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Question on OPM as 0

Sep 25, 2013 9:18 PM

Tags: #knockout #opm

Hi here


I went through PDF 1.7 reference and Pitstop Pro to understand  the difference between OPM 0 and OPM 1.

I have clearly understanding on none-zero overprint mode now.


However,I got questions on OPM 0.

1)If the default OPM from Illustrator is OPM 1,and there is also "knockout" definition when the overprint checkbox is not selected ,how could I generate OPM 0 definition from Illustrator or other creative application? So far I could just change the definition in PitStop Pro.


2)What's the result difference between the definition of OPM 0 and knockout?

Looks like they are  the same, when viewing the different objects as OPM 0 and knockout in Acrobat Pro and also in creative application,with overprint viewing is selected.


Many thanks


  • Currently Being Moderated
    Sep 25, 2013 11:32 PM   in reply to kevinyuan

    - OPM always only can impact DeviceCMYK vector, text or imagemask objects

    - it controls whethere these objects migth overprint Cyan, Magenta, Yellow or Black

    - overprint will only happen, if one or more colorant values for the DeviceCMYK object are 0.0

    - it will only kick in if op or/and OP are already set to true


    this implies the following:

    - if op/OP are false, it does not mater what value OPM has

    - if op/OP are true but OPM is zero, a DeviceCMYK text, vector or imagemask object will always knock out Cyan, Magenta, Yellow and Black, even if some or all of the colorant values for the DecviceCMYK object are 0.0




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  • Currently Being Moderated
    Sep 27, 2013 12:42 AM   in reply to kevinyuan

    OPM only has an effect if op/OP is alerady true. Once op/OP are false it does nto matter what value OPM has. SO for an application like Illustrator thereis no need to set OPM to zero if op/OP are false.


    Why would you need OPM equal zero?



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  • Currently Being Moderated
    Sep 27, 2013 12:46 AM   in reply to kevinyuan

    Illustrator only works in the OPM 1 world. It has done since long before PDF was invented. It is rare for an application to give control over OPM, generally it works in only one overprint model.


    There are three cases to think about with overprint.


    1. Overprint off. This is often called knockout. OPM is ignored.


    2. Overprint on, OPM 0.


    3. Overprint on, OPM 1.


    Be sure you understand this and what effect they will have.

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  • Currently Being Moderated
    Sep 27, 2013 2:47 AM   in reply to kevinyuan

    Not at all. OPM has meaning. It just has no meaning when overprint is off.


    "Overprint off" is the same thing as "knockout".


    OPM neither turns overprinting on, nor turns it off. That is a separate setting. You still do not seem to be working with the THREE cases I described. Look closely to see which one(s) are the ones you are testing. If you read my cases carefully you will see that So if OPM 0 has the same behavior,why does it still call overprint instead of knockout? is not right.

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