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Inspection
Compare two images
Drop in a before and an after. Every pixel that changed is highlighted, and you get a number for how much of the image moved.
Difference map
What tolerance is for
Two JPEGs of the same picture at different quality settings differ in almost every pixel by a small amount, so a comparison with zero tolerance lights up the whole frame and tells you nothing. Tolerance sets how far apart two pixels have to be before the difference counts. At around 8 you see real changes and ignore compression noise; at 0 you see everything including the noise.
Two practical uses. Checking whether a compression setting actually damaged anything visible: compress, compare, and look at where the differences cluster, which is usually edges and flat gradients. And design review, spotting the one padding change in a screenshot that nobody can find by eye.
If the two images are different sizes the smaller one sets the comparison area, since there is no meaningful way to compare pixels that do not correspond.
Common questions
It says 100% different but they look identical.
Almost always a size or alignment mismatch: a one pixel offset makes every pixel differ. Check the dimensions reported for each file.
Can I see them side by side with a slider?
Not here. The difference map answers the question a slider is usually being used to answer, and answers it with a number.
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