Printing a transparent PDF document with alpha channel results in artifacts on the photo
Me and my friend are kinda struggling a lot. We have a "overlay" PDF file containing a PNG bitmap image (no vectors). We use this PDF for automatic montages in PDFlib together with generic photos to create another print PDF. This PDF is printed afterwards.
However, the source overlay PDF seems to have issues with transparency. On screen, everything looks ok, even the PDFlib output PDF looks alright, but after printing the document out, there are artefacts clearly visible on the transparent part of the overlay PDF. Other parts on the photo do not have any artifact issues, except these transparent ones.
Is there a way to prevent these artifacts from appearing? Now, as I said, we put a PNG image into inDesign and export it to PDF (v1.6) afterwards.
I will attach 2 images of, one is before the actual printing (no artifacts visible at all) and the other one is scanned picture of the printed document. Artifacts are clearly visible in that one. (Sorry for the crappy scanner quality that darkened the photo, but it will do. It looks exactly like the input, except the artifacts.)
I believe this is called "drop-off" or "ink drop-off" and it is especially problematic in gradients.
My basic understanding is that you are providing art with values that the ink dot cannot reproduce. The smaller the possible dots (the finer the line screen), the less apparent it is, but it is always there.
In some cases, you can avoid the worst of it by dithering your gradients so you never present a hard-edge--the camouflage method. In your particular case, it is the high color contrast and the fairly solid white area that is highlighting it. You'll notice that in the area around the ski boots, you barely see it when looking at it casually. Further, the problem is all over the page in your example, but you only object it in certain contexts.
I found one web hit which discusses the problem cogently in the context of flexographic printing and offers a way to help deal with it:
An accepted method of ensuring no point in the artwork goes below a cut-off level causing [...] ink drop-off, is to use the Curves tool to adjust the image so that the white point moves up to the cut-off level.
I am not a pre-press guy though, so I can't speak to how it is best handled or developed, and I am not sure if the solutions differ in the context of standard offset, etc. My gut says the problem is universal to separated line screens and varies only in degree between specific methods.
If you have control over the choice of line screen, increase the line count: ask for 133lpi or 150lpi or more. You might also get better results using stochastic plates, but not all printers do this