I like the chunking approach. It is efficient, reliable, and has low
overhead for reasonably sized chunks. Also fits well in a typical
implementation that writes a chunk of data at a time.
-Carl
"Zehler, Peter" <PZehler at crt.xerox.com>
Sent by: owner-ifx at pwg.org
03/05/2003 05:00 AM
To: "'Rick Seeler'" <rseeler at adobe.com>, ifx at pwg.org
cc:
Subject: RE: IFX> PDF/is Issue.
Rick,
Why not just increase the size of the length field signature? Could this
be done by the addition of data or comments in the length object or by
adding another object? I don't know pdf very well. I don't think we need
0% probability of confusion just a statistically insignificant chance.
Pete
Peter Zehler
XEROX
Xerox Architecture Center
Email: PZehler at crt.xerox.com
Voice: (585) 265-8755
FAX: (585) 265-8871
US Mail: Peter Zehler
Xerox Corp.
800 Phillips Rd.
M/S 128-30E
Webster NY, 14580-9701
-----Original Message-----
From: Rick Seeler [mailto:rseeler at adobe.com]
Sent: Tuesday, March 04, 2003 1:29 PM
To: ifx at pwg.org
Subject: IFX> PDF/is Issue.
During prototyping of PDF/is the following problem arose:
How does the Consumer know when the end of a data stream (See section
3.2.7 of [pdf]) is reached? Normally, in a PDF, the Consumer would
consult the stream length field. The problem here is where to put the
length field. If the length were placed before the stream, the Consumer
would know how long the stream is. This requires the Producer to know the
stream's length before writing it to the Consumer. If, instead, the
length were written at the end of the stream, this would solve the
Producer's problem but the Consumer would not know how to find the length
since they can't identify, 100% of the time, where the stream ends and
where the length object is.
An example will illustrate:
First, the normal case...
stream
sdljfiwefnwfubrevurewliysnhr;hgawebfz;h;uwre (lots of binary data
here)....
84trhdvfyu7wgf4.nbdrgur4uaru4gb
endstream
12 0 obj
3456 <- the length of the previous stream.
endobj
But, what if the data looked like this...
stream
sdljfiwefnwfubrevurewliysnhr;hgawebfz;h;uwre (lots of binary data
here)....
endstream <- the binary data could have a string of bytes that
looked like this.
84trhdvfyu7wgf4.nbdrgur4uaru4gb
endstream
12 0 obj
4567 <- the length of the previous stream.
endobj
Of course, you could look to bytes after the appearance of the word
'endstream' to see if this is really the end of the stream; but you can
always come up with a stream that could match your parsing algorithm's
expectations (although with decreasing percentage of occurrence).
Possible solutions:
1) Write all data using ASCII85 encoding (See Section 3.3.2 of [pdf]).
This will increase stream lengths by 25%. ASCII85 has a stream delimiter
which would solve this problem -- the end of the stream can be known for
certain and the length field can be placed after the stream.
2) Require the Producer to write the stream length before any stream (the
streams would stay binary). The Producer can use banding to break up
large images into small enough chunks so the Producer can cache the stream
before sending.
3) Offer a combination of 1 & 2. The Producer would cache streams if
possible, but may use ASCII85, if necessary.
4) Producer must make certain all streams must not contain a series of
bytes "\0D\0Aendstream" in the stream data. This is how the spec is
defined currently -- but this may be too onerous for the Producer.
Any other ideas? I'm personally leaning toward solution #3.
-Rick
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