Why “quality 80” is the wrong control
Every compressor asks you to pick a quality number. That is a question about the encoder, not about your photo. Quality 80 is invisible on a busy portrait and produces obvious banding on a flat gradient — the same setting, wildly different results.
This tool asks the inverse question. It compresses, decodes its own output, measures how much the image actually changed, and searches for the lowest quality that still looks untouched. You get the smallest file that passes, and the score that proves it.
How the measurement works
The metric is SSIM, which compares local luminance, contrast and structure rather than raw pixel differences. It is measured at native resolution across tiles sampled from across the frame — downscaling first would average away the very artefacts we are looking for.
| Threshold | SSIM | What it means |
|---|---|---|
| Untouched | 0.999 | Effectively identical. Largest files. |
| No visible difference | 0.995 | Default. Nothing findable at 100% zoom. |
| Balanced | 0.985 | Artefacts findable if you go looking. |
| Aggressive | 0.97 | Visible on gradients. Fine for thumbnails. |
Click any result thumbnail to flip between the original and the compressed version.
Your photos carry more than you think
A phone photo records the camera, the timestamp, and very often the exact coordinates where it was taken. Re-encoding discards all of it, so compressing here strips that metadata whether you ask or not. When a photo contains GPS data, the result says so — because almost nobody knows it is there.
Limits
- PNG output is lossless, so quality search does not apply. For photographs, JPEG or WebP will be dramatically smaller.
- Compression uses the browser's built-in encoder. Dedicated encoders such as MozJPEG produce roughly 10–20% smaller files, at the cost of a multi-megabyte download.
- HEIC isn't supported. No browser decodes it. On iPhone: Settings → Camera → Formats → Most Compatible.
- Very large images can fail on iPhone. Mobile Safari caps canvas memory around 16 megapixels.