white on black = ?

hAVE you ever gotten a job back from your printer and notice something missing? Say a line of white text? You're sure that the text is in the file you sent to your printer. So where did it go?

Overprinting vs Knockout

The two terms refer to a property of color in vector graphic objects, and are mutually exclusive. The difference between overprinting and knockout is akin to photoshop's multiply and normal blend mode, respectively.

When an object is set to overprint, it will let all other colors underneath it come through. When an object is set to knockout, the object will ignore the color information of everything underneath it. A concrete example may help here.


Here we have two lines of text in illustrator, set over a solid field of gray.


The fill property of both lines of text have been set to overprint, found in the attribute palette.


Now turn on overprint preview...


...and see how different things look.

So what exactly happened here?

The second, latte-colored line of text is made of 10% Cyan, 20% Magenta, and 30% Yellow. Because it is set to overprint, the 50% Black field underneath it comes through. As a result its color composition actually becomes 10% Cyan, 20% Magenta, 30% Yellow and 50% Black. That's why it now looks darker.

The first line of white text is made of 0% of all colors. As a result, when it is set to overprint, the text becomes 50% Black - the exact same color as the color surrounding it. That's why it looks as if it's disappeared.

If the lines of text were set to knockout instead, the printing will look like the first screen cap, with none of the gray underneath coming through.

Practical considerations

• If you are looking at your work in illustrator or acrobat, turn on overprint preview. This is how it will look once printed
• White fill and stroke should never overprint. You might as well set the color to 'none' in that case
  
• Die-lines should always overprint. This is because the die-line will never print on the final piece. But if it's set to knockout, it will knock out all the color underneath it. So what the printer gets is white lines printing where the die-lines would be, which is just as bad as the die-lines actually printing (and that's bad).