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Trouble with colour coverage

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I'm about printing a book made in Publisher. The printer asks me to lower the colour coverage in some pictures, below 300 %. One image is especially problematic. In Photoshop it shows something like 155-160 % in a black area, shown in the red circle in the attached image file (the image is only a cut-out of a bigger image). When I place this image on a page in Affinity publisher and export this page to PDF the same area in the PDF file shows something like 330 % in the same area. I thought the colour coverage in the image file would correspond to the coverage  in PDF file, but I've obviously got it wrong. But what is my mistake? Thanks for any help (or explanation).


Win 10, Affinity Publisher 1.10.1



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Did you measure the ink percentages with the image already in CMYK and with the correct profile applied? You example image looks like you didn't: Those luminous greens of the island's vegetation would get pretty muted in CMYK (unless you make use of additional spot colors, but then the question is what your proofing setup is). If the sampling was done on an RGB image, the control points of the picker may have just given you wrong or at least inaccurate results. In general, adjusting ink limits of photos is not a trivial task, but in fact shouldn't be necessary because the color profile takes care of this on document export.

If you really have to do this manually, you could probably use a Channel Mixer adjustment to gradually lower the ink levels, consistently across all CMYK channels to avoid introducing a color cast. This will brighten the photo slightly, but since total area coverage typically only exceeds specifications in dark areas, you can limit the effect to the image's shadows using Blend Ranges or by applying a luminosity mask to the adjustment layer. Ideally, you'll only go so far until TAC meets the specifications so the visible impact will be as low as possible.

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I ran a test with your image (that had SWOP (Coated) ICC profile), and made a copy of it using ISO Coated 300 profile (assigned), then created Publisher documents where I had ISO Coated 300 and ISO Coated v2 document color profiles, and placed these images there and exported to PDF/X-1a:2003, PDF/X-3 and PDF/X-4.

Unlike e.g. InDesign, Affinity apps do not by default discard conflicting CMYK color profiles, so if your placed images contain a CMYK color profile that is different than the document where they are placed, the colors get converted to the target color profile, in case the target color profile is ICC based and embeds all source color profiles. This means that the dark areas in the placed images (like yours with below 200% TAC) will become approximately the max TAC of the target profile, something you definitely do not want...

You can avoid this most easily if you export using PDF/X-1a:2003, as that will create a device-CMYK PDF with no embedded profiles, which means that conflicting ICCs will not cause translations but the source CMYK values will be passed through.  Another option would be making sure that all your source CMYK images have the same color profile as the Publisher document so that CMYK to CMYK color conversion does not happen.

Attached are PDFs that demonstrate this. The upper image is my copy with ISO Coated 300 image and the lower is yours with SWOP (Coated). My image works in PDFs with ISO Coated 300% profile because it does not conflict; your image only works with PDF/X-1-based export.






UPDATE: Below are screeshots showing this:

a) PDF/X-4 based export where ICC profile conflicts with the lower image (yours), but not mine (ISO Coated 300):


b) Both images have different profile than the PDF, but as that is exported with PDF/X-1a:2003 and is not ICC-based but device-CMYK, source CMYK values are passed through:


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I forgot to mention one obvious method of avoiding the problem: not including a color profile at all in your CMYK images, as this way they will get assigned with the document profile whatever it is and will not be translated at export time, whatever export method which does not explicitly change the color profile. So in the example below exported to ISO Coated v2 (max TAC about 330%) both the image with ISO Coated 300% and one that your image had embedded (SWOP Coated) are translated while the third image (one that did not have an embedded color profile) at the bottom will not: 


[IMO this is also logically the most obvious method to avoid export time CMYK-to-CMYK translations as I do not really understand why PDF/X1-a based export method now survives the translation, as logically it should happen during the processing even if the exported file is not ICC-based.]

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