Image comparison

Any image type can be opened here. Three ways to look: side by side, an overlay with a slider for the right image, and a difference image. Zoom and pan are shared. Fit, 1:1 (one image pixel per screen pixel) and pinch work on both panes at once.

Local only. Pixels are compared on this Mac. Images are not uploaded. Why that matters

Side by side view of two beach illustrations. Orange rectangles mark a cloud, a sail, sunglasses, a shell, a ball and other objects that moved or changed. Region 1 of 7 is in the toolbar. The footer reports 45109 pixels differ by more than 8, in 7 regions. — Side by side, regions on. The header shows size, format, colour space and whether the file has alpha.

When pixels are compared

Only when both images have the same width and height. If they do not, the window says so, and only side by side is available, because positions do not correspond. Nothing is resized or aligned to force a fit.

Both images are converted to 8-bit sRGB with premultiplied alpha, then compared channel by channel, alpha included. Colours outside sRGB are clamped. Orientation metadata is applied. Only the first frame of an animation is used. A fully transparent pixel equals any other fully transparent pixel.

Above 25 million pixels: shown, not compared.
Above 120 million pixels: not opened.

Exact or tolerance

These start from Viewers → Images. You can also switch in the window.

Mode	Default	Behaviour
Exact pixels	On	Any channel difference counts. Pixel-identical is stated only when every channel of every pixel matched. Tolerance and smallest-region are not applied.
Tolerance and regions	Off	Differences up to a limit are ignored. The result always says it is not an exact match, because those small differences were not counted.

The difference measure is either the largest channel difference or a weighted perceptual value. Tolerance is 0–255 (default 8). Changed pixels within the merge distance (default 6 px) form one region. Regions smaller than the minimum (default 4 px) are dropped in tolerance mode. Up to 5,000 regions are listed, top to bottom, left to right, and Previous / Next Difference walks them.

Difference image

Changed pixels are red. Small ignored differences are yellow. Everything else is a faded copy of the left image.

Difference view of the same beach pair. A cloud, a sail, sunglasses, a shell, a ball and a missing starfish are painted solid red on a pale copy of the left drawing, each boxed in orange. — Difference view. Red is “these pixels changed”. The orange boxes are the same regions as in side by side.

Overlay

Overlay selected, the right image partly transparent over the left, slider visible, one obvious mismatch (the cloud or the sail) showing through. Include the zoom percentage and Fit / 1:1.

Hover

The pointer position is shown in the header. When the images are the same size, you get both colours at that pixel.

Sometimes comparing document content isn't enough — visual differences matter too. DiffMan allows you to compare documents as images, helping you detect changes in layout, formatting, fonts, and other visual elements that text comparison might miss. You can view pages side by side, use a transparency overlay, or highlight pixel-level differences to spot even subtle changes. This is especially useful for reviewing contracts, reports, and other documents where visual consistency matters.

Image comparison stays available after the three-month introduction. It is not gated by the document licence.