Different colors, common inks

An important thing to remember when talking about overprint is the concept of “common inks”, meaning inks which the background and the foreground objects have “in common”. We talk about inks, not colors, because the overprint and knockout concepts are relevant in print only, not when you view the PDF documents on screen.

The principle here is as follows: If there are common inks, the foreground inks win, even if their tint value is lower than the corresponding one of the background ink.

Suppose you have a CMYK object set to overprint on a spot color background. Their inks are as follows:

There are no common inks because cyan, magenta, yellow and black are not specified in the spot color background. And if colors are not specified in either one of the objects, they cannot be common. Note that they are not specified, which is not the same as 0 %. Consequently, the cyan foreground ink will be printed over the Red spot color background. The resulting color in the intersection will be as follows:

C: 100 % M: 0 % Y: 0 % K: 0 % Spot color: Red

Let’s convert the Red spot color of the background to CMYK with the following tint values:

The resulting color in the intersection will be as follows:

C: 100 % M: 0 % Y: 0 % K: 0 %

The cyan foreground object has been set to overprint, but still, the magenta and yellow in the background are knocked out. Confused? Remember the basic principle: If there are common inks, the foreground inks win, even if their tint value is lower than the corresponding one of the background ink and even if this tint value is equal to zero.

Let’s put the tint values of the above example in a table:

See what happened? All inks are now common, even if they have a tint value of 0 %. The foreground inks win and therefore, the 0 % magenta and yellow of the foreground object will be used, not the 100 % of the background.

• The devil in the overprint and knockout detail
• Common pitfalls of overprint behavior