Stacking two drawing revisions in Photoshop or GIMP and setting the top layer to Difference does show change: identical pixels turn black and anything that moved or changed lights up. But the result does not tell you whether a line was added or removed, it falls apart if the sheets are even slightly misaligned, and it works one sheet at a time. Drawing Revision Delta aligns each pair automatically, shows the earlier revision in red and the later in blue, and does it for a whole set on your PC.
This guide explains how the blend mode trick works, where it is good enough, and when a purpose built comparison saves real time.
Compare two whole drawing sets and see which sheets changed, on your own PC.
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How does Difference blend mode work?
Both programs document the mode plainly. Adobe’s Photoshop blending modes reference says Difference looks at the color information in each channel and subtracts either the blend color from the base color or the base color from the blend color, depending on which has the greater brightness value. Blending with white inverts the base color values, and blending with black produces no change.
The GIMP manual’s inversion modes page says Difference subtracts the pixel value of the upper layer from the lower layer and takes the absolute value of the result. It notes that the mode can be used to compare two layers: if they are identical the difference is zero, which shows as black. It also says the mode is commutative, so the order of the two layers does not matter.
For drawings, that means:
- Where both revisions have the same black line on the same white paper, the result is black.
- Where one revision has a line and the other has white paper, the result is bright.
- Everything unchanged disappears into black, so changes stand out.
How do you compare two drawing sheets this way?
The usual recipe, in either program:
- Export or rasterize both revisions of the sheet at the same resolution.
- Open the earlier revision, then paste or place the later revision as a new layer on top.
- Set the top layer’s blend mode to Difference.
- Nudge the top layer until the title block border and grid lines go black.
- Scan the sheet for bright marks.
On two clean plots from the same CAD file at the same scale, this can work well. It is free if you already own the software, and it shows small graphic changes that are easy to miss by eye.
Where does the trick break down on real drawing sets?
It cannot tell added from removed
Because Difference is an absolute value, a wall that was added and a wall that was removed look the same: a bright line. GIMP’s manual says the order of the layers does not matter, which is exactly the problem. You still have to flip between the two originals to learn which revision has the line.
Misalignment lights up everything
If one PDF was plotted with a slightly different margin, or a scan is a fraction of a degree off, every line shows a bright double edge. Nudging by hand fixes shift, but not rotation and scale together, and not the slight stretch that different plotters and scanners introduce. On a dense floor plan the result becomes noise.
It is one sheet at a time
A 60 sheet set means 60 rasterize, stack, nudge and inspect cycles, plus working out which sheet in the new set matches which in the old. New and deleted sheets are easy to miss.
Text changes are hard to read
A dimension that changed from 2x6 to 2x8 shows as a small bright smudge. You can see something changed but not what.
The title block always lights up
Dates and revision letters change on every reissue, so the title block is bright on every sheet and competes with real changes.
How does Drawing Revision Delta handle the same job?
Drawing Revision Delta starts from the two sets rather than two images. You pick a folder or files for the earlier revision and one for the later revision. PDFs, scanned TIFFs and other images are accepted, and multi-page files are split into sheets.
- Sheet matching. Sheets are matched across the two sets by the sheet number in the title block. When a sheet has no text layer, the app reads the title block with the text recognition built into Windows. You can correct any pair by hand.
- Alignment. Each pair is aligned for shift, rotation and scale, then corrected locally for the slight stretch that different plotters and scanners introduce.
- Ranking. The Pairs table shows how much of each sheet changed and sorts the most changed sheets first. Sheets that are new in the later set, or missing from it, are listed.
- Direction of change. The Overlay view shows red for content only in the earlier revision and blue for content only in the later one. Blink and Side by side views are there too, with revision clouds around each change.
- Words, not smudges. Where both PDFs have text, changed words are listed in plain terms, for example 2x6 to 2x8.
- Title block mask. A mask over the title block ignores date and revision letter edits.
See removed content in red and added content in blue, with every pair aligned for you.
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How do the two approaches compare?
| Task | Difference blend in Photoshop or GIMP | Drawing Revision Delta |
|---|---|---|
| Shows that pixels changed | Yes | Yes |
| Shows added vs removed | No, the result is an absolute difference | Yes, red for earlier only, blue for later only |
| Alignment | Manual nudging | Automatic shift, rotation, scale and local stretch |
| Whole sets | One sheet at a time | Folders or multi-page PDFs, matched by sheet number |
| New or deleted sheets | You find them yourself | Listed |
| Changed text | Visible as marks only | Listed as words where both PDFs have text |
| Title block noise | Always bright | Masked |
| Output | A layered image | Clouded PDF of the later set, PDF report, CSV and PNG |
When is the blend mode trick still the right tool?
Use Difference blend when:
- You are checking one or two sheets, both from the same CAD source at the same scale.
- You already have Photoshop or GIMP open and need a quick yes or no.
- You only need to know whether anything changed, not what or which direction.
Switch to a dedicated comparison when there are many sheets, when any of them are scans, when you need to tell added from removed, or when someone else needs a record of what changed.
What do you get to hand to the team?
Drawing Revision Delta exports the later revision’s sheets, unchanged, with a revision cloud around every change, saved as one PDF. The clouds are PDF markups, not drawn into the sheets. It also writes a PDF change report that opens with the changed sheet list, then one section per sheet, with a SHA-256 hash of every input file, plus a sheet summary and a list of every change as CSV files, and PNG snapshots.
That is the part a layered Photoshop file cannot give you: a document an estimator, superintendent or reviewer can open and act on. Files stay on your PC; the app has no account, no sign-in, no telemetry and no ads, and it reads only the files and folders you pick.
Frequently asked questions
Why is my Difference result full of double lines?
The two layers are not aligned exactly. Small differences in shift, rotation, scale or plotter stretch make every line appear twice.
Does it matter which layer is on top?
Not for the result. GIMP’s manual describes Difference as commutative, which is also why it cannot tell you which revision a line belongs to.
Can Drawing Revision Delta compare scans to CAD PDFs?
Yes. Scanned TIFFs and other images are accepted and aligned to the PDF. See our guide on comparing scanned drawings to CAD PDFs.
Will it flag every date change in the title block?
No. A mask over the title block ignores date and revision letter edits.
Try Drawing Revision Delta
Drawing Revision Delta is a one-time purchase with a free trial in the Microsoft Store; the listing shows the current price and system requirements, including Windows 11. Run it on the last two revisions you compared by hand and look at the Pairs table first. More detail is on the Drawing Revision Delta product page, and our guide on how to compare two drawing revisions covers the review process.
Replace sheet by sheet blend mode checks with one comparison of the whole set.
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Last updated 24 October 2026. Published by Automata Labs, a software company in Canada.