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are thawed, the program reads in that geometry from the reference drawing
as required.
When demand loading is turned on, the program places a lock on all reference
drawings so that it can read in any geometry it needs to on demand. Other
users can open those reference drawings, but they cannot save changes to
them. If you want other users to be able to modify an xref that is being demand
loaded into another drawing, use demand loading with the Copy option.
If you turn on demand loading with the Enable with Copy option, the program
makes a temporary copy of the referenced drawing and demand loads the
temporary file. You can then demand load the xref while allowing the original
reference drawing to be available for modification. When you turn off demand
loading, the program reads in the entire reference drawing regardless of layer
visibility or clip instances.
Layer and spatial indexes were added in AutoCAD Release 14 and AutoCAD
LT 97. If you externally reference a drawing saved in a release previous to this,
you do not see the same performance benefit as drawings saved with the
indexes. For maximum performance, use demand loading with referenced
drawings saved with layer and spatial indexes turned on in AutoCAD Release
14, AutoCAD LT 97, or more recent versions.
Work with Layer and Spatial Indexes
To receive the maximum benefit of demand loading, it is recommended that
you save any drawings that are used as xrefs with layer and spatial indexes.
A layer index is a list showing which objects are on which layers. This list is
used when the program is referencing the drawing in conjunction with demand
loading to determine which objects need to be read in and displayed. Objects
on frozen layers in a referenced drawing are not read in if the referenced
drawing has a layer index and is being demand loaded.
The spatial index organizes objects based on their location in 3D space. This
organization is used to efficiently determine which objects need to be read in
when the drawing is being demand loaded and clipped as an xref. If demand
loading is turned on, and the drawing is attached as an xref and clipped, the
program uses the spatial index in the externally referenced drawing to
determine which objects lie within the clip boundary. The program then reads
only those objects into the current session.
Spatial and layer indexes are best used in drawings that will be used as xrefs
in other drawings where demand loading is enabled. Drawings that are not
Reference Other Drawing Files | 479
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