Nelle specifiche PDF troviamo operatori di sovrastampa analoghi a quelli che troviamo in PostScript (vedi questa pagina).

PDF 1.2 ha un parametro OP per ogni oggetto. Se OP vale false, l'oggetto buca (knockout), se OP vale true l'oggetto sovrastampa (overprint). Da PDF 1.3 esiste anche il parametro op. Se op ha un valore (true, false) questo è il valore della sovrastampa della traccia e OP è il valore della sovrastampa del riempimento. Altrimenti vale OP sia per stampa che per riempimento. Per default questi parametri sono false.

PDF 1.3 introduce la modalità di overprint OPM per distinguere i due tipi di overprinting. Se OPM vale 0 si tratta di full overprint (default); se vale 1 è non zero overprint. In Acrobat 4 e successivi questa modalità viene chiamata Illustrator Overprint Mode. Se attivata OPM è posto uguale a 1, altrimenti vale 0.

Insomma sia in PostScript Level 3 che in PDF 1.3 ci sono tre possibili definizioni di sovrastampa:

• knockout, overprint off (bucatura);
• full overprint (sovrastampa);
• non zero overprint (sovrastampa non zero).

Aandi Iston di Quite ha raccolto le regole per la sovrastampa in Acrobat.

Here are the requirements for overprinting of process colours to work in this new world. Of course, it is more convenient if your RIP has a switch.

1. The PostScript file you Distill must actually ask for overprinting. This may seem obvious, but some releases of QuarkXPress [which ones] do not write overprint requests into composite PostScript, while they may do so into EPS (but distilling EPS has other issues).

2. PDF 1.3 format (or later) is required. There can be no Acrobat 3.0 compatibilily.

3. The Distiller option "Illustrator Overprint Mode" must be on. Less obviously, you must also ensure that no colour management applies to CMYK, since colour managed CMYK will not honour overprint. This requires that "Colour: unchanged" be selected in Distiller, and may require you to turn off colour management of CMYK in some other applications [details?]

So far, so good. If you get this far, you should have OPM=1 set in the file, and overprint requests marked.

4. You must output the PDF file with Acrobat 4.0 or above. You may also be able to use a RIP which consumes PDF directly, but in that case it depends on whether the RIP understands OPM=1.

In realtà deve essere Acrobat 4.0.5 perché Acrobat 4 ha un bug. That's the reason why companies like Lantana had to implement an "Black overprint" feature in Crackerjack which helped to circumvent this. Adobe fixed this bug with version 4.0.5.

5. You must output to a PostScript 3 RIP. Acrobat's technique for process-as-spot will not work on a level 2 RIP.

6. Acrobat must know to output PostScript 3. In Windows, this is a choice when generating PostScript. On the Macintosh, it can come from the PPD, so be sure the PPD is right.

There may be other conditions involved too. There is an "Overprint preview" in Acrobat 5.0; and there is an option to set colour management on output. And there is transparency, which throws all these concepts up in the air too. I haven't investigated any of these in detail. [Any suggestions?].

There is also the question of what happens to process WHITE (0,0,0,0) which is marked as overprint. By the rules of OPM=1, this must vanish (and it does). Quark may do something different.

%!
0 1 .5 0 setcmykcolor clippath fill
true setoverprint
1 0 0 0 setcmykcolor 100 100 200 200 rectfill
showpage

Distill using "Leave Color Unchanged" and "Illustrator Overprint Mode".

Open the PDF. Go to Save As, PostScript File. In Settings, leave PostScript at "Language Level 2", check "Apply Overprint Preview", and set Color Profile to "Same As Source (No Color Management)". (The color profile must be set this way so that the output is not color managed.)

Inspect the resulting PostScript and you will see that the rectangle is drawn as follows:

[/DeviceCMYK] cs 1 1 0.501961 0 sc

This is very close to what is wanted, which is "1 1 0.5 0 sc". The error resulting from Overprint Preview is pretty small. This works for level 1 as well.