Hi Mads
Sources for Moiré can be from a variety of sources. Since you 100% view did not show any issues I strongly suspect that this is all caused by the specific resampling percent and algorithm used.
If you have a repeating pattern in the source pixels and resampling is done in a fashion that resizes so the sampling is done at about the same frequency as the pattern it is not hard at all to get Morie.
Since I did not have your full original, I took your 100% sample, flipped it vertical and tiled and then horizontal and tiled. The I created a Photoshop pattern and did a fill on a 4000 x 4000 pixel image. Noticing that some of the repeating vertical white lines in your original were at every 7 pixels, I resized by 1/7 using nearest neighbor reampling option and the image below is what resulted at 100% viewing.
I suspect that the resizing done somewhere along the line (could even be in your printer driver) could cause this. (click on thumbnail)
[ATTACH]51419[/ATTACH]
Very quick and easy to generate Moiré.
There are a variety of ways to avoid the issue.
- create a version of your image resized in Photoshop (or most desired editor) and try a couple different resampling algorithms.
- If you print is of a size where you will not be able to see the fine threads anyways, do a 1 pixel or so blur on the image so there is less of a pattern in the first place to create the Moiré pattern by resampling
- Check the options for resizing in the PDF options. A fair chance that is involved as well.
Hope the above suggestions help you make some progress.
