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Compare two images

Find what changed between two pictures with a slider, a difference map or a fade, all on this device.

Compares two images pixel by pixel with a slider, a difference map or a fade, and shows the percentage of pixels that differ, saving the map as a PNG. It is free, needs no account and adds no watermark, and your files are not uploaded: everything runs in your browser on your device. The size limit is your device’s memory, and phones hold less; give the page a short while after one visit to save itself for offline use, and then it works offline.

Drop two images to compareor click to choose files. Batches welcome.
    ADrop image A
    BDrop image B
    Working…

    Done

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    Your files are processed on this device and never sent to a server. Reload the page to clear everything from memory.

    How it works

    1. Drop two images, or choose one for A and one for B.
    2. Pick Slider, Difference or Fade, and set the sensitivity threshold.
    3. Read the percentage of pixels that differ, and save the difference map as a PNG if you want it.

    In your browser, not on a server

    Your browser decodes both pictures (libheif or the Squoosh decoders step in for HEIC, AVIF and JPEG XL) and draws them onto canvases of the same size, shrunk in halving steps when they pass 4,096 pixels. A short loop then compares every pixel’s red, green, blue and alpha values and flags those that are further apart than your threshold. The slider and fade views are two canvas draws; the difference map is saved as PNG.

    Compared with upload-based tools: Online image comparison sites upload both pictures to a server and some cap the size. Here neither picture leaves your device, and the threshold and difference map are free.

    Verify it yourself. Give the page a short while after one visit to save itself for offline use, then turn off Wi-Fi or switch to airplane mode and run the tool again. Or open your browser's developer tools (F12, or Cmd-Option-I on a Mac), pick the Network tab, and watch it while you convert. The panel above does the same count for you.

    Questions

    What does it compare?

    Pixels, not meaning. Each pixel in A is checked against the pixel in the same place in B. A photo shifted by a few pixels, or the same scene taken twice, shows as mostly different even though a person would call them alike. It cannot tell whether two photos show the same scene.

    Why do two JPEGs of the same picture show small differences?

    JPEG changes colours slightly every time it is saved, which leaves faint noise around edges. Raise the sensitivity threshold until the noise disappears and only real changes are left. At 0 any change counts.

    What happens when the images are different sizes?

    With “Fit B to A’s size” on, B is scaled to A’s width and height before comparing, and a warning appears when the two have different aspect ratios because B is then stretched. Turn it off to compare at B’s own size, with anything outside it counted as different.

    Is there a size limit?

    Very large pictures are compared at no more than 4,096 pixels on the long side, and the page says when that happened, so a 24 megapixel pair does not freeze a phone. The difference map you save is that size too.

    Are my images uploaded?

    No. Both images are decoded and compared on a canvas in your browser tab, and the difference map is saved from there. Your files are not uploaded, and it works offline after one visit and a short wait.

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