[pdftex] Howto activate overprint (/knockout)

[JEBs]

>J> I'm developing a PDF document which shall be printed by a professional
>J> printing company.
>
>J> The whole document has a light-yellow background so that I need to 
>J> activate (most times, but not always) the "black overprint" option as
>J> well known/available in PS or PDF.
>
>J> First I though it should be definable on color level but while searching
>J> the whole web (and this ML) I couldn't find anything related which 
>J> helped me except PDF native codes (/op /OP /OPM ...).
>
>J> Now I'm sure that I'm on the right way but guess that I do something
>J> stupidly wrong.
>
>J> The way I tried to get it working is:
>
>J> ----- snip ----- snip ----- snip ----- snip
>J> ...
>J> \pagecolor....
>
>J> \pdfobj{<<
>J> /op true
>J> /OPM 1
> >>>}
>J> \edef\opObj{\the\pdflastobj}
>J> ...
>J> ...
>J> \color{black}
>J> {\huge{\textbf{\pdfrefobj\opObj Test 5 6 7}}}
>J> ...
>J> ----- snip ----- snip ----- snip ----- snip
>
>J> but it don't work. If I look into the PDF I can see the created object
>J> as i.e. "8 0 obj ..." but it's never used ("8 0 R").
>J> However I figured a lot with all the stuff like "\pdfliteral{<< /Type
>J> /ExtGState /op true /OPM 1 >>}" too, but never got a working result .
>
>
>J> Does anybody has an idea or knows a way ?
>
>J> Thanks in regards
>J>     JEBs
>J> _______________________________________________
>J> pdftex mailing list
>J> pdftex at tug.org
>J> http://tug.org/mailman/listinfo/pdftex
>
>I am aware and have actually made the same thing work with the perl
>module pdf::api2
>There it is possible to set objects to overprint (I used it for color
>seperation).
>Search on CPAN and you'll find it. Be aware that though the library is
>very powerful the documentation is rather poor since it is a "real
>time project" which is used and developed on the demands of a real
>project and therefore documentation is last.
>  
>
Yes, I'm aware that it will work and I know that it could be done with 
the PDF internal "/op", "/OP" and "/OPM" parameters.
But my problem is how to integrate them in pdflatex.
I figured a lot with \pdfobj, \pdfrefobj and \pdfliteral but couldn't 
get a working solution.
I get the object in but don't get them used/referenced nor working.
I'm sure that I'm doing something stupidly wrong but don't know what :/.
In addition I couldn't find any real useful docs/samples about the use 
of the extended \pdf... commands.

Any more suggestions ?

Cya
    JEBs