Re: Problems with .PDFPIC caused by pdfinfo

[Keith Marshall]

Ref: https://savannah.gnu.org/bugs/index.php?55107

On 01/10/2021 01:10, Deri wrote:
> I did try to help Keith with this previously, but I was mildly "told
> off" (on list) for sending my help off list. I've learned my lesson.

Thanks, Deri.

IIRC, the reason for the "mild telling off" was that, by replying off
list, you denied us the potential benefit from other list members who
may have been willing to review the issue, and so contribute to the
debugging effort.  I am pleased that, on this occasion, you have kept
this on-list; even if the majority of list members aren't sufficiently
interested to assist, there may be some who will, and any assistance
will be gratefully accepted, and very much appreciated.

> I attach a couple of pdfs with which the current code has problems.
> 
> Picture.pdf
> 
> [derij@pip groff-psbb]$ ./psbb ../../Picture.pdf
> ../../Picture.pdf: bounding box = (0,0)..(0,0)

This is caused by the nested /Group dictionary, within the /Page object;
the current groff-psbb lexer is confused by it, and ends up in the wrong
state, when it eventually encounters the /MediaBox key.  Adding one more
rule (for "<<") to the PDF dictionary state scanning model gets us to:

   $ ./psbb Picture.pdf
   Picture.pdf: bounding box = (0,0)..(592,842)

> [derij@pip groff-psbb]$ pdfbb ../../Picture.pdf
> Processing '../../Picture.pdf'
> ../../Picture.pdf: CropBox: 162.085,623.346,340.825,716.546  (178.74,93.2)

The psbb lexer doesn't handle the /CropBox key.  Should it?  Should
/CropBox override any extant /MediaBox?

> croptest.pdf
> 
> [derij@pip groff-psbb]$ ./psbb ../../croptest.pdf
> psbb:t-psbb (t-psbb.cpp):193: PDF file '../../croptest.pdf' is
> malformed; no trailer found

Since croptest.pdf lacks both a trailer dictionary, and a free-standing
cross reference table, (both are hidden away within a /XRefStm object,
with a compressed cross reference table), croptest.pdf is _incompatible_
with applications which do not support this feature of PDF-1.5 (and
later).  The groff-psbb prototype implementation (currently) does not
offer this level of PDF-1.5 support; thus, this behaviour is expected.

> [derij@pip groff-psbb]$ pdfbb ../../croptest.pdf
> Processing '../../croptest.pdf'
> ../../croptest.pdf: MediaBox: 0,0,595,842  (595,842)

Well, this agrees with the result I've shown above, for Picture.pdf,
with groff-psbb modified to properly handle nested dictionaries; some
further (non-trivial) development effort will be required, to support
concealment of trailer dictionaries and cross reference tables within
/XRefStm objects.

-- 
Keith