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"

Monika Gause

Community Expert

I am unable to reproduce that. How do you do it?

To select "Clip complex regions" with PDF 1.3 when Saving as PDF, I must lower the raster/vector proportion below 100% (for example 99%). This enables me to select "Clip complex regions", otherwise the option is grayed out. Once I save with "Clip complex regions" selected, the size of the resulting object around the red rectangle is still larger than the red rectangle, pretty much exactly as what the pictures posted here show.

Alternatively, if I select the green background and the red rectangle in Illustrator and flatten them with Clip complex regions selected within Illustrator, Illustrator appears to show the correct boundaries for the red rectangle. But once I save the flat AI file to PDF (no flattening during the conversion) and run Preflight, the red rectangle object is still larger, with pixels from the green background, just like the pictures posted here.

Why should it be an issue to move the slider to 99% vector?

I don't know where exactly I got lost, but I'm lost. I don't know what we're talking about anymore. Why do you need to solve what? As soon as someone comes up with a solution or a workaround you post just another "but what if". So what exactly are we trying to solve here? Or are we just discussing something theoretical for the sake of finding a fly in the ointment?

And maybe the fact that it needs to be PDF 1.3 doesn't help either. What about PDF/X-4?

Mylenium

Legend

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.

Monika Gause

Community Expert

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

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