Tuesday, 26 July 2011

Revit - 2D vs. 3D AutoCAD Backgrounds

Did you know there are two very different types of CAD backgrounds?
  • Flattened “2D” backgrounds that are overlaid onto a view
  • Model element “3D” backgrounds that have a physical location within the model
The easiest and preferred method to link in a dwg is as a 2D background.

How can you tell them apart?
Select a CAD background.. if you are able to specify foreground / background from the options bar, then it is a 2D background.

When would you want to use a 3D background?
3D doesn’t necessarily mean a dwg will be given depth; what it really means is that the dwg will be created as a model element
  • 3D backgrounds will be visible in all views where they are within the view range. Therefore if you link a CAD plan in, all floor plan views will automatically show the background!
  • If you want to see the dwg in a 3D Isometric or Perspective view
  • If you want to see dwg elements that were modeled in 3D
  • Because 3D backgrounds have a physical location, they can be obscured by other elements such as walls, ceilings or finish floors
  • A 3D background is not flattened, so any linework that has a height (Z value) other than 0” may show in unexpected places or not at all.
  • A 3D dwg linked into a floor plan will not show on an RCP even if you “copy/paste” it into the view UNLESS you give it a positive offset (such as 9’-0”) to bring it into the RCP view range
  • The seemingly random, bulgy, coloured linework in an elevation is usually a 3D dwg being seen from the side. These can be easily hidden by turning off “Imported Categories” through VG.
Setting an Offset
CAD Obscured Hidden Line by Ceiling
Same CAD visible in wireframe

When would you want to use a 2D background?
In most cases a 2D background is the easiest to work with because they cannot be obscured by other elements
  • A specific background (such as furniture) that does not need to automatically show in all views
  • RCP backgrounds
  • Elevations, Sections or other views where there is no real workplane for the dwg to be placed on
  • Sending the dwg link to foreground will show the CAD on top of all model elements. Sending the dwg link to background will place it behind model elements.
  • 2D views WILL NOT show up in all views automatically, however they can be copy/pasted into other views (the link will still update)

So how do you choose to link a dwg as 3D or 2D background?
Actually it doesn’t specifically say annotation or model, 2D or 3D… the option is found in the dialog box when you are first linking the background as “Current view only”
  • Checked will create a 2D background
  • Unchecked will create a 3D background


Yogi said...

Hi David ...I always follow your informative blogs just wanted to also include something in this context. When we bring in CAD dwgs using Current View checked/unchecked it allows a user to control assigning the DWG to a workset or not. Now I always tell users is that just to have a good control over these temporary CAD links insert using the unchecked option. So you can turn ON/OFF that individual workset.



Brandon Crane said...

I like to use 3d links as backgrounds on multistory buildings; the link can be set to a level & offset which is used to easily control which views show the link.

However for this to work, your CAD file must be flattened before linking into Revit; otherwise the link may show up on multiple levels (stacked views).

To flatten the CAD file, open it in autoCAD and use the 'flatten' function.