Inspiring

Question

Transparent object in PDF bigger than transparent object in AI

Forum|Forum|6 years ago
December 1, 2019
2 replies
2950 views

Hi,

I'm not sure whether this post belongs in Illustrator or Acrobat but I'll start here since it starts in Illustrator.

I have a green rasterised stroke in the background (so, we will consider that as a picture). On top of that picture, I have placed a red rectangle (vector), for which I have set the Blending mode to 100% Multiply. I then Saved as PDF from Illustrator (PDF 1.3) and view the resulting file in Acrobat. In Acrobat, I also run Preflight. I have attached a picture of what I observe in Preflight for the red object. The red object, along with the green background right beneath it, involve transparency and are therefore flattened and result in an image object in PDF. What I don't understand is why the border of this new object, indicated by the blue dashed line in Acrobat (this blue line is provided by Preflight), is now slightly bigger than the area involving transparency, which is only limited to the red rectangle.

Can somebody explain why that is and if there is something I can do within Illustrator or the flattener settings when saving to PDF to make sure the resulting object in PDF is exactly the size of the red rectangle?

This is for CS4. Thanks.

How to

This topic has been closed for replies.

Ton Frederiks

Community Expert

Monika and Mylenium are right.

Anti-aliasing makes the image larger.

The width and height of your red rectangle contain fractional pixels (if you look at the pixel size) and the X/Y coordinates contain fractional pixels too. This and a resolution other than 72 will give additional pixels.

Monika Gause

Community Expert

But shouldn't matter when you flatten transparency with the option "Clip complex regions"

S

sPretzelAuthor

Inspiring

The flattened object that results just takes those 1-2 pixels from the background image, in addition to the red rectangle. It doesn't alter the image in any way. That's not anti-aliasing (the background picture does have anti-aliasing however, before the flattening in PDF).

I tried Clip complex regions (but I have to lower vector percentage in transparency flattener options below 100% otherwise that option is grayed out, if I recall correctly). It made no difference. Does it change anything for you Monika? Does it keep the rasterised object the same size as the rectangle?

Mylenium

Brainiac

Image objects may contain padding to accomodate antialiasing and of course account for overprinting. Nothing wrong there. You can of course turn this stuff off when generating and viewing the PDF and see if it makes a difference at the risk of getting a less printer-friendly file that may show issues.

Mylenium

S

sPretzelAuthor

Inspiring

Hi Mylenium. The red rectangle, which is the only transparent object here, is a vector and is therefore not subject to anti-aliasing. There is no overprinting at work here.

"You can of course turn this stuff off when generating and viewing the PDF". How? The only setting that is relevant in the Illustrator Save as PDF menu is the Transparency flattener options and I always end up with this flattened object in PDF being bigger than the red rectangle, no matter what I've tried.

S

sPretzelAuthor

Inspiring

If those vertical and horizontal lines don't fall exactly on the pixel grid, then they will be subject to anti-aliasing.

Hi Monika,

The picture shows what I see in PDF. The rectangle has straight borders in PDF, just as it has in AI. I don't see any evidence of anti-aliasing. The pixel pattern for the green background picture in PDF is also just like what I see in AI. The blue border (from Preflight) does exactly follow the pixel pattern of the green background. It looks as though it is allowing 1-2 pixels around the red object. So the flattened object resulting from the red/green object transparency interaction in PDF is up to 2 pixels larger on each side than the red rectangle.

These extra 1-2 pixels create a problem when I have other objects that go all the way to the edge of the red rectangle without crossing it. In AI, those objects are not subject to transparency but in PDF, they become trapped in this extra 1-2 pixels and Acrobat alters them (for example, a stroke in AI that finds itself in this crossfire becomes a filled object in PDF).

As a side note, Flattener Preview in Illustrator does not show me these extra pixels in what gets flattened. I have to convert to PDF to see them in Preflight!

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