PDF revision guide
Visual PDF comparison: alignment and tolerances
Raster comparison is useful only when its sensitivity is visible. Fonts, antialiasing and tiny placement shifts can create pixels that changed without representing a meaningful content revision.
Start with bounded alignment
Search only a small translation window and record the selected offset. Unlimited alignment can hide a real layout change or match unrelated content.
Separate pixel delta from coverage
The pixel threshold decides whether one sample differs; the coverage threshold decides whether the total difference rises above the recorded noise floor.
Ignore margins deliberately
Headers, crop marks or scanner edges may justify a small ignored margin. A large hidden margin can conceal a real change, so write the value into the receipt.
Keep regions inspectable
Cluster changed pixels into bounded regions and let a person view them on the page. One percentage without locations is difficult to audit.