How to Read a PDF Redline Overlay

A DrawDiff redline overlay uses three colours to show every difference between two PDF revisions. This guide explains what each colour means, how to interpret complex results, and how to use the additional tools to investigate specific changes.

The Three Colours

Green — Original Only

Green pixels appear where content exists in the original drawing but not in the revised drawing. This means the content was removed or deleted in the revision. On an engineering drawing, a green line or dimension means that feature was present in the previous revision and is no longer there.

Red — Revised Only

Red pixels appear where content exists in the revised drawing but not in the original. This means the content was added or is new in the revision. A red dimension, note, or symbol is something that wasn't on the previous drawing.

Cyan — Unchanged

Cyan pixels appear where content is identical in both drawings — present in both the original and the revision at the same position. Most of a typical drawing revision will appear cyan, with changes highlighted in green and red.

Reading a Changed Value

When a dimension or annotation value changes — say, a toleranced dimension changes from 25.0±0.5 to 25.0±0.2 — the redline shows both the old and new text overlaid:

The old value (25.0±0.5) appears in green — it was in the original
The new value (25.0±0.2) appears in red — it's in the revision
The parts that didn't change (the "25.0±" prefix) appear in cyan

This pattern — green and red in the same area with cyan nearby — is the signature of a changed value. Zoom in to read both the old and new values.

Tip: Ctrl+drag over any region to open the Region Inspector, which shows the overlay, original, and revised side by side at full resolution. Useful for reading changed dimension values clearly.

Common Patterns and What They Mean

A line or symbol in green only, nothing red nearby

A feature was removed from the drawing in the revision. Could be a deleted dimension, removed weld symbol, erased note, or a line that was cleaned up.

A line or symbol in red only, nothing green nearby

A new feature was added in the revision. Could be a new callout, added dimension, new detail view, or additional annotation.

Green and red in the same area, slightly offset

Something moved slightly between revisions. A line, dimension, or text block was repositioned. The green shows where it was, red shows where it is now.

Large areas of green and red with very little cyan

The view position or scale changed between revisions, or the drawings are not aligned. Use the Alignment Tool to pick reference points and rerun the comparison aligned to those points.

Faint speckled noise across the whole drawing

Rendering artefacts from PDF compression or anti-aliasing. Increase the Diff Threshold slider in Compare Settings to filter out sub-pixel differences below the threshold. A value of 30–60 typically eliminates rendering noise without missing real changes.

Using Find Differences for a Systematic Review

For a complete, systematic review of all changes, click ◎ Find Differences in the toolbar after running the compare. DrawDiff clusters all changed pixels into numbered regions and lists them in a panel on the right. You can:

Jump to any numbered region by clicking it in the list
Toggle the numbered boxes on and off for a cleaner view
Export a JPG with the numbered overlay baked in — useful as an attachment to a change log or review record

The numbered list gives you a structured way to work through every change systematically, rather than hunting across the drawing visually.

Customising the Colours

The default green/red/cyan colour scheme can be customised in the Colour Settings section of the sidebar. If your drawing has a coloured background or you prefer different colours for accessibility reasons, you can set any RGB value for each of the three layers. Changes take effect when you re-run the compare.

The Difference Between DrawDiff Redlines and Manual Redlines

A traditional manual redline is an engineer or designer marking up a drawing — circling changes, annotating what's different, using red pen (hence the name). This is time-consuming and depends on the reviewer noticing every change.

A DrawDiff redline is computed — the software finds every pixel that differs between the two PDFs, with no chance of a change being overlooked. It's not a replacement for human judgement in reviewing whether a change is correct or appropriate, but it ensures that nothing is missed in the detection of what changed.

Generate your own redline — free, no account, no upload.

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