Re: RFC: Syntax for page numbers in file: links?

[Carsten Dominik <carsten.dominik@gmail.com>]

On Jan 3, 2010, at 1:42 AM, Jan Böcker wrote:

> Hello List,
>
> During the discussion in the "protocol for PDFs?" thread, I realized
> that docview: links are an almost exact copy of file: links, the only
> single difference being that file: takes a line number and docview:
> links take a page number.
>
> Up until now, (docview:) links to PDF files have been the only link  
> type
> known to me which took a page number. With the upcoming integration of
> Xournal and Org that Daniel M. German is working on, there will  
> already
> be two of those.
>
> Daniel also wants to add org-store-link functionality to evince, so he
> needs a link type with page numbers for PDFs, too. I could (and have  
> in
> a proof-of-concept version) modify org-docview.el to open PDFs with an
> external viewer, but I think what really should happen is the  
> following:
>
> - We need a syntax to specify an (optional) page number for file:
>  links, and make the line number optional as well.
>  This might look something like this:
>
>  - file:<path>[:page][::line]
>  - file:~/some-file.txt::23    some-file.txt, line 23
>  - file:~/document.pdf:4       document.pdf, page 4
>  - file:~/document.odf:3::5    document.odf, page 3, line 5
>
>  This is only what comes to my mind immediately about one possibility
>  of encoding this.
>
> - org-docview-store-link should return links in the new syntax
>  (e.g. file:~/some-doc.pdf:5 instead of docview:~/some-doc.pdf::5)

Yes, I think that would make sense.

> - the same syntax could also be used to integrate evince and Xournal

I have yesterday implemented modifies for file link:

file+sys: now forces opening a file with the system's open command
file+emacs: forces opening in Emacs.
I guess it would make sense to make more of these, so that
one could select a specific viewer for selected files, what
do you think?


>
> - org-file-apps should allow to specify how to pass a page number to
>  an external program. Unlike the file name, this is an optional
>  argument, as a link may not specify a page number at all.
>
>  I do not know how to do this in an elegant way, maybe let
>  the user specify multiple entries - one for links with a page
>  number, one for links without.

This is not easy at all, but I am sure it can be done.

>
> - the same should apply to line numbers - maybe someone will integrate
>  Org with Open Office or some heretic wants to open all text files in
>  zile, who knows?

Line numbers is a rotten concept - I am not even sure we
should support it at all.

>
> - if a link with a page number is opened, org-file-apps specifies
>  "Visit with Emacs", and emacs decides to open it in doc-view-mode,
>  doc-view-goto-page should be called with the page number. (I imagine
>  this would be possible using the right hooks?)

You need to do this in the org-docview code.

>
> - Line number and page number also seem somewhat arbitrary as the only
>  pieces of data available to point to a specific location inside a
>  document. There are HTML fragment identifiers, which AFAIK work in
>  http: links but do not in file: links.
>  One might also imagine specifying a precise position in a PDF
>  using a line number and a percentage to indicate how far the
>  page should be scrolled down.
>
>  So if it is feasible at all, we might want an even more general  
> syntax
>  to specify a "target location", maybe attributes such as
>  [[file:/path/to/file][page=5,line=3][Description]]
>  [[file:notes.txt][search="The quick brown fox"][Description]]
>
>  This may obviously create more problems with backward compatibility.

I am not sure if it makes sense to handle more that a page number,  
really.


> Luckily, there has been no release featuring docview: links yet, so
> there still is a chance to correct this mistake without breaking too  
> much.

Well, the next release is not far off....

> If there will be no special syntax for page numbers, we could also
> interpret the line number as a page number for PDF and Xournal files,

Yes, why not.

> but that number would still need to be passed to an external  
> application
> and cause doc-view-goto-page to be called when visiting a PDF in  
> emacs.
>
> I really do not want to create another file: link type with
> org-docview.el, which is why I ask y'all to comment!
>
> - Jan
>
>
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- Carsten