Add an original and a new version of a picture, for example a photo before and after compression, retouching or resizing, and compare them three ways:
- Slider: the two pictures on top of each other with a divider you drag (or move with the arrow keys). Left of the divider is the original, right of it the new one.
- Side by side: both at once, the same part of each.
- Difference: black where the pixels are equal, brighter where they differ (amplified four times, so faint compression noise becomes visible).
Zoom to 2×, 4× or 8× and drag to move around: both pictures move together, so you always look at the same pixels. Next to the pictures the tool shows each file's size and dimensions, how much smaller the new one is, and measured differences: PSNR, SSIM, mean difference and the share of pixels that changed visibly. Export the comparison as one PNG (split at the divider, side by side or the difference map) for a review or a bug report. Both pictures stay on your device.
What the numbers mean
- PSNR (peak signal-to-noise ratio, in dB) measures the average error between the pictures: higher is closer. Identical pictures have infinite PSNR. Around 40 dB and above, differences are usually invisible at normal size; below about 30 dB they usually show.
- SSIM (structural similarity, 0 to 1) compares local patterns of brightness, contrast and structure in 8 × 8 windows, which follows what the eye notices better than PSNR. 1 means identical; above 0.98 is usually indistinguishable.
- Mean difference is the average change per colour channel on the 0 to 255 scale.
- Pixels visibly changed counts pixels where any channel moved by more than 8 levels.
The two can disagree, and that is informative. JPEG's chroma subsampling, which stores colour at half resolution, lowers PSNR a lot on saturated pictures while SSIM, computed on brightness, barely moves. The article on comparing compressed images has measured examples. The numbers are computed on a copy of at most 1,024 pixels on its longest side so they stay quick; the views and the export use the pictures at up to 4,096 pixels.
