External references

An external reference (xref) is another DWG or DXF drawing displayed inside the current one without copying it: an architectural plan under the building services, a survey base, a project's common title block. If someone changes the referenced drawing, the change shows when it is reloaded. A PDF underlay is the same with a page of a PDF: a plan received as a PDF that serves as a background to draw on.

Attaching a reference

Enter XATTACH or click Attach DWG… in the XREF palette.
Choose the file and specify the insertion point, the scale and the rotation.

With the Overlay option the reference is an overlay: it is shown in this drawing, but not in the drawings that attach this one (useful to avoid carrying bases from one drawing to another). The reference's layers appear as reference|layer: they can be turned off, frozen or recolored like any other, and with VISRETAIN = 1 those changes are kept when it is reloaded. The reference content is shown somewhat faded (XFADECTL) to tell it apart from your own objects.

The references of a reference (nested ones) are also loaded and drawn; nested overlays are not.

The External References palette

XREF opens the External References palette, with the tree of DWG references (and their nested ones), the PDF underlays and the raster images of the drawing. For each reference it shows its status, type, size, date, saved path and the file that was found, and it has buttons to:

Button	What it does
Reload	Reads it again from disk
Unload	Stops showing it without removing it from the drawing
Detach	Removes it from the drawing with all its inserts
Bind	Turns it into a normal block of the drawing (BINDTYPE)
Change Path…	Chooses another file or fixes its path

The same is done on the command line with -XREF, which accepts wildcards in the names. If a reference's file changes while the drawing is open (for example, because someone else saved it), the status bar warns about it with a button to reload it.

Where files are looked for

As in AutoCAD, for each reference the program tries, in this order:

the saved path, if it is absolute;
the saved path, relative to the drawing's folder;
just the file name, in the drawing's folder.

That is why, to take a project to another computer, it is enough to copy the drawing and its references together. How the path of new ones is saved (full, relative or no path) is decided by REFPATHTYPE. For security, network paths (\\server\...) are not read, and nothing from a referenced file is ever run: only its geometry is read.

Clipping

XCLIP shows only part of a reference or a block, inside or outside a rectangular, polygonal or polyline boundary. XCLIPFRAME decides whether the boundary is shown.

PDF underlays

PDFATTACH attaches a page of a PDF; at scale 1 it measures the same as on paper.
PDFCLIP clips the underlay, PDFADJUST changes its fade or contrast or turns it monochrome, and PDFLAYERS turns the PDF layers off or on.
LinceCAD turns the page into an image by itself, without external programs. That is why object snaps do not work on what is drawn in the PDF, and with a lot of zoom the pixels show.

Not available yet

Editing references in place (REFEDIT); the Block Editor does not open external references.
XCLIP clip depth (3D).
DWF and DGN underlays.

Alternatives

To bring the content of another drawing in permanently, insert it as a block with INSERT or -INSERT, or bind it with -XREF: its objects become part of the current drawing.
To copy only layers, styles or blocks from another drawing, use ADCENTER.