Overprint stroke in InDesign is a specialized printing setting that dictates how the outline (stroke) of an object interacts with underlying colors during the print production process. When enabled, instead of the default behavior where the top color cuts out or "knocks out" the colors beneath it, the overprint stroke will print directly on top of the underlying inks. This results in the colors blending together, rather than the top color appearing in isolation over a void.
Understanding Overprint Stroke
By default, InDesign (and most design software) is set to knockout. This means that when one colored object is placed over another, the shape of the top object acts like a cookie-cutter, removing the portion of the underlying color that it covers. The top object then prints into this "knocked out" area, ensuring its color remains pure and unaffected by what's beneath it.
However, when you activate the overprint stroke setting for an object, this default behavior is overridden. The ink color of the stroke will print directly on top of any colors already present on the page. Consequently, you will observe a blending of the stroke's color with the background color, creating a new, combined color effect. This is distinctly different from the pure, unblended state you would achieve with knockout printing.
Why Use Overprint Stroke?
Overprint stroke is a powerful tool primarily used in professional print production for specific technical and aesthetic reasons:
- Trapping: This is the most common and critical use. Trapping is a technique used to compensate for potential minor misalignments (misregistration) of printing plates on press. By overprinting a very thin stroke of one color on top of another, you can ensure that tiny gaps (often appearing as white lines) don't show up between abutting colors. For instance, a small overprint stroke can "spread" one color slightly into an adjacent one, preventing a visible gap if the plates aren't perfectly aligned.
- Creating Rich Black: When you want a dark, deep black stroke to truly stand out and cover everything underneath without creating knockout issues, especially when the rich black is composed of CMYK inks, overprinting it ensures maximum coverage and depth.
- Special Effects & Varnishes: Overprinting can be used for creative effects, such as applying a spot color or a clear varnish as a stroke. This allows the varnish or spot color to add an effect to the underlying design without obscuring or altering the appearance of the original artwork.
- Registration Marks and Dielines: In some workflows, registration marks, crop marks, or dielines (for custom cuts) are set to overprint so they appear on top of all colors without affecting the final design.
How to Apply Overprint Stroke in InDesign
Applying an overprint stroke in InDesign is straightforward:
- Select the Object: Use the Selection tool to select the text frame, shape, or graphic whose stroke you want to overprint.
- Open the Attributes Panel: Go to Window > Output > Attributes.
- Check Overprint Stroke: In the Attributes panel, locate and check the "Overprint Stroke" checkbox. If you also want the fill of the object to overprint, you would check "Overprint Fill" as well.
Importance of Overprint Preview
It is crucial to use Overprint Preview in InDesign before sending files to print. Since overprinting changes how colors interact, simply looking at your document in normal view will not accurately show the final printed result.
- To enable Overprint Preview, go to View > Overprint Preview.
When Overprint Preview is active, InDesign simulates how overprinting will look when the document is printed, allowing you to catch any unintended blending or issues before they become costly printing errors. Always verify your overprint settings with this view enabled.
Overprint vs. Knockout: A Comparison
Understanding the fundamental difference between overprint and knockout is essential for accurate print production:
| Feature | Knockout (Default) | Overprint (Stroke) |
|---|---|---|
| Interaction | The top color "cuts out" the underlying colors. | The top color prints directly on top of underlying colors. |
| Result | Only the top color is visible in its pure form. | The top color blends with the bottom color(s), creating a new hue. |
| Transparency | Creates an opaque layer that obscures what's beneath. | Creates a translucent effect where colors combine. |
| Primary Use | Standard printing, ensuring color accuracy. | Trapping, rich black, special effects, utility marks. |
Proper use of overprint stroke ensures predictable and professional results, preventing common printing problems like white gaps or unexpected color shifts.
