of face normal problems in a particular file, verify that the File Link Settings: DWG Files dialog Weld switch is on, and then reload the drawing. Weld forces nearby faces to share edges and vertices. Welding can still result
in groups of face normals that are flipped in 3ds Max, in which case, turn on Unify Normals as well.
Linking Files with High Numbers of Linear Line Segments
Two-dimensional elements in drawing files, such as lines, polylines, circles, and arcs, are represented as splines in 3ds Max. These splines carry much more information at each vertex than typical AutoCAD 2D structures. Since some drawing files contain large quantities of 2D data, exercise caution when linking files containing a high number of discrete line segments. There are two ways these elements can be left out of your 3ds Max scene:
If you need this type of 2D geometry in your visualization, try to use polylines instead of connected lines to get cleaner geometry in 3ds Max and to reduce the final size of your scene.
3D Solids objects that you link from a drawing file into 3ds Max are tessellated; that is, turned into mesh objects with faces. The fineness of the tessellation is controlled by the Maximum Surface Deviation For 3D Solids setting on the File Link Settings: DWG Files dialog. A high value results in coarser tessellation. 3ds Max uses less memory in the scene, but poor approximations of curved surfaces could result. For acceptable performance, keep this value as high as possible.
Splines are not rendered in 3ds Max unless they have rendering parameters applied. Normally, you have to collapse a shape to an editable spline object in order to apply rendering parameters; however, this is not possible with spline objects from AutoCAD.
Instead, you can apply a Renderable Spline modifier to the spline. This lets you set rendering properties without having to collapse to an editable spline.
External References and Block Names
A linked AutoCAD or AutoCAD Architecture drawing can include xrefs that reference files but use the same block names. 3ds Max keeps the blocks distinct by prepending xref names to block names.
In Revit, a DWG, DXF or RVT file can be linked to the project. This kind of link is called a RVT Link. When the project is exported to a DWG file, this type of link is represented in the exported drawing as an external referenced drawing. In this case, more than one drawing file may be created, with one referencing the other(s).
An xref file that contains a sequence of nested references that refers back to the xref file is considered a circular reference. 3ds Max resolves xrefs until it detects a circular reference. For example, if you have the circular reference A|B|C|A, 3ds Max detects and breaks the circularity between C and A. This is consistent with the way AutoCAD and AutoCAD Architecture handle circular xref dependencies.
3ds Max treats overlay xrefs in the same way as AutoCAD when resolving xrefs.
For more information regarding overlay xrefs, refer to your AutoCAD User Reference.
Cloning Actively Linked Objects
If you want to clone actively linked objects, you should only use the Copy option. Creating references or instances of actively linked objects is not recommended, as reliability issues can arise when the instanced or referenced object is deleted in the linked file.
When you copy actively linked objects, linked through the File Link Manager, the copies are automatically converted to editable mesh or editable spline objects. If your selection contains several objects which instance another object, the resulting copies also instance the same object.
Compound Objects Containing Actively Linked Objects
If you use an actively linked object as part of a compound object, you should always choose Copy when you specify how the linked object is transferred to the compound object. Choosing Reference or Instance can cause instability in 3ds Max.
Attaching Actively Linked Objects to an Editable Mesh, Poly, Patch, or Spline Object
Actively linked objects should not be attached to editable objects, as this introduces instability to 3ds Max. Instead, make a copy of the actively linked object, and attach the clone to the editable object.
Creating Hierarchies with Actively Linked Objects
Creating parent-child links between actively linked objects AND 3ds Max objects can cause unpredictable results. For this reason, 3ds Max does not allow you to link an actively linked object to a 3ds Max object. However, you can link a 3ds Max object to an actively linked object.
The existing hierarchies of linked objects cannot be broken in 3ds Max. This would compromise the structure of Blocks and Styles. Any changes must be made in the original DWG file.
Similarly, actively linked objects cannot be included in the creation of Group or Assembly objects in 3ds Max.
Actively linked objects should not be included in any IK animation chains, as they will cause unpredictable results. Likewise, animation controllers should not be applied to actively linked objects.
