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Topic: Re: transparency (alpha) in pdf (Via Email)
Conf: (P-PDF) What's Wrong with my PDF?, Msg: 142277
From: LeonardR
Date: 11/5/2005 01:32 AM

At 08:12 PM 11/4/2005, p-pdf-whats-wrong-with-my-pdf Listmanager wrote:
>look at this screenshots.

I was wondering if this was the issue or not...Seems to be
the week for this.

Here is a description that I wrote to someone else on the
same topic...

The usual cause of this type of issue is that transparency requires a
blending space, and if you add transparency to a page you may create
a requirement for a blending space for the page. This leads to the
creation of a page level transparency group, which does affect
rendering of the whole page. The usual way to avoid this is to wrap
the transparent elements in an isolated transparency group. 7.3.6 of
the PDF 1.6 manual is essential reading here.

[EMPHASIS BEGIN]In fact, when there is transparency on a page, and
the page does not specify a top level blending space, Acrobat always
uses the CMYK working space. [EMPHASIS END] Acrobat does this because
it values consistency between screen and print, and because Adobe
figures print users are typically pickier about color than people who
design for the screen.


So there you have it! Because you haven't specified a
blending space, Acrobat may assuming any other ColorSpace as the
blending space - and they've chosen CMYK. So all your RGB colors are
being converted to CMYK - thus the shift.

In order to avoid the shift, you'll need to add additional
structures to the PDF to specify a blending space for your colors.

If you need more details - let me know.


Leonard

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Leonard Rosenthol
Chief Technical Officer
PDF Sages, Inc. 215-938-7080 (voice)
215-938-0880 (fax)