Imposition Aside...

I got involved in another PDF project the other day...  This involves imposition.  However, there were a few catches.

First off, the imposition itself was fairly simply - single sheet front and back.  Unfortunately the imposition has to occur over a stream of front/back sheets, i.e., the underlying sheets change.  Secondly pages to impose are chosen not by sequence but by bookmark - and bookmarks can also have an indicator which says to leave that position blank.

So basically you have a stream of documents to impose:

  DOCID_01 DOCID_03 DOCID_99 DOCID_12

And so on.  The blanks can appear like this:


  DOCID_01 blank DOCID_03 DOCID_99 DOCID_12 blank

where "blank" is a special document ID meaning skip document pages here (pages per document is constant over the run).

Other than that we're basically dealing with cell positions for each page of the document.

I used our pdfExpressXM software as a platform for this.  It supports the multi-stream imposition (though I had to fix bug relating to relocating resources).  Internally the imposition model looks like this:

    item2={source2=+1,clip={0,0,792,612},ctm=    { 0, -1, 1, 0, 41.04,     1431 }}
    item4={source2=+3,clip={0,0,792,612},ctm=    { 0, -1, 1, 0, 693.36,    1431 }}
    item6={source2=+5,clip={0,0,792,612},ctm=    { 0, -1, 1, 0, 40.04,     1143 }}
    item8={source2=+7,clip={0,0,792,612},ctm=    { 0, -1, 1, 0, 693.364,   1143 }}
    item10={source2=+9,clip={0,0,792,612},ctm=   { 0, -1, 1, 0, 40.04,      855 }}
    item12={source2=+11,clip={0,0,792,612},ctm=  { 0, -1, 1, 0, 693.364,    855 }}
    item14={source2=+13,clip={0,0,792,612},ctm=  { 0, -1, 1, 0, 40.04,      567 }}
    item16={source2=+15,clip={0,0,792,612},ctm=  { 0, -1, 1, 0, 693.364,    567 }}
    item18={source2=+17,clip={0,0,792,612},ctm=  { 0, -1, 1, 0, 40.04,      279 }}
    item20={source2=+19,clip={0,0,792,612},ctm=  { 0, -1, 1, 0, 693.364,    279 }}

    item1={source2=+0,clip={0,0,792,612},ctm=    { 0, 1, -1, 0, 1305.36,   1157.4}}
    item3={source2=+2,clip={0,0,792,612},ctm=    { 0, 1, -1, 0, 653.04,  1157.4}}
    item5={source2=+4,clip={0,0,792,612},ctm=    { 0, 1, -1, 0, 1305.36,    869.4}}
    item7={source2=+6,clip={0,0,792,612},ctm=    { 0, 1, -1, 0, 653.04,   869.4}}
    item9={source2=+8,clip={0,0,792,612},ctm=    { 0, 1, -1, 0, 1305.36,    581.4}}
    item11={source2=+10,clip={0,0,792,612},ctm=  { 0, 1, -1, 0, 653.04,   581.4}}
    item13={source2=+12,clip={0,0,792,612},ctm=  { 0, 1, -1, 0, 1305.36,    293.4}}
    item15={source2=+14,clip={0,0,792,612},ctm=  { 0, 1, -1, 0, 653.04,   293.4}}
    item17={source2=+16,clip={0,0,792,612},ctm=  { 0, 1, -1, 0, 1305.36,      5.4}}
    item19={source2=+18,clip={0,0,792,612},ctm=  { 0, 1, -1, 0, 653.04,     5.4}}

    item21={source1=+0,clip={0,0,1440,1345.68},ctm={1,0,0,1,0,0}}
    item22={source1=+1,clip={0,0,1440,1345.68},ctm={1,0,0,1,0,0}}

There are two "sources" defined source1 and source2.  source1 is the stream of backgrounds and source2 is the stream of items to impose.  Each itemn entry defines a location where an imposed item is placed.  A clip and CTM (transformation matrix) is also supplied. 

There is also a way to specify the cycle for each input stream - cycle being the number of pages to step each time (items are offset, e.g.,  "source1=+2", from the current page in the cycle).

The interesting part of this is that the stream of imposed pages is "virtual", i.e., defined by the document id stream I described above.

The internal architecture decodes the documents ID's into a stream of pages.  The XM architecture is defined such that page number that are out of range may occur in the input page stream.  When such an occurrence is found a blank is produced.  So the document ID stream gets converted to something like:

  5 6 99999999 99999999 11 12 1 2

Where 99999999 basically causes a blank page to be produced.