If you've ever needed to onion skin comparison, you've probably tried at least three apps that ask you to sign up, upload your file to a server you don't trust, or pay a monthly fee. There's a simpler answer. OptiPix.art Image Comparison lets you compare two images with a slider, side-by-side, diff, and onion skin with pixel-level statistics for visual regression testing — and every byte of your image stays in your own browser tab.
This guide walks through exactly how to onion skin comparison in under a minute, what to watch out for, and which other free tools on OptiPix can speed up the rest of your workflow. It's written for QA engineers, designers, photographers, and developers running visual regression tests, but anyone can follow along.
Why use a browser-based tool for onion skin comparison?
Cloud apps for onion skin comparison all share the same problem: your file leaves your computer. That means it travels across the internet, gets buffered on a server you've never audited, and then sits in someone else's logs for at least a few minutes. For sensitive material — screenshots with passwords, personal photos, work-in-progress designs — that's a real risk. Even when the company is reputable, the file is still copied somewhere outside your control.
OptiPix's Image Comparison is different: it walks both pixel buffers in parallel and highlights per-channel differences above a configurable threshold. There is no upload step at all. You drop a file into the browser, the work happens in JavaScript and WebAssembly on your own CPU, and the result downloads back to disk. The OptiPix server only ships the HTML and JavaScript bundles — your image data never leaves the tab. That makes it safe for medical images, contracts, ID documents, NDA screenshots, and anything else you wouldn't email to a stranger.
The trade-off used to be that browser tools were slow or low-quality. That's no longer true. WebAssembly, modern Canvas APIs, and efficient JavaScript libraries make it possible to do real image processing in the browser at speeds that rival native apps for most everyday tasks — including diff, before/after, and regression test workflows.
Step-by-step: Onion Skin Image Comparison
Here's the exact process. It takes about 30 seconds end-to-end.
- Open the tool. Go to OptiPix Image Comparison. The whole tool loads in your browser the first time, then works offline on subsequent visits — bookmark it.
- Add your image. Drag and drop a JPEG, PNG, WebP, or HEIC file into the drop zone, or click to pick from your file system. You can also paste a screenshot directly with Ctrl+V (Cmd+V on Mac).
- Adjust the settings. Each control is documented inline. Sensible defaults work for most images, but you can fine-tune diff, before/after, and the other parameters listed in the side panel.
- Preview the result. OptiPix shows the output live as you change settings. Check the before/after view to make sure the result is exactly what you want.
- Download or copy. Click the download button to save the result locally, or use the copy-to-clipboard button (where available) to paste it directly into another app like Slack, email, or Notion.
That's it. No account, no email verification, no watermark, no rate limit, no upload progress bar. Just you, your file, and the result.
Tips for the best Onion Skin Comparison results
A few things matter more than people think when you onion skin comparison:
- Start with a clean source. If your input is already compressed, blurry, or rotated, the result inherits those flaws. When possible, work from the original file, not a screenshot of a screenshot.
- Use the right format. diff workflows generally prefer PNG for accuracy and JPEG or WebP for delivery. OptiPix lets you choose your output format on every tool.
- Mind the resolution. Bigger isn't always better — a 4000-pixel-wide source can be slower to process and harder to inspect than a 1500-pixel version. Resize first if you don't need full resolution. The Image Resizer handles this in two clicks.
- Iterate. The first run is usually 90% of the way there. Tweak one setting at a time until you're happy, instead of changing everything at once.
- Save your settings. Most OptiPix tools remember your last-used settings in localStorage so the next session starts where you left off.
If you regularly onion skin comparison, treat OptiPix like a workflow. Bookmark the tool, learn the keyboard shortcuts, and combine it with related tools below for batch jobs.
Pair it with these tools
Most onion skin comparison workflows don't end at one step. Here are the OptiPix tools that pair best with the Image Comparison:
- Image Compressor — the most common companion. Use it before or after the Image Comparison to clean up the input or polish the output.
- Format Converter — useful when you need to convert formats, resize, or reduce file size after you finish the main task.
- Exif Remover — covers an adjacent use case. If your file needs more than one transformation, this is usually the next step.
All three are free, all three run in your browser, and all three respect the same no-upload privacy guarantee. Together with the Image Comparison they cover most everyday image workflows without ever opening Photoshop.
Try the Image Comparison free at OptiPix.art — your files never leave your device. Open OptiPix Image Comparison now, drop your image, and you'll have your result before a cloud app would have finished uploading.