Here are some tips for choosing File Linking options and avoiding common pitfalls.
Face normals can be a source of confusion when linking to AutoCAD, AutoCAD Architecture, or Revit drawing files. In Autodesk VIZ, 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, Autodesk VIZ 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, Autodesk VIZ 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 Autodesk VIZ.
If this happens, try one of these four options:
During file link or import of the DWG file, turn on the Unify Normals switch in either the Basic panel of the File Link Settings dialog or the Geometry Options group of the AutoCAD DWG/DXF Import Options dialog.
If the drawing is already linked or imported, assign a Normal modifier to the object that is not displaying properly. Turn on the Unify Normals switch to force all the normals to face the same direction. If the object then appears to be “inside-out”, also turn on the Flip Normals switch.
Turn on the Force 2-Sided switch in the Render Scene dialog (to render the faces correctly), and turn on Force 2-Sided in the Viewport Configuration dialog (to display the faces correctly in the viewports.)
Note: Using the Force 2–Sided options can result in slower performance, particularly when rendering. Using either of the Unify Normals options is the preferred method of handling face normals.
If you are experiencing a high volume of face normal problems in a particular file, you should verify that the Weld switch is turned on in the File Link Settings dialog, and then reload the drawing. Weld forces nearby faces to share edges and vertices. This can still result in groups of face normals that are flipped in Autodesk VIZ so the Unify Normals switch should also be used.
Note: The disadvantage of welding is that it can be time-consuming when you attach and reload the linked file. The time penalty is much greater when Autodesk VIZ is creating objects that have very large numbers of individual faces.
Two-dimensional elements in drawing files, such as lines, polylines, circles, and arcs, are represented as splines in Autodesk VIZ. 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 Autodesk VIZ scene;
by freezing their layers in AutoCAD, AutoCAD Architecture, or Revit before you start Autodesk VIZ and before each subsequent reloading process.
by excluding specific layers during the File Link Attach/Reload process or Import process so you do not have to freeze layers in the drawing. This is the preferred workflow.
If you need this type of 2D geometry in your visualization, try to use polylines instead of connected lines to get cleaner geometry in Autodesk VIZ and to reduce the final size of your scene.
3D Solids objects in a drawing file will be tessellated (that is, turned into mesh objects with faces), when you link them into an Autodesk VIZ scene. The fineness of the tessellation is controlled by the Surface Deviation For 3D Solids setting in the File Link Settings dialog. A high value results in coarser tessellation. Autodesk VIZ uses less memory in the scene, but poor approximations of curved surfaces could result. For acceptable performance, keep this value as high as you can.
Tip: You can change the value of the Surface Deviation For 3D Solids control at any time by turning on Show Reload Options on the Files panel of the File Link Manager dialog, and then adjusting when you reload the file.
Splines are not rendered in Autodesk VIZ unless they have rendering parameters applied to them. 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.
A linked AutoCAD or AutoCAD Architecture drawing can include xrefs that reference files but use the same block names. Autodesk VIZ 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. Autodesk VIZ resolves xrefs until it detects a circular reference. For example, if you have the circular reference A|B|C|A, Autodesk VIZ detects and breaks the circularity between C and A. This is consistent with the way AutoCAD or AutoCAD Architecture handles circular xref dependencies.
Autodesk VIZ treats overlay xrefs in the same way as AutoCAD when resolving xrefs.
For more information regarding overlay xrefs, refer to your AutoCAD User Reference.
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.
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 the software.
Actively linked objects should not be attached to editable objects, as this introduces instability to the software. Instead, make a copy of the actively linked object, and attach the clone to the editable object.
Creating parent-child links between actively linked objects AND Autodesk VIZ objects can cause unpredictable results. For this reason, Autodesk VIZ does not allow you to link an actively linked object to a Autodesk VIZ object. However, you can link a Autodesk VIZ object to an actively linked object.
The existing hierarchies of linked objects cannot be broken in Autodesk VIZ. 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 Autodesk VIZ.