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enter image description hereI work in CMYK mode, it's an exterior navigation table for a client and I put two pictures as backgrounds. The upper one with blue bg is working perfectly after tracing, the lower one with the tattooed leg looks like it has a light outline and there are even a few blank spots in the image, I tried filling them in with livepaint but I'm worried about how this image will print. I did use the image in the previous table I was doing and I don't recall the image acting this way. BTW the vector looks good in Illustrator, it doesn't look good in PDF.

Can someone explain to me what went wrong?

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I suspect that this is just a poor preview of the PDF, in which case it should print fine, but there are a couple of things that you can try:

  • Rasterise the image: The PDF looks like it's struggling with the complexity of the traced version so rasterising it (300dpi at finished size) should eliminate this problem with no visible drop in quality.
  • Try different PDF settings: When you save the design out as PDF, make sure that you are selecting an option like 'high quality print' or 'press ready' for the PDF. You may also need to dig around in the settings to turn of some optimisation. There will be a certain amount of trial and error, but once you find a setting that works, you can save that as a custom option.

Like I said at the start of my answer, it will probably print OK as it is, but it's always a good idea to make sure. I'd go with the rasterising option myself.

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    It worked, now it displays correctly in illustrator and adobe reader as well, thank you again! – Ilona Takácsová Oct 3 '16 at 11:44
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It is a known bug in most 2D vector engines. It is because the algorithm conflates transparency as alpha, so its known as a conflation artefact. See here for a better explanation. Illustrator GPU preview is the first generation of Adobe engine that does not have this problem in one way or another. Though the software rasterizer does, illustrator is tuned so that opaque objects will not show this effect but transparent ones will.

When a printer rasterizes, it does point samples. When one does this then one nolonger does conflation and the effect goes away. Offcourse it is possible to have a totally broken printer driver that does just the same as illustrator. Again transparency may cause some porblems with pixel quantisation.

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