A visual overlay compares rendered pages
A visual overlay starts from how each PDF page looks after rendering. The two page images are combined so areas that match behave differently from areas whose pixels differ.
This approach is useful for layout changes, moved elements, altered graphics, signatures, stamps and other visible differences.
1into1 uses a difference compositing view
In overlay mode, 1into1 renders both PDF pages and combines them using the browser canvas difference compositing operation together with an adjustable blend level.
MDN describes difference compositing as subtracting the corresponding layer colors so changed areas become visually distinct. The result is a visual aid rather than a text-aware change report.
A text diff analyses document content differently
A semantic or text comparison tries to understand changed text, formatting or document structure rather than only comparing the rendered appearance.
Adobe Acrobat Pro, for example, can analyze two PDF versions and report categories of differences. That is a different capability from a browser-local visual overlay.
Overlay works best when pages align
Visual overlays are easiest to interpret when the two versions have similar page dimensions, page order and overall layout. If content reflows heavily, many pixels can change even when the underlying meaning changed only slightly.
For heavily restructured documents, side-by-side reading may be more useful than relying on the overlay alone.
Use both views for different questions
Use side by side when you want to read both versions in context. Use overlay when you want to spot visual movement or changed appearance on corresponding pages.
If you need an automated legal redline or a machine-generated list of wording changes, use a dedicated semantic comparison system rather than treating a visual overlay as equivalent.
Official sources reviewed
- MDN — globalCompositeOperationDefines the Canvas 2D difference compositing operation used for high-contrast visual blending.
- Adobe Acrobat — Compare two versions of a PDF fileExplains automated PDF difference analysis, comparison reports and side-by-side result review in Acrobat Pro.