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I'm running into a problem when using a clipping mask or opacity mask in Illustrator CC 17.1. Either type of mask leaves a slight outline around the masking object, creating a noticeable break in what should be a uniform background. Some details:

example 1

example 2

I've created a ring in 4 quadrants. Each quadrant contains patterns/objects that need to be isolated from one another. Over top of the artwork of each quadrant, I Paste In Place a copy of the quadrant's shape whose colours for background and stroke are both set to none, the stroke value field is blank, not set to 0. Where the same colour in the adjacent areas of each quadrant meet there is a faint hairline. These are meant to be seamless and I don't know why this is happening. When viewing as Outlines, all vectors are overlapping perfectly to my eye.

I've read posts in this forum and others about this being an anti-aliasing issue, that it won't print with the faint outlines, but not every application of this graphic will be going to print. I've exported EPS files, PDFs and various other formats. I didn't notice any hairlines in a TIFF but the library of files I will be providing to my client will consist of vector and other raster files like PNGs and JPGs that do show the faint outlines. When I place a PDF or EPS into an Illustrator or InDesign layout, the hairlines remain so that documents sent electronically, not printed, have these noticeable lines. They also remain when I place an EPS into Photoshop which is unacceptable for building graphics for the web.

Can anyone tell me if there's a solution for making seamless transitions between masked objects in Illustrator? Or this just an inherent flaw with Illustrator (or perhaps my approach to building these graphics)?

Some additional info:

  1. Just saved PDF and EPS copies. Both show the hairlines in the following: QuickLook, Reader, Preview, when placed into Illustrator / InDesign / Photoshop, and when opened directly in Photoshop (with the exception of the EPS). Only the EPS opened directly with Photoshop was free of the hairlines.

  2. When exporting to JPG and PNG, using the Type Optimized setting retains hairlines but they disappear if Art Optimized is chosen. So that's one little trick I just learned.

So there are ways that I can save them without hairlines, that's great. But once I distribute PDFs and EPS files for the client/third parties to place into publications for electronic distribution the hairlines will appear in those documents. I could ask them to place hi-res raster files instead but it doesn't give them the flexibility that vector graphics afford.

Is there a similar option to 'Art Optimized' I'm overlooking when exporting PDFs and EPS files? As a test, under EPS Options > Transparency I tried both the High Resolution and For Complex Art presets and it didn't make any difference.

marked as duplicate by Scott adobe-illustrator Jul 24 '14 at 10:53

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  • It's an on-screen anti-aliasing issue. Won't appear when printed. – Scott Jul 24 '14 at 10:54
  • I understand that it's not going to show up when printed but since this graphic is meant to be viewed on-screen as well I'm left with the problem of EPS and PDF files placed into page layouts still having the hairlines. I can't be the only one building graphics and making use of masks, can I? I'm not running into this problem placing other people's logos created in Illustrator. – dugost Jul 24 '14 at 16:36
  • A quick solution is to place an object behind the artwork filled with a color you want to anti-alias to, such as a black-filled rectangle.The hairlines will disappear that way as well. – Scott Jul 24 '14 at 16:38
  • Thanks for the tip but that isn't doing the trick for clipping masks unfortunately. In the example graphics I posted, I placed a black shape behind the objects hoping it would darken the light hairline where black segments meet. However, it still appears in my layout and in an exported EPS. In fact, it has made a new darker hairline appear where segments with light backgrounds meet. – dugost Jul 24 '14 at 16:51
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I don't think this is a problem with Illustrator. What you really want is one continuous black shape. You can unite the different parts together, which will solve your problem. I think there are people out there who would want a slight division between two separate black objects. It's logical. If you don't want two object you should really use the Unite function.

  • That's just it, though: I need separate but seamless objects. If someone wants a division between adjacent elements of the same colour they would add a space or stroke, not a mask. Masks are meant to isolate elements cleanly. It's definitely specific to AI. The same issue doesn't happen in Photoshop, for instance. I've read many posts complaining of these hairlines in AI, and asked others make objects with the same results. Uniting the shapes would also make it tough to change colours/attributes of distinct elements if the client requests changes or if I need to create alternate versions. – dugost Jul 24 '14 at 7:02
  • @dugost You can Unite copies of the black shapes and keep the originals for backup/edit. – Komental Jul 26 '14 at 8:53
  • @dugost I can't replicate this problem. Objects next to each other, masked black objects, RGB, CMYK ... everything works fine for me. Maybe your shapes are not aligned properly. Maybe they are placed with some gaps or maybe they had outlines that you disabled, maybe it's your graphics driver. I use CS5 with no such issues. – Komental Jul 26 '14 at 9:06
  • I appreciate you trying to replicate the results and, to be honest, I've never noticed this before using CC so maybe it's specific to this version or at least post-CS5 versions. I had a friend with CC work with an example file I provided and he was able to get the same results as I did. The position of the anchors are exactly the same for the mask path as they are for the underlying background wedge of the same shape. – dugost Jul 26 '14 at 23:54

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