PDF Overprint

What is Overprint?

In traditional printing, when two colored objects overlap, the press will knock out the color underneath. This means it doesn’t print any ink on the lower layer in that shape, printing only the top color on clean paper.

Overprint is a setting that instructs the press to print the top ink directly on top of the bottom ink, mixing the two colors.

Overprinting is commonly used to prevent white gaps along the borders of black text (using black overprint), but it can cause problems if applied to other colors.

The Print Risk: Disappearing White Text and Color Mixing

The most common overprint error is Overprinting White:

• Disappearing Elements: White ink does not exist in standard CMYK printing (the paper itself is white). When an element is colored white ($0%\text{ CMYK}$), the press expects it to “knock out” the color underneath so the white paper shows through.
• Invisible Prints: If a white logo or text is set to Overprint, the press prints the background color underneath it and then applies “nothing” ($0%$ white ink) on top of it. The white logo or text disappears entirely from the print.
• Color Shifts: If a yellow object is set to overprint a blue background, the yellow ink will mix with the blue ink, printing green instead.

How Filecheck Audits Overprint Flags

Overprint issues are hard to catch because they often look correct in standard PDF viewers, but fail during physical printing. Filecheck inspects the file’s graphics state dictionaries:

1. ExtGState Analysis: We check for stroking (OP) and non-stroking (op) overprint parameters.
2. White Overprint Check: If any path, vector shape, or text is set to white ($0%\text{ CMYK}$ or $0%\text{ Gray}$) and has the overprint flag enabled, Filecheck triggers an alert.
3. Prevention: This check prevents expensive reprint runs due to disappearing logos or text.