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When I put PNG images into an InDesign document, sometimes they have black borders on the exported PDF. Usually this doesn't seem to be a problem in print but in a recent proof, I did see some of the issue surfacing there too (albeit not quite as bad). However... Even if it's not as bad in print, it'd be nice to not occur in the PDFs too as these get sent out to customers via email too.

Now I know what people are going to say here... PNGs are a web format and not for print! I agree! I'm slowly converting people over to TIF/PSD here.

The catch here is that I don't get this problem with all of the PNG files. Some are absolutely fine. I've noticed that in the links panel of InDesign that some have black backgrounds and some have white backgrounds (presumably the alpha layer doesn't show up in the links panel). Why are some of these PNG files showing up perfect and others not? I can't seem to see the difference between the files in Photoshop.

To illustrate this issue, screenshots and photos follow:


Here's a PNG placed into an InDesign Document:

enter image description here

Note the black background that this file has in the links panel.

Here's the same section on a PDF export of this document (PDF/X-4:2014, Average downsampling to 300dpi for images over 350, compression:JPEG, quality:Maximum):

enter image description here

And here's what it looks like when zoomed in:

enter image description here

Finally, here it is on the proof:

enter image description here

The right side of the bottle has dark bits along the edge.


Now here is another PNG in the same document:

enter image description here

There's a white background in the links panel for this file.

And here's how it looks in the same PDF:

enter image description here

Zoomed in:

enter image description here

No black borders at all on the PDF and no black edge on the print either:

enter image description here

Why?

  • So, you're asking why a PNG with a black background is showing the anti-aliasing associated with the black background??? What would you expect it to do? – Scott Apr 27 '15 at 17:44
  • Hi Scott. Why do some files have black backgrounds, in the links panel, at all? The files, when opened anywhere else, do not have black backgrounds. – marcusdoesstuff Apr 28 '15 at 10:45
  • None of these photos were shot on a black background either! There has never been black in the background, why now? Hidden Photoshop feature? – marcusdoesstuff Apr 28 '15 at 10:50
  • Here's a screenshot of an offending file in Photoshop: imgur.com/tTgD6sF – marcusdoesstuff Apr 28 '15 at 11:41
  • There is no visible difference between this one and the other except in the links panel of InDesign. I tried messing with the colour settings first as I noticed one was Adobe RGB and the other was sRGB but any way I changed them, copied them over, etc resulted in them being the same in InDesign. – marcusdoesstuff Apr 28 '15 at 11:47
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There's a short answer here and that's to use TIF files and your problems will go away...

But if people would still like to know more, I'll try to explain what's actually happening. It is likely that these files are different since being created in different programs. It doesn't appear that one is "wrong" as such (I'll explain that later). It just seems to be a way that PNGs work and how they clash with PDFs. Different programs have different default settings and some must just add a black background.

In Photoshop, using Layer > Layer Mask > From Transparency we can split the transparency layer off from the image and we see that this particular file has a black background underneath:

enter image description here

This isn't really that much use to us right now but this reveals the hidden black background in Photoshop (InDesign isn't just imagining it!). The alpha channel (transparency layer) is slipping out of alignment with the rest of the image when embedded into a PDF, causing us to be able to see some of that (usually) hidden black background when viewed on screen. It's unlikely to cause much effect in final print (though as we've seen, there is a little)... It's mostly an on screen issue.

As the hidden black background is already there, copying the image into a new document won't do anything. However we can replace that background with a different colour so when the transparency alpha layer slips, we don't see it as easily. Any colour can go in here but we want to use a colour that isn't contrasting to the image and whatever you're placing it on top of. I've added a flat white layer below the image and masked everything again here:

enter image description here

Below is a comparison of the different versions. The mystery black background version on the left and modified version with a white background in the middle. You can see how there's far less issues with the white background when the image is so light.

enter image description here

I mentioned that the hidden black background wasn't exactly wrong... The reason is that if an image with a hidden white background is placed on a dark page, you see a white halo instead as the transparency alpha layer slips:

enter image description here

And also, finally, that very shiny, perfect TIF. :D

  • This is probably all going to end up being utterly useless for me once I get these files all in TIF in future. I'm going to run actions via Bridge to fix the few files that need it for now, and not need to re-link any files, for now though. Interesting learning how PNGs work, how to separate transparency etc. :) – marcusdoesstuff Apr 29 '15 at 10:11
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I had a similar issue some time back. I noticed that I was exporting the PNGs from Adobe Photoshop without going in detail to the advanced setting. Later, I tried using the PSD instead of PNG and it worked perfectly okay. I will have to check the layer mask thing if similar issue comes up.

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