MultiPLine (MPL) — polyline-based MLINE replacement with per-line layers, presets, and auto-sync [Free + Pro]

[SkillAmplifier]

Hi all,

 

I've been a lurker on CADTutor for years and learned 

a lot from this community, so I wanted to share 

something I built and get feedback from people who 

actually know LISP.

 

Background: I'm a civil CAD drafter working on US 

land development projects — grading, drainage, 

easements, corridors. MLINE has always been the 

tool I wanted to use for drawing parallel offset 

lines but couldn't, mainly because the objects are 

locked, there's no lineweight control per component, 

and the .mln style management across a team is a 

nightmare. So I wrote a replacement.

 

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WHAT IT DOES

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Command MPL works like PLINE — you draw a centerline 

and it generates a set of configurable parallel 

LWPOLYLINEs around it. Every satellite line is a 

real polyline: fully grip-editable, trimmable, 

offsettable, joinable. No locked geometry, no 

exploding required.

 

Each satellite line carries its own:

- Offset distance (positive = left, negative = right)

- Layer (Pro version)

- Color, linetype, lineweight, linetype scale

 

Configuration is handled through a DCL dialog 

(MPLEDIT). Settings save as user defaults and, 

in the Pro version, as named presets stored in 

%APPDATA%\MplineAuto\presets.dat.

 

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TWO VERSIONS

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Lite (free .lsp):

- MPL, MPLEDIT, MPLSYNC

- Manual sync after edits — run MPLSYNC, 

  window-select the group, done

- Groups are tracked via XDATA (app tag: MPLINE_PIPE)

 

Pro (compiled .vlx, $29.49):

- Everything in Lite plus:

- Command reactor for auto-sync — watches masters 

  before/after every command, diffs vertex lists, 

  rebuilds only what changed

- Named preset library with Save/Update/Delete 

  from the dialog

- Per-satellite layer assignment

- MPLADD — promote existing LWPOLYLINEs or LINEs 

  into MPL groups

- MPLON / MPLOFF — toggle reactor at runtime

- XDATA app tag: MPLINE_AUTO

 

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IMPLEMENTATION NOTES

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The interesting part was the Pro reactor. I went 

through a few approaches before landing on a global 

command reactor that snapshots the master cache 

before a command fires and diffs it after. This 

avoids object reactors entirely (which caused IDispatch 

arity issues in testing) and means the reactor 

doesn't interfere with Express Tools or COPY/MIRROR 

operations on non-MPL geometry.

 

Groups copied with COPY or MIRROR produce new masters 

on the next MPLSYNC call — the reactor doesn't 

auto-register copies, which is intentional to avoid 

unexpected geometry multiplication.

 

Happy to discuss the approach if anyone has thoughts 

or has solved similar problems differently.

 

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LINKS

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Video walkthrough: https://youtu.be/DXpyy1JWtXs

 

Free Lite download: 

https://skillamplifier.gumroad.com/l/hgpujs

 

Pro listing: 

https://skillamplifier.gumroad.com/l/nsidsv

 

Blog post with full documentation and use cases:

https://skillamplifier.wordpress.com/2026/05/16/multipline/

 

Full disclosure: I'm the developer. Posting here 

because I'd genuinely value feedback from experienced 

LISP users, and the Lite version is free so there's 

no risk trying it.

 

Thanks for any thoughts.

 

Zlatislav