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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> <TITLE>Message</TITLE> <META content="MSHTML 6.00.2715.400" name=GENERATOR></HEAD> <BODY> <DIV><SPAN class=280495411-05032003><FONT face=Arial color=#0000ff size=2>Rick,</FONT></SPAN></DIV> <DIV><SPAN class=280495411-05032003><FONT face=Arial color=#0000ff size=2>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.</FONT></SPAN></DIV> <DIV><SPAN class=280495411-05032003><FONT face=Arial color=#0000ff size=2>Pete</FONT></SPAN></DIV> <DIV> </DIV> <UL> <UL> <UL> <UL> <P><FONT face=Impact>Peter Zehler</FONT> <BR><FONT face=XeroxPeopleNet color=#ff0000>XEROX</FONT> <BR><FONT face=Tahoma size=2>Xerox Architecture Center</FONT> <BR><FONT face=Arial size=2>Email: PZehler@crt.xerox.com</FONT> <BR><FONT face=Arial color=#000000 size=2>Voice:    (585) 265-8755</FONT> <BR><FONT face=Arial color=#000000 size=2>FAX:      (585) 265-8871</FONT><FONT face=Arial size=2> </FONT><BR><FONT face=Arial size=2>US Mail: Peter Zehler</FONT> <UL> <P><FONT face=Arial size=2>        Xerox Corp.</FONT> <BR><FONT face=Arial size=2>        800 Phillips Rd.</FONT> <BR><FONT face=Arial size=2>        M/S 128-30E</FONT> <BR><FONT face=Arial size=2>        Webster NY, 14580-9701</FONT> </P></UL></UL></UL></UL></UL> <BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px"> <DIV class=OutlookMessageHeader dir=ltr align=left><FONT face=Tahoma size=2>-----Original Message-----<BR><B>From:</B> Rick Seeler [mailto:rseeler@adobe.com]<BR><B>Sent:</B> Tuesday, March 04, 2003 1:29 PM<BR><B>To:</B> ifx@pwg.org<BR><B>Subject:</B> IFX> PDF/is Issue.<BR><BR></FONT></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>During prototyping of PDF/is the following problem arose:</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2></FONT></SPAN> </DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>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.</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2></FONT></SPAN> </DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>An example will illustrate:</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>First, the normal case...</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2></FONT></SPAN> </DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>stream</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>sdljfiwefnwfubrevurewliysnhr;hgawebfz;h;uwre (lots of binary data here)....</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>84trhdvfyu7wgf4.nbdrgur4uaru4gb</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>endstream</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>12 0 obj</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>3456    <- the length of the previous stream.</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>endobj</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2></FONT></SPAN> </DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>But, what if the data looked like this...</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2></FONT></SPAN> </DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>stream</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>sdljfiwefnwfubrevurewliysnhr;hgawebfz;h;uwre (lots of binary data here)....</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003>endstream            <- the binary data could have a string of bytes that looked like this.</SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>84trhdvfyu7wgf4.nbdrgur4uaru4gb</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>endstream</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>12 0 obj</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>4567    <- the length of the previous stream.</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>endobj</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003></SPAN> </DIV> <DIV><SPAN class=358045117-04032003>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).</SPAN></DIV></FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2></FONT></SPAN> </DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>Possible solutions:</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>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.</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>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.</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>3) Offer a combination of 1 & 2.  The Producer would cache streams if possible, but may use ASCII85, if necessary.</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>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.</FONT></SPAN></DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2></FONT></SPAN> </DIV> <DIV><SPAN class=358045117-04032003><FONT face=Arial size=2>Any other ideas?  I'm personally leaning toward solution #3.</FONT></SPAN></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <P><FONT size=2>-Rick<BR></FONT></P> <DIV><FONT face=Arial size=2></FONT> </DIV></BLOCKQUOTE></BODY></HTML>