When creating pdfs with InDesign CS6 my target pdfs show sometimes strange patterns in the pdf. These patterns are sometimes black horizontal lines straight over the whole page, sometimes a square around a graphic I included, or some others. I experienced these patterns only on A4-pages which are completely filled up with one colour and a semi-transparent image (meaning only a part of the square-sized image is transparent) copied over it afterwards.
My problem is that I have problems with reproducing it. These patterns only show up when (in my current case) I am viewing the resulting pdf in Foxit PDF Reader at a specific size. When I zoom in, they are gone. When watching the pdf with the integrated Windows pdf reader, I can not see any lines regardless of the zoom level. Same goes for the integrated reader in Ubuntu. In InDesign I can not see these lines either. According to a friend he can see the lines when watching the pdf with the Adobe Reader.
Where do these lines come from? And how can I remove them? Or is it a simple bug of the viewers (Adobe InDesign -> Adobe Reader: Bug? Seriously?)?
These lines come about because of flattened transparency effects in the PDF and the method used by the PDF reader to render them on a low-resolution device like a monitor. “Transparency” includes any kind of glow or shadow effect, any blend mode other than Normal and any opacity other than 100%.
Any legacy PDF format based on Postscript, such as PDF/X-1a, requires InDesign to flatten transparency, because there the Postscript language doesn’t recognize it. PDF versions earlier than 1.5 are based on Postscript, so will require transparency to be flattened.
What happens under the hood is that parts of the page are rasterized at a very high resolution, and what you see as weird white lines are the result of your PDF reader trying to anti-alias the edges of these rasterized regions — the “seams” where they meet — to cope with the very low resolution of the screen. “Low resolution,” where PDF is concerned, very much includes so-called “High DPI” and “Retina” displays. (You can see these blocks if you open the PDF in Illustrator. When you try to select things with the Direct Selection tool you’ll see these image blocks that don’t seem to have any relationship to the objects in your original layout.)
Because the anti-aliasing is based on the resolution of your monitor, the ghost lines show up or disappear at different zoom levels depending on whether the “seams” fall on a (screen) pixel boundary or not. You’ll note that they never get thicker, no matter how far you zoom in, because their size is a function of your monitor, not of the content of the PDF.
Because a PDF goes through a much higher-resolution raster image processor when going to press, these lines don’t appear in print. You can also turn them off by changing the view settings in some PDF readers.
If your PDF will mostly be seen on screen, and printed, if at all, only to desktop printers, you can export as an Interactive PDF, which is designed for screen rendering. You could also choose “High Quality Print” or just change the PDF version in the export dialog to 1.5 or later in the Print PDF settings. In any of these cases, the PDF will contain “live” transparency, no flattening will occur, and you won’t see the hairlines when you view them.