Image diff
Highlight the pixels that differ between two same-size images, on your device.
Runs on your device. The file is never uploaded.
Image diff compares two images pixel by pixel and writes image-diff.png: the first image, with every differing pixel repainted magenta, plus a count and a percentage. A pixel counts as different once the mean of its red, green and blue gaps passes 24. Two images of unequal size are refused, and the alpha channel is never compared.
Questions
What do I get back?
A PNG called image-diff.png showing the first image with every differing pixel painted in the highlight colour, plus a line of text giving the counts, such as "a.png vs b.png: 1284 of 480000 pixels differ (0.27%)". Pixels that match are copied from the first image unchanged, so you can see where the changes sit.
Why does it say the images differ in size?
Because it compares pixel by pixel at the same coordinates, which only means anything when the two pictures have identical dimensions. The refusal names both sizes and tells you to resize one to match before diffing. Use Resize image for that; a scaled comparison would report differences everywhere and be useless.
How many images do I drop, and does order matter?
Exactly two, and yes. Any other count stops with "image-diff needs exactly 2 images, got 3". The first file is the base that shows through in the output, and the second is compared against it. The difference count itself is the same either way round.
What does the threshold do?
Per-pixel difference threshold, default 24, is compared against the mean of the absolute red, green and blue differences for that pixel. Anything above it is highlighted. Raise it to ignore compression noise between two JPGs, lower it towards 0 to catch every changed pixel. The range is 0 to 255. Highlight colour defaults to #ff00ff and takes six hex digits.
Is this a perceptual diff?
No. It is a straight per-channel comparison with a flat threshold, with no perceptual colour model, no anti-aliasing allowance and no shift detection, so an image moved one pixel across lights up almost entirely. The alpha channel is not compared at all, only red, green and blue. Everything runs in a Web Worker on your device.