Knockout/overprint configuration in PDF file

Question

I am confused about the concepts of overprinting and knocking out in the context of printing a PDF file. While I think that I understand the difference between these two concepts per se, I am not clear on whether the choice between overprinting and knocking out is a property that is set in the PDF file, be it for the PDF file as a whole or for each element in the document separately, or whether it is up to the printer to decide whether to overprint or to knock out and this choice does not concern the file itself.

The reason I ask is that I have a PDF file where the colors of the text look different than expected when opened in Acrobat Reader, both with “overprint preview” turned on and off, but when opened in Okular, they look fine with “overprint preview” turned off but almost like in Acrobat Reader with “overprint preview” turned on. In the program with which I created the source for the PDF file, there is a property that can be set to “overprint” or “knockout” for every element in the document individually, and for the text in question, it is set to “knockout”, so it seems at least somewhat logical that the text looks correct in Okular with “overprint preview” turned off.

But I don't understand whether this property is also present for the elements in the PDF file, meaning whether the PDF file itself contains information about whether particular elements should be overprinted or knocked out by elements that lie above them. From what I could find on the internet, it seems so. But then, why is there a setting in Acrobat Reader for switching overprint preview on and off, and what does this setting do? Does it cause everything to overprint, overriding the PDF file's configuration? And why does my PDF file look identical (and not as expected) in Acrobat Reader both with “overprint preview” turned on and off and similar in Okular with “overprint preview” turned on, but as expected in Okular with “overprint preview” turned off? Is this a bug in Acrobat Reader? Is the PDF file configured correctly?

I have attached the PDF file and also screenshots of how the PDF looks like in Acrobat Reader versus how I expect it to look, i. e. how it looks like in Okular with “overprint preview” turned off. Any help is appreciated.

AnandSri · Accepted Answer

Hello @AnandSri!

I am not sure, but I believe that the problem is actually entirely unrelated to overprinting. Instead, my current suspicion is that the color blend space for the text in question (the text has the color blend mode "exclusion" applied) is set to a CMYK color space in the PDF file, while in the original program, the blended colors were apparently calculated in RGB mode (regardless of whether the source colors themselves were RGB or CMYK – in my case, they were RGB, but I have tried it with CMYK and the result was the same). I have read that the result of a color blend can vary depending on the color blend space and that the difference is especially noticeable with the exclusion blend mode.

I cannot confirm that the PDF file is incorrectly configured in this way because I have not yet found out how to determine the color blend space of the text in the PDF file, but I have tried combining the text and the background color using the exclusion blend mode in a different program where the blending seems to have been performed in CMKY mode, and the resulting color was pretty much the same color that Acrobat Reader renders the text in my PDF file in.

Is my understanding correct that a PDF/X-4 document is permitted to contain transparent elements or elements with a color blend mode applied where the color blend space is an RGB space (or where even the source colors themselves are RGB colors), meaning that the blending operation is performed in RGB, and that only the color resulting from such a blend is intended to be converted to the document's destination color space (a CMYK space)?

Thank you for your help,

Max

Hello Max,

Thank you for sharing all the details.

Yes, your understanding is correct: PDF/X-4 allows blending in RGB, and only the flattened/printed result should be converted into CMYK. If your file looks wrong in Acrobat but correct in Okular (with overprint preview off), it’s Acrobat that is interpreting the blend mode using the document’s CMYK blend space instead of RGB. This is not a bug, but Acrobat is adhering strictly to the PDF standard for blending spaces.

Recommended Solution:

Check the Blend Color Space in the authoring app (Scribus, InDesign, etc.): If your design intent was RGB blending, set the transparency/blend space explicitly to RGB. Export as PDF/X-4 with “Preserve RGB Numbers” where possible.

Verify in Acrobat Pro Preflight: Open your PDF in Acrobat Pro → Print Production > Output Preview. Check transparency and blend spaces for the text objects.

If needed, force RGB blend space: In Scribus or your source tool, ensure the “Transparency Blend Space” is set to RGB before exporting. Or export to PDF/X-4 with explicit RGB transparency, then let the printer handle conversion to CMYK.

Note:

Acrobat/Reader has a feature called Overprint Preview (Preferences > Page Display > Overprint Preview). Options:

Never → ignores overprint instructions.

Always → always simulates overprinting.

Automatic → uses overprint preview if the document uses spot colors. This does not override the PDF’s instructions, but rather controls whether you see the effect on screen. See this article for more information: Previewing output (Acrobat Pro).

Learn more about PDF/X, PDF/A, and PDF/E-compliant files (Acrobat Pro) in this article.

I hope this helps.

Thanks,

Anand Sri.

maximilian_6016 · Answer

Nevermind, I think I found the problem.

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