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

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

RE: IFX> PDF/is Issue.

From: Poysa, Kari (Kari.Poysa@usa.xerox.com)
Date: Wed Mar 12 2003 - 09:04:25 EST

  • Next message: Carl Kugler: "RE: IFX> PDF/is Issue."

    Tom, The Length being discussed here actually is the byte count of the
    streams of Image XObjects that belong to the Page. So if the Page is
    comprised of more than one image (a.k.a banding), then the sender does not
    need to cache even a full page's worth of compressed data in order to be
    able to write the Image XObject's stream length in the stream dictionary.
     
    Full PDF allows the writer to enter an indirect object reference into the
    required Length entry. This makes it easy to implement writers because the
    separate object for the length can be written after all of the image data
    has been written. The PDF files are then read in the reverse order starting
    from the end of the file. This works well if one has a file system to store
    the complete PDF file. So requiring the Length to be a direct value in the
    stream dictionary most likely would cause existing writer SW to have to be
    modified. One could not keep writing the same kind of files and claim them
    PDF/is compliant.
     
        --- Kari ---

    -----Original Message-----
    From: Hastings, Tom N
    Sent: Tuesday, March 11, 2003 5:49 PM
    To: Poysa, Kari; 'Rick Seeler'; 'Carl Kugler'
    Cc: ifx@pwg.org
    Subject: RE: IFX> PDF/is Issue.

    Kari,
     
    I think you summed up the argument about tradeoff simply between the Sender
    and the Receiver when you said:
     
    "If we require the reader to be able to cache a page's worth of uncompressed
    data, surely we can require the writer to cache a page's worth of compressed
    data [in order to determine the length and send that length in the stream]."
     
    I assume that PDF has the notion of a length for each page, right? So we
    require that the Sender put in a length field for each page of data at the
    front of each page of data. Can that length field be sent with the data in
    some manner, so that the Sender doesn't have to know the lengths of all of
    the pages before sending any?
     
    Tom

    -----Original Message-----
    From: Poysa, Kari [mailto:Kari.Poysa@usa.xerox.com]
    Sent: Friday, March 07, 2003 15:04
    To: 'Rick Seeler'; 'Carl Kugler'
    Cc: ifx@pwg.org
    Subject: RE: IFX> PDF/is Issue.

    Rick, I bet this solution can be implemented, but it does have some problems
    for the reader that unfortunately I did not see earlier. The difficulty
    really is whether we want to make life easy for the streaming writer or the
    reader.
     
    If the length follows the image stream, the reader must scan the filtered
    stream to find the end of the stream. This can make the reader
    implementation both cumbersome and slow, especially if the stream has to be
    fully decoded during the PDF file parsing, instead of simply extracting the
    correct amount of binary data and passing it to a separate decompression
    module. The PDF file parser would have to know details of the compressed
    streams which should really be of no interest to the PDF file parser module
    and makes creating applications from 3rd party components harder.
     
    In addition, if the reader attempts to decode the stream, how much data
    should be cached and decoded at a time? If the end of stream is not found at
    first attempt, one has to pass additional data to the decoder and continue
    decoding from where previous data ended. This can delay achieving robust
    implementations. The alternative, searching for the "endstream" text, is not
    100% reliable (although very close) and is a wasted step since no
    decompression is achieved yet.
     
    This issue is really at the heart of what "streamable" means, and also has a
    big impact on what kind of low resource applications PDF/is can be used for.
    I think we should consider it a "MUST" for the writer to prefix the stream
    with its length, since the goal is to make the file format streamable
    especially at a low resource reader. If we require the reader to be able to
    cache a page's worth of uncompressed data, surely we can require the writer
    to cache a page's worth of compressed data.
     
    I do understand Ira McDonalds note about streaming writers (see separate
    Email). Possibly this issue whether to prefix or postfix image streams with
    their lengths should be a negotiable capability between the sender and
    receiver?
     
        --- Kari ---

    -----Original Message-----
    From: Rick Seeler [mailto:rseeler@adobe.com]
    Sent: Thursday, March 06, 2003 2:37 PM
    To: 'Poysa, Kari'; 'Carl Kugler'
    Cc: ifx@pwg.org
    Subject: RE: IFX> PDF/is Issue.

    Kari,
     
    Yes, the stream length should precede the stream, if possible (this is
    allowed). But, in the case where the stream may be long, this may not be
    possible for the Producer. In that case, the length should be an indirect
    object reference to the length that should come immediately after the
    stream.
     
    As for your idea of scanning for "endstream" that's followed by the size
    object. This still has the same problem as scanning for "endstream" but
    just has more data and a smaller likelihood of occurrence.
     
    Given that, and what I discussed in my previous e-mail on this subject (to
    Rob Buckley), I think the best approach might be to:
    1) The Producer MUST always write the stream length of all 'Content Streams'
    and 'ICC Profile' streams immediately in the object dictionary (before the
    stream).
    2) When writing image streams, the Producer MAY either write the stream
    length before or after the stream, as they prefer.
    3) When an image stream is length succeeded (indirect object), the Consumer
    SHOULD decode image streams to determine the stream length, when possible.
    But, the Consumer MAY (at their peril) scan for the 'endstream' marker.
     
    How does this sound as a solution?
     
     

    -Rick

    -----Original Message-----
    From: owner-ifx@pwg.org [mailto:owner-ifx@pwg.org] On Behalf Of Poysa, Kari
    Sent: Thursday, March 06, 2003 7:15 AM
    To: 'Carl Kugler'
    Cc: ifx@pwg.org
    Subject: RE: IFX> PDF/is Issue.

    In my opinion the goal should be to write the stream length immediately to
    the stream dictionary.
     
    Also, the likelihood of "endofstream" to exists in the data is small. We
    could also require that if a low resource streaming writer is not able to
    add the length directly into the stream directory, then the PDF object for
    the length MUST immediately follow the stream object. This way, the reader
    can scan for "endofstream" (but of course only if the length was not in the
    stream dictionary) and make sure that it is the correct "endofstream" by
    verifying that it is immediately followed by something that looks like a
    length object. Could reader implementers comment on this?
     
    I think introducing an additional filter like ASCII85 just for spotting the
    end of stream adds unnecessary complexity to both writer and reader,
    increases file sizes and also requires more memory and processing as the
    stream cannot be passed directly to a decompressor.
     
        --- Kari ---

    -----Original Message-----
    From: Carl Kugler [mailto:kugler@us.ibm.com]
    Sent: Wednesday, March 05, 2003 10:50 AM
    Cc: ifx@pwg.org
    Subject: RE: IFX> PDF/is Issue.

    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@crt.xerox.com>
    Sent by: owner-ifx@pwg.org

    03/05/2003 05:00 AM

            
            To: "'Rick Seeler'" <rseeler@adobe.com>, ifx@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@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@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...
      
    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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