Best AutoLISP to Draw a Centerline Between Two Lines (No X Error)

Program Description

The CenterlineDraw (CLD) AutoLISP routine is a fast, production-ready tool that automatically creates a precise centerline between two selected lines. It solves a common drafting issue: incorrect centerlines caused by reversed or inconsistent line directions.

Unlike basic scripts that can generate a crossed “X” result, this routine evaluates both possible endpoint pairings and selects the correct one using 2D distance comparison logic. The result is a clean, accurate centerline whether the source lines are parallel, angled, reversed, or imported from unreliable geometry.

Another practical advantage: this routine creates standard LINE entities, not associative objects. That means:

no dependency on newer AutoCAD object types
no unexpected behavior when modifying geometry
full compatibility with legacy workflows and downstream software

How to Use the CLD Command (Step-by-Step)

1. Save the File

Paste the AutoLISP code into a text editor and save it as:

CenterlineDraw.lsp

Make sure the extension is .lsp

or download the script here.

2. Load into AutoCAD

Type:

APPLOAD

Then press Enter.

Official reference: https://help.autodesk.com/view/ACD/2024/ENU/?guid=GUID-APPLOAD

3. Load the Script

Select CenterlineDraw.lsp and click Load

4. Run the Command

Type:

CLD

Press Enter.

5. Select Two Lines

Select the first line
Select the second line

If you press ESC, AutoCAD cancels cleanly (no script error).

6. Result

The routine:

reads endpoints using DXF group codes (10 / 11)
evaluates correct pairing using distance comparison
calculates midpoints using mapcar
draws a centerline on the current layer

Why This Routine Works Reliably in Real Drawings

Eliminates the “X” Centerline Error

Most scripts fail when lines are drawn in opposite directions. This routine compares both pairing configurations and selects the correct one using total distance:

Start–Start / End–End
Start–End / End–Start

This guarantees correct orientation.

Ignores Bad Z Values (Real-World Fix)

Imported drawings often contain incorrect elevations. This routine uses a 2D distance function, ignoring Z values during pairing.

This prevents:

twisted centerlines
incorrect midpoint selection
geometry distortion

Lightweight (No ActiveX / COM)

The routine uses:

entget
DXF codes
native AutoLISP functions

No vlax / COM calls.

Benefits:

faster execution
fewer compatibility issues
stable across AutoCAD versions

Reference: https://help.autodesk.com/view/ACD/2024/ENU/?guid=GUID-DXF

Clean Geometry Output (Key Advantage)

AutoCAD’s native CENTERLINE command (introduced in 2017) creates associative centerline objects.

This routine does not.

It creates a simple LINE entity, which means:

no associativity
no automatic updates
no hidden constraints

This is often preferred in:

fabrication drawings
exports to other CAD systems
legacy environments

Fast Midpoint Calculation

Midpoints are computed using mapcar, keeping the code:

simple
readable
efficient

Corrected AutoLISP Code (Stable Version)

This version avoids forced exits and lets AutoCAD handle cancellation cleanly.

“`lisp id=”c9d2k1″ (defun dist2d (a b) (distance (list (car a) (cadr a)) (list (car b) (cadr b))) )

(defun midpoint (a b) (mapcar ‘(lambda (x y) (/ (+ x y) 2.0)) a b) )

(defun c:CLD (/ ent1 ent2 data1 data2 pt1 pt2 pt3 pt4 dA dB midPt1 midPt2)

;; Select first line (while (not ent1) (setq ent1 (car (entsel “\nSelect first line (or press ESC to cancel): “))) (if (and ent1 (/= “LINE” (cdr (assoc 0 (entget ent1))))) (progn (princ “\nNot a line.”) (setq ent1 nil)) ) )

;; Select second line (while (not ent2) (setq ent2 (car (entsel “\nSelect second line (or press ESC to cancel): “))) (if (and ent2 (/= “LINE” (cdr (assoc 0 (entget ent2))))) (progn (princ “\nNot a line.”) (setq ent2 nil)) ) )

;; Cache data (setq data1 (entget ent1) data2 (entget ent2))

;; Extract points (setq pt1 (cdr (assoc 10 data1)) pt2 (cdr (assoc 11 data1)) pt3 (cdr (assoc 10 data2)) pt4 (cdr (assoc 11 data2)))

;; Compare pairing options (setq dA (+ (dist2d pt1 pt3) (dist2d pt2 pt4))) (setq dB (+ (dist2d pt1 pt4) (dist2d pt2 pt3)))

(if (< dA dB) (setq midPt1 (midpoint pt1 pt3) midPt2 (midpoint pt2 pt4)) (setq midPt1 (midpoint pt1 pt4) midPt2 (midpoint pt2 pt3)) )

;; Create centerline (entmake (list ‘(0 . “LINE”) (cons 8 (getvar “CLAYER”)) (cons 10 midPt1) (cons 11 midPt2) ) )

(princ “\nCenterline drawn.”) (princ) ) “`

Practical Use Cases

Mechanical drafting: shafts, symmetry axes
Architecture: wall centerlines, layout alignment
Civil drawings: offsets, alignments
Steel detailing: beam center tracking

Works especially well with:

imported DWG files
inconsistent geometry
reversed line directions

Limitations

Only supports LINE entities
Does not extend beyond endpoints
No automatic linetype assignment

These choices keep execution fast and predictable.

FAQ

Does it support polylines?

No. Only LINE objects are supported. You would need to extract segments or convert geometry.

Why not use AutoCAD CENTERLINE?

AutoCAD’s CENTERLINE command creates associative objects that:

update automatically
depend on source geometry

This routine:

creates a simple LINE
avoids associativity
works better for exports and legacy workflows

What happens if lines are not parallel?

The routine still works. It creates a centerline between corresponding midpoints, not a geometric bisector.

What if my drawing has wrong elevations?

No issue. The routine uses 2D distance checks, so Z values are ignored during pairing.

Why use entmake instead of LINE command?

Because entmake:

avoids OSNAP interference
runs faster
creates consistent geometry

Can this be extended?

Yes. Common upgrades:

polyline support
CENTER linetype assignment
automatic layer creation
centerline extension

This routine removes a repetitive manual step and prevents small geometric mistakes that typically go unnoticed until late in a project.

Best AutoLISP to Draw a Centerline Between Two Lines (Anti-X Error Script)