[NTG-pdftex] [pdftex-Bugs][4294] issue with isolated/non-isolated page transparency group

pdftex-bugs at sarovar.org pdftex-bugs at sarovar.org
Wed Sep 2 00:43:40 CEST 2009

Bugs item #4294, was opened at 2009-09-01 15:43
Status: Open
Priority: 3
Submitted By: John Owens (jowens)
Assigned to: Nobody (None)
Summary: issue with isolated/non-isolated page transparency group 
Category: None
Group: None
Resolution: None

Initial Comment:
(The last bug didn't show up. So I am posting a web link to the
three files referenced below: http://tinyurl.com/n4jjm4 )

Howdy, I'm attaching 3 files. One is a tar.gz of a LaTeX document.
When we first submitted the paper built from the document, we used
dvips and Distiller. Now we're using pdflatex for the final version.
Both pdfs are attached. However, strangely, page 4 on the pdflatex
pdf, when viewed in Apple Preview on Snow Leopard (10.6), has
substantially lighter fonts than the other pages. This is probably
Preview's problem, not pdflatex's problem, and I've filed an Apple
bug. However, from talking to a PDF expert (summary below), it appears
that pdflatex is doing something weird, so I thought I'd bring it to
your attention.

The base page transparency group for page 4 is nonisolated, where for
all the other pages the /I key is not present (and thus they are
implicitly isolated). The PDF is correct, although a little
weird--there is no point in even specifying the isolation key of a
page's transparency group unless it a) contains non-normal blend modes
_and_ b) will be rendered in non-device-like contexts (e.g., as an
object in a larger piece of artwork). So I would argue that pdflatex
should probably not even bother specifying the /I key, especially
given that it appears to trigger a bug--or at least a dramatically
different code path--in Preview.

This _should_ have no effect, as the value of the /I key for the page
group is supposed to be ignored (and treated as if it were true).
Furthermore even if Preview were ignoring that requirement, the page
doesn't have any blend modes, so mathematically it shouldn't be
producing a different result.

But my guess is this: Preview is rendering page 4 into a nonisolated
transparency group using an initial, opaque, all-white backdrop, when
it should be completely ignoring the /I key and rendering into a
completely transparent backdrop. This shouldn't lead to different
results unless a blend mode is present. But,the isolated and
nonisolated code paths are somehow sufficiently different that they
generate different pixel values--due to precision or perhaps an actual
bug. But most of what I say is conjecture.

See chapter 7.3.6 ("Page Group") of the PDF Reference Manual, sixth

The Distiller version doesn't even have transparency group objects on
the pages (which is normal). So I wouldn't expect it to have the same
issues in Preview, and it doesn't.

(This is LaTeX source for a paper, so though I know you'll need the
source to do your investigations and bug fixes, kindly don't do
anything else with it beyond that.)

$ pdflatex --version
pdfTeX 3.1415926-1.40.10-2.2 (TeX Live 2009)
kpathsea version 5.0.0
Copyright 2009 Peter Breitenlohner (eTeX)/Han The Thanh (pdfTeX).
There is NO warranty.  Redistribution of this software is
covered by the terms of both the pdfTeX copyright and
the Lesser GNU General Public License.
For more information about these matters, see the file
named COPYING and the pdfTeX source.
Primary author of pdfTeX: Peter Breitenlohner (eTeX)/Han The Thanh (pdfTeX).
Compiled with libpng 1.2.39; using libpng 1.2.39
Compiled with zlib 1.2.3; using zlib 1.2.3
Compiled with xpdf version 3.02pl3

$ uname -a
Darwin classico-2.local 10.0.0 Darwin Kernel Version 10.0.0: Fri Jul 31 22:47:34 PDT 2009; root:xnu-1456.1.25~1/RELEASE_I386 i386 i386


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