TutorialApril 8, 20265 min read

Image Comparison Tool - Find Every Pixel Difference Online

Designers, developers, and QA engineers all run into the same question dozens of times a week: what changed between these two images? Maybe it's a before/after of a layout fix, a screenshot diff for a regression bug, two scans of a contract, or two versions of a brand asset. Eyeballing it never works - the human eye is great at spotting big changes and terrible at spotting small ones. OptiPix's free Image Comparison Tool gives you five different visualization modes plus pixel-level statistics, all in your browser.

What makes OptiPix's Image Comparison different

There are a few visual diff tools out there, but most are either built for a single workflow (e.g. screenshot regression in CI) or they upload your images to compute the diff. OptiPix is general-purpose, browser-based, and gives you every comparison view in one place.

Five comparison modes: in a single tool: slider, side-by-side, overlay (with opacity), pixel diff, and onion skin.

Real pixel-level diff: - magenta highlights for changed pixels, dimmed grayscale for unchanged.

Difference statistics: - exact count of changed pixels, percentage changed, average diff, max diff, and "% identical."

Adjustable threshold: - ignore tiny anti-aliasing differences and only highlight meaningful changes.

Manual alignment offset: - slide one image relative to the other to fix small registration mismatches between scans.

100% client-side: - both images load into local canvases and never leave your browser.

Key features

Slider mode: - drag a vertical line to wipe between images A and B

Side-by-side mode: - see both at the same scale

Overlay mode: with an opacity slider - fade between A and B

Diff mode: - pixel-level highlight with adjustable threshold

Onion skin: - A at 50% over B for ghostly comparisons

Pixel statistics: - changed count, %, average diff, max diff, % identical

X/Y offset sliders: for manual alignment of slightly-misaligned images

Download the diff visualization as PNG

How to use it

1. Open the Image Comparison tool.

2. Drop Image A on the left and Image B on the right.

3. Switch between Slider, Side-by-side, Overlay, Diff, and Onion skin to pick the view that surfaces what changed.

4. In Diff mode, raise the threshold to ignore noise and pick the magenta-highlighted regions. Download the diff as PNG to attach to a bug report.

Who is it for

QA engineers: comparing screenshot baselines to verify a UI fix

Designers: showing clients exactly what changed between two revisions

Photographers: reviewing edits versus the original RAW

Forensics and document analysts: verifying that two scans of a contract match

If the two images are different sizes or formats, run them through our Image Resizer and Format Converter first to normalize them. And once you've identified the regions that changed, the Image Annotator is the perfect next step for circling them and writing notes for your team.

Try the Image Comparison tool free at OptiPix.art - your images never leave your browser.

Try Image Compressor free - your files never leave your device

100% private, offline, no signup - try OptiPix now.

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