Three ways to compare
The slider lays the second image over the first and lets you reveal it from left to right. It is the clearest way to judge a retouch, a color correction, a compression level or an upscaled photo.
Side by side shows both images next to each other, each in its original proportions, useful when you want to see the whole of each version. Difference compares every pixel and paints what changed in yellow to red on a pale gray copy of the first image.
Numbers for design reviews and QA
Under the images you get the share of changed pixels, their exact count, the largest difference on a scale of 0 to 100, and the area that contains all changes, given in pixels. A dashed frame marks that area on the map. These numbers make visual regression checks of screenshots, banners, exports and app screens quick to report.
Threshold ignores small differences. At 0 every tiny change counts, including JPG compression noise; around 5 to 15 hides that noise and keeps real changes such as moved elements, new text or another color.
Images of different sizes
If the second image has another size, it is scaled to the size of the first so the pixels line up. A note under the results tells you when this happened, and also when the proportions differ and the second image had to be stretched. For exact pixel checks, compare files with the same dimensions.
Swap changes which image is first. The difference map can be downloaded as a PNG to attach to a bug report or a design review.