Re: Ghostscript/GhostPDL 9.22 Release Candidate 1

[Ken Sharp]

At 13:42 19/09/2017 +0200, David Kastrup wrote:

> So the mechanisms mostly out of our own control are Ghostscript in its
> ps2pdf facility, various TeX engines when including lots of
> ps2pdf-generated PDF files into a main document.

To me this is where the problem lies, PDF is good as a terminal document 
format, and that was its original aim. Its not good as an intermediate 
format, or for inclusion in more complex documents.

I feel the correct answer to this is not to use PDF as an intermediate 
format, it seem to me you should stick with a typesetting format because 
that allows you to determine that fonts which are named the same, are in 
fact the same, and you don't need to include them multiple times. In fact 
for a layout format, you wouldn't normally include the actual fonts at all, 
of course.


>   For this use case, we
> want a process that avoids excessive font duplication.  The process so
> far involved an additional Ghostscript run removing most of the
> duplicates from the TeX-generated PDF (someone please correct me if I
> got this wrong).

This only works because all the PDF files you are using (so far) embed the 
whole font, don't use subsets, and use the same Encoding (or use different 
names so that they are clearly different fonts). Were you to start using 
PDF files (from whatever source) where that is not the case, and I quoted 
OpenOffice as an example, then you might run into the problem with the bug 
you are exploiting.

By not using the PDF object number as a unique identifier, Ghostscript only 
uses the font name. If you get two different fonts (subset or otherwise) 
Ghostscript will assume they are the same font. If they are differently 
encoded (say that 'A' is encoded at position 0x42 in the first font, but 
0x42 in the second font has a 'B') then Ghostscript can't tell and will 
simply drop the second font.

The result of this is that you will get the wrong text in the output PDF 
file. Again, this isn't a theoretical problem, we have had numerous bug 
reports on this count which we have done our best to work around. In the 
end there was no alternative but to use the object number as the unique 
identifier (NB we actually use the object number and the filename, in case 
we get two files with the same font using the same object number....)

The only way you find out this has happened is when you carefully read the 
text, of course.


> We don't really have a way to forego Texinfo for our printed manuals.
> Given the comparative importance of TeX for document preparation,
> however, I think it would be good to figure out how to keep at least one
> viable way open of making this work and figure out a migration path of
> the involved tools to how you would optimally would want to have things
> working.
>
> I don't think that TeX can (or should) preserve object ids when
> including external PDF files, so figuring out some other reasonably
> robust identity associated with fonts would seem important.

Well I know nothing about TeX. It seems to me however, that it *must* 
preserve the object IDs in some sense, because otherwise you wouldn't be 
ending up with multiple copies of fonts. If it didn't preserve the object 
numbers, then it would assume that the first 'Times' is the same as the 
second 'Times' and would collapse them into a single reference. Exactly as 
you are using Ghostscript for at present.

If your PDF files contain ToUnicode CMaps then its possible to identify 
properly which glyph is actually intended by each character code in each 
font. Doing that would allow you to optimise the use of fonts, because you 
could alter the character coding of each usage so that it was consistent 
across the documents and only required a single instance of the font in 
question.

I'd have to experiment to find out, but it would nit surprise me to 
discover that when you include a PDF file in TeX what it actually does is 
convert it into an EPS or PostScript program and then concatenates all the 
documents together.

That would mean TeX could use PDF files as a kind of 'black box', and would 
mean that the fonts would be included multiple times, just as you say is 
happening.


> > PDF was never intended as a means of transferring, or 'containerising'
> > content, its not trivial (or even possible in general) to extract
> > content from, or simplify, PDF files.
>
> And yet I seem to remember Adobe has a specification for how to write
> PDF intended for embedding, haven't they?

Err, no, I don't think so. You can embed files untouched (including PDF 
files) inside a PDF, just as other file types. But that's not really what I 
meant when I said 'containerising'.

You can also have PDF Collections (I can't recall if that's the correct 
name) but again that isn't what I meant when I talk about transferring 
content, because you aren't transferring the content, you are including the 
whole thing, not just its content.

I was thinking more like writing a .docx file as an RTF or a spreadhseet as 
a comma separated file. Transferring the content without the associated 
container.



                    Ken