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DiffCheck.lsp — Auto-Mark Revision Clouds for Design Changes

Stop circling changes by hand. Select old and new, get revision clouds instantly.

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What it does

Every design revision requires marking what changed with revision clouds — for review, for building permits, for your own sanity. Everyone does this by hand. DiffCheck does it in seconds.

• Select Region A (old) and Region B (new) in the same DWG
• Automatically aligns the two regions using Spatial Anchor Voting
• Compares every object and finds what's different
• Draws red revision clouds around changes — ready for submission
• Nearby changes are merged into clean grouped clouds
• O(N log N) performance — 1400+ objects in seconds

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The Problem

Design revision submitted
→ Circle all changes with revision clouds
→ Did I get them all? Did I miss that wall I moved?
→ Reviewer finds unmarked changes
→ Rejected, redo

Manual cloud marking is tedious, error-prone, and happens every single revision cycle. AutoCAD's built-in DWG Compare only works between two separate files and outputs color overlays — not revision clouds you can submit.

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The Solution

DiffCheck compares two regions inside the same DWG and outputs real revision clouds on a dedicated layer. Toggle the layer, print, submit.

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Installation

1. Download DiffCheck.lsp
2. In AutoCAD, type APPLOAD
3. Select and load the file
4. Commands DFC, DFCC, DFCT are now available

Tip: Add it to AutoCAD's Startup Suite for automatic loading every session.

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How to Use

Step 1 — Run the command

Type DFC:

1. Window-select Region A — the previous version
2. Window-select Region B — the revised version
3. Done — red revision clouds appear around every change

Select Region A (old):
  238 objects selected
Select Region B (new):
  238 objects selected
  Auto-align votes: 31
  Generating Signatures & Sorting (Ultra Fast)...
  Matching & Grouping...
  ── Results ──
  Matched (Unchanged): 220
  Changes detected:    18
  All differences marked on Region B (DIFF_CLOUD layer).
  Time: 1.23s

Step 2 — Review and submit

The clouds are on the DIFF_CLOUD layer. Toggle visibility, adjust if needed, print.

Step 3 — Clean up

Type DFCC to erase all revision clouds when done.

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Commands

Command	Description
DFC	Run comparison, generate revision clouds
DFCC	Clear all revision clouds
DFCT	Adjust merge distance, padding, and arc size

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Supported Entity Types

Type	Signature Method
LINE	Normalized endpoints
CIRCLE	Center + radius
ARC	Center + radius + angles
LWPOLYLINE	Vertices + bulges + closed flag
TEXT / MTEXT	Insertion point + height + content
INSERT (Block)	Name + insertion point + scale + rotation + attribute tags & values
DIMENSION	Type + measurement value + display text

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How It Works

1. Select Region A (old) and Region B (new)

2. Spatial Anchor Offset
   Extract feature points from LINE, CIRCLE, INSERT, TEXT
   → Consensus Voting to find the best displacement vector

3. Signature Generation
   Each entity → deterministic string
   based on type + geometry (rounded to tolerance)
   Blocks include attribute tag=value pairs

4. Sorted Merge  O(N log N)
   Sort both lists → single-pass linear scan

5. Box Merging
   Nearby diff bounding boxes → merged groups

6. Revision Clouds
   One cloud per group on DIFF_CLOUD layer

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Configuration

Adjust with DFCT or modify at the top of the file:

Variable	Default	Description
*dc:tol*	2.0	Coordinate rounding tolerance (drawing units)
*dc:pad*	20.0	Padding around each bounding box
*dc:arc*	30.0	Arc segment length for revision clouds
*dc:merge*	50.0	Max gap for merging nearby difference boxes
*dc:maxbox*	0.4	Giant box filter — ignores objects larger than 40% of region

Tips:

• Too many false positives? Increase *dc:tol* (try 5.0 or 10.0)
• Clouds too large? Decrease *dc:merge*
• Auto-alignment failed? The tool prompts you to click two matching reference points manually

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Notes

Item	Details
Hatch (fill patterns)	Skipped — seed points are unstable across edits
LEADER / MLEADER	Skipped in current version
Auto-alignment failed	Prompts for manual 2-point alignment
2D only	Z coordinates are not compared
Block attributes	✅ Compared — tag names and values are included in signature

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Compatibility

AutoCAD Version	Status
2014 and above	✅ Supported
Below 2014	Not tested

Also works with BricsCAD, GstarCAD, and other AutoLISP-compatible CAD platforms.

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Troubleshooting

Too many clouds / false positives? Increase tolerance with DFCT or set *dc:tol* to 5.0. Dimension text micro-shifts are the most common cause.

Offset looks wrong? If auto-alignment gets less than 3 votes, you'll be prompted to click two matching reference points. Pick a column center or wall corner that exists in both regions.

Nothing happened after running? Check that both regions contain supported entity types. Objects on locked or frozen layers may not be selected.

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Version History

Version	Notes
v21.1	Added block attribute deep comparison (tag names & values)
v21	O(N log N) sorted merge, spatial anchor voting, localized box merging, giant element filter, manual alignment fallback

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Support This Project

If DiffCheck saved you from hand-circling revision clouds, consider buying me a coffee ☕

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License

MIT License — Free to use, modify, and distribute.

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Made with ❤️ for architects who are tired of circling clouds by hand.