Inside stroked object in AI is qualified as filled object in PDF !!

[sPretzel]

An object (like a circle) with stroke aligned to centre in Illustrator remains a stroked object when saved as PDF. However, the same object with stroke aligned to inside in Illustrator becomes a filled object when saved as PDF. What? If this is standard behaviour, please explain to me the logic behind it. Otherwise, is this a bug?

This is in CS4.

[Lukas Engqvist]

I believe that the mathemetics behind a stroke means it is always centered, though in creating software you can have options to have the stroke inside or outside. This means that a stroke that is not centred has to be expanded/flatened to preserve appearance. I don't think this is a bug but a limitation of exporting between formats. #lostintranslation.

[sPretzel]

That is a real problem if it is how Illustrator handles strokes. A printer does not print a stroke the same way it prints a fill. As a result, there can be a visible difference for the same stroke size, whether it is centre-aligned or inside-aligned! Awful.

Can someone at Adobe confirm that this is intended by design in Illustrator and not a bug?

[Ton Frederiks]

Please show the awful visible difference...

[sPretzel]

This is a difference you see in print, not in the files. The visible difference may vary from printer to printer. The PDF and AI file look the same, as far as the "size" of the stroke goes. But you can see in preflight within the PDF that the object with the inside stroke is no longer a stroked object but has become a filled object.

Again, I'd like confirmation from Adobe that this "feature" is intended by design in Illustrator, as Lukas implied, and that it is not a bug.

[Ton Frederiks]

There is no such thing as stroke inside or outside in PDF, so you need to simulate it.

It is not a bug.

[sPretzel]

I am not concerned about stroke inside or outside or centred in PDF, I am concerned that what is a stroke (aligned to inside) in AI becomes a filled object in PDF, whereas a stroke (aligned to centre) in AI remains a stroked object in PDF. A stroke in AI should remain a stroke in PDF, no matter the stroke's alignment, and it does not. Perhaps you misunderstood what I described.

[Ton Frederiks]

"A stroke in AI should remain a stroke in PDF, no matter the stroke's alignment, and it does not."

Why? If that kind of stroke alignment does not exist in PDF how can it remain a stroke?

I think you need to do a feature request to change the PDF specification.

[sPretzel]

Hi Ton. I don't know enough about the PDF specification to be able to tell how strokes are positioned in PDF. If you know what part of the PDF specification addresses that, please point me to it and I will read it. I have not tested this but as Lukas pointed out, it seems that a stroke aligned to inside in AI goes to PDF exactly as the expanded version of that stroke in AI would go to PDF. That may sit well with you but not with me, as it creates problems in print.

[Ton Frederiks]

https://www.adobe.com/devnet/pdf/pdf_reference_archive.html

I am still curious to see an example of your print problems.

[Test Screen Name]

A lot of things that can be designed in llustrator don’t have a simple direct conversion to PDF. This is one of them. If you were to read the 1000 page PDF specification, on the subject of stroking, it would be clear, eventually, that if a path exists, the stroking function will produce a shape centred on the path. In effect, your design choices do as much damage as outlining text or expanding a stroke. We don’t have to have inside knowledge of what the code in long-obsolete software does, to know this must happen. 

[sPretzel]

Hi Test_Screen_Name. I have read a small portion of the pdf 1.3 specification. There are mentions of the stroke being centred, such as in this paragraph on page 152.

Stroking The S operator paints a line along the current path. The stroked line follows each straight or curved segment in the path, centered on the segment with sides parallel to it. Each of the path’s subpaths is treated separately.

I assumed that a stroke in Illustrator would remain a stroke in PDF. I think that's a fair assumption! It's true that I could alter the design to have the path centred, but that in turn, will have other unwanted consequences like dimensions and positions of objects. Perhaps Illustrator could have made it clear that a stroke aligned to inside or outside is only a stroke in Illustrator, and may be expanded in other file formats.

I didn't understand your last sentence.

Ton, I can't share pictures unfortunately. The print shows all the inside strokes (now filled objects in PDF) as noticeably thicker than the same strokes for which the path is centred (and remained stroked objects in PDF). Another printer may show matching results, it varies by printer.