<strong>Knockout model (default)</strong> — In the knockout model, every object being placed on the page removes any previously defined ink from that area on each separation. A black text character on a yellow background: in knockout mode, the yellow ink is removed from the area under the character, and only black ink is printed there. This ensures that ink mixing does not occur (the black character has known ink properties, not a mix of black + yellow).

<strong>Overprint model</strong> — With overprint enabled on the black text character, the yellow ink under the character is not knocked out. Black ink is added on top. On press, the black ink absorbs most light; the yellow shows through where the black ink is thinner (at edges, in small text). In practice, for dense black text, overprint is often used without visible difference — but for thin rules or small text, the ink interaction can be visible.

<strong>Why overprint matters for printing</strong>: Multi-colour offset printing involves separate printing plates, one per ink colour. In theory, plates align perfectly; in practice, there is a small registration tolerance (0.1–0.5mm typically, worse for newsprint). If a thin black rule on a yellow background is knocked out, a slight misregistration shows a white halo around the rule (where the yellow was knocked out but the black rule shifted). Overprinting the black rule prevents this: the yellow is always present, so no white halo appears even with misregistration.

<strong>PDF graphics state for overprint</strong> — Overprint is part of the graphics state, set via the <code>op</code> (overprint for stroking) and <code>OP</code> (overprint for filling) parameters in an ExtGState dictionary, applied with the <code>gs</code> operator: <code>&lt;&lt; /Type /ExtGState /op true /OP true &gt;&gt;</code>.

<strong>Rich black</strong> is a composed black made from multiple inks: typically C=60% M=40% Y=40% K=100% (or various other recipes). Rich black absorbs more light than 100% K alone, producing a denser, deeper black appearance — desirable for large black areas in high-quality print.

<strong>The overprint risk with rich black</strong>: If rich black text is placed over a coloured background and overprint is enabled, the CMYK components of the rich black overprint onto the background inks. For small text, the ink mixing can make the text appear fuzzy. The correct approach: rich black is appropriate for large fills; use 100% K black (with overprint enabled) for text to avoid this problem. InDesign and Acrobat provide overprint preview to visualise this before printing.

<strong>"Black overprint" setting</strong> — Design applications like InDesign have a preference for "Black Overprint" which automatically sets overprint mode for 100% K black objects. This is the industry standard behaviour: black text and thin black rules always overprint, preventing registration halos without ink mixing problems.

<strong>Registration colour</strong> — A special colour used for crop marks, registration marks, and other printer's marks: a Separation colour with /All as the colorant name (100% of every ink). Registration marks must be visible on every separation, so they cannot overprint (they are already on all separations). The PDF specification's /All Separation colour space is specifically designed for this.

<strong>Overprint preview</strong> — Acrobat's "Overprint Preview" (View > Preview > Overprint) renders the document with overprint simulation enabled. This is the only way to see how overprint will affect output on screen; without it, overprint objects render as if in knockout mode.

<strong>PDF/X and overprint</strong> — PDF/X (ISO 15930) does not restrict overprint settings — overprint is a legitimate print production feature. However, PDF/X preflight checks (in tools like callas pdfToolbox or Enfocus PitStop) typically flag overprint settings on white objects as potential errors: "white knockout" (a white object with overprint enabled) is a common production error — it produces a visible content gap when white ink does not exist on the press (most presses don't have white ink unless specifically added).

<strong>Common overprint errors</strong>: - <strong>White overprint</strong> — A white object with overprint enabled. Since white has 0% of all inks, nothing prints where the object is, but the background shows through. The white object appears invisible on press. - <strong>Unintended overprint on coloured objects</strong> — Some design applications propagate overprint settings from master pages or styles. A CMY coloured background with overprint enabled will add its inks to underlying content, producing unexpected colour shifts. - <strong>Overprint in spot colour varnish</strong> — This is intentional: a spot varnish uses /All or a custom colourspace with overprint enabled so the varnish layer deposits on top of all existing inks without knocking them out. See <a href="/glossary/pdf-spot-colour" target="_blank" rel="noopener">PDF Spot Colour</a> for varnish implementation.