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Stop circling changes by hand. Select old and new, get revision clouds instantly.
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
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.
DiffCheck compares two regions inside the same DWG and outputs real revision clouds on a dedicated layer. Toggle the layer, print, submit.
- Download
DiffCheck.lsp - In AutoCAD, type
APPLOAD - Select and load the file
- Commands
DFC,DFCC,DFCTare now available
Tip: Add it to AutoCAD's Startup Suite for automatic loading every session.
Type DFC:
- Window-select Region A — the previous version
- Window-select Region B — the revised version
- 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
The clouds are on the DIFF_CLOUD layer. Toggle visibility, adjust if needed, print.
Type DFCC to erase all revision clouds when done.
| Command | Description |
|---|---|
DFC | Run comparison, generate revision clouds |
DFCC | Clear all revision clouds |
DFCT | Adjust merge distance, padding, and arc size |
| 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 |
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
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
| 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 |
| AutoCAD Version | Status |
|---|---|
| 2014 and above | ✅ Supported |
| Below 2014 | Not tested |
Also works with BricsCAD, GstarCAD, and other AutoLISP-compatible CAD platforms.
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.
| 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 |
If DiffCheck saved you from hand-circling revision clouds, consider buying me a coffee ☕
MIT License — Free to use, modify, and distribute.
Made with ❤️ for architects who are tired of circling clouds by hand.