The simplest way to compare two photos is to just place them next to each other, and that works fine for a general impression. It falls apart the moment the comparison needs to be precise, because your eye has to jump back and forth between two separate positions on the screen, re-locating the same detail in both images every time. A slider avoids that entirely by keeping both images pinned to the exact same position on screen and only changing which one is visible on each side of the divider. The same pixel of the photo stays in the same place on your screen the whole time, so any change jumps out the instant you sweep across it, rather than requiring you to mentally align two separate frames.
That same pixel alignment is why this tool always treats one image as the reference frame and fits the second image into it, rather than just stacking two arbitrarily sized images and hoping they line up. If the two photos are not the same size to begin with, no comparison mode can make them perfectly aligned by magic, and the tool is upfront about that with a note whenever it detects a mismatch, rather than silently distorting one image to paper over the difference.
What the difference threshold is actually measuring
Difference mode compares each pixel's red, green and blue channels between the two images and takes whichever channel changed the most. If that change is at or above the threshold you set, the pixel gets painted in the highlight color. If it falls below, it stays as a dimmed grayscale version of the original, there for context but not treated as a meaningful change. A very low threshold catches nearly everything, including the kind of tiny pixel-level shifts that ordinary JPEG recompression introduces even when nothing about the image actually changed on purpose. Raising the threshold filters that noise out and leaves only the differences large enough to matter, which is why the default sits at a moderate value rather than the most sensitive setting available.
Why Difference mode resizes the second image differently than every other mode
Every other mode in this tool fits the second image inside the first image's frame without changing its proportions, which is the right call when the goal is a fair visual look at both photos. Difference mode has a stricter requirement, every pixel in image A needs a corresponding pixel in image B to compare against, which is only possible if both images share the exact same width and height. So for that mode specifically, the second image is stretched to match rather than letterboxed, and if the two images started at different aspect ratios that stretching itself will show up as a difference across the whole frame. For the cleanest possible difference result, starting from two images that are already the same dimensions, using the resize tool first if needed, gives a far more meaningful result.
