A PDF file may contain different types of objects, such as text, line
art and images. Some object types have an overprint behavior which depends on the
overprint mode, others don’t. We can distinguish the following:
| Overprint-mode-dependent object
types |
Overprint-mode-independent object
types |
| CMYK text |
CMYK images |
| CMYK line art |
CMYK shadings |
| CMYK image masks |
|
In other words, overprint modes only have an effect on CMYK text, line
art and image masks.
There are two “overprint modes”:
- Standard overprint mode, also
known as “OPM 0”
- Illustrator overprint mode,
also known as “OPM 1” or “nonzero overprint
mode”
The difference between both modes lies in the effect of the tint value
0 for one of the CMYK inks:
- In standard overprint mode (OPM 0), the tint value 0 of one of the CMYK inks
in the foreground object has a knockout effect on the color rendering of the
underlying object. In other words, a foreground ink with a C, M, Y or K value of
0 % erases the corresponding background ink. Here, the “foreground inks win” principle applies.
- In Illustrator overprint mode
(OPM 1), the tint value 0 is neutral: it is
ignored (as if it were “not specified”) and has therefore no effect on the color
rendering of the underlying object.