IFX Mail Archive: RE: IFX> PDF/is Issue.

IFX Mail Archive: RE: IFX> PDF/is Issue.

RE: IFX> PDF/is Issue.

From: Buckley, Robert R (RBuckley@crt.xerox.com)
Date: Thu Mar 06 2003 - 10:20:42 EST

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    Rick et al.,
    I would go with #2. In fact in our prototyping, we include the codestream
    length explicitly in the image object, rather than as an indirect object
    reference, to address the problem you describe. I would not support #1,
    simply because it would increase file size.

    -----Original Message-----
    From: Rick Seeler [mailto:rseeler@adobe.com]
    Sent: Tuesday, March 04, 2003 1:29 PM
    To: ifx@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...
    sdljfiwefnwfubrevurewliysnhr;hgawebfz;h;uwre (lots of binary data here)....
    12 0 obj
    3456 <- the length of the previous stream.
    But, what if the data looked like this...

    sdljfiwefnwfubrevurewliysnhr;hgawebfz;h;uwre (lots of binary data here)....
    endstream <- the binary data could have a string of bytes that
    looked like this.
    12 0 obj
    4567 <- the length of the previous stream.
    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
    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.



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