bug#58041: [PATCH] docview: Use svg images when using mupdf for conversion

[Visuwesh]

[வியாழன் ஜனவரி 12, 2023] Stefan Monnier via "Bug reports for GNU Emacs, the 
Swiss army knife of text editors" wrote:

>> --- a/lisp/doc-view.el
>> +++ b/lisp/doc-view.el
>> @@ -921,7 +921,7 @@ doc-view-shrink-factor
>>  (defun doc-view-enlarge (factor)
>>    "Enlarge the document by FACTOR."
>>    (interactive (list doc-view-shrink-factor))
>> -  (if doc-view-scale-internally
>> +  (if (or doc-view-scale-internally doc-view-mupdf-use-svg)
>>        (let ((new (ceiling (* factor doc-view-image-width))))
>>          (unless (equal new doc-view-image-width)
>>            (setq-local doc-view-image-width new)
>> @@ -941,7 +941,7 @@ doc-view-shrink
>>  (defun doc-view-scale-reset ()
>>    "Reset the document size/zoom level to the initial one."
>>    (interactive)
>> -  (if doc-view-scale-internally
>> +  (if (or doc-view-scale-internally doc-view-mupdf-use-svg)
>>        (progn
>>      (kill-local-variable 'doc-view-image-width)
>>      (doc-view-insert-image
>
> Hmm.... `doc-view-mupdf-use-svg` means "use SVG when the backend
> is mupdf" but we don't know here whether the backend is mupdf, so this
> will misfire when using something else than mupdf, no?

Ah yes, of course.  Somehow, I managed to completely forget about the
other file formats supported by doc-view like djvu (since I only use
file formats that use mupdf in the end).  How about the below revised
patch?

0001-Use-internal-image-scaling-when-using-SVG-images-in-.patch
Description: Text Data

This does the expected on my end with doc-view-scale-internally to nil:
when visiting a djvu file and zooming in, it regenerates the images;
when visiting a docx file and zooming in, it modifies the :width image
property (i.e., no regeneration).

>>> I wasn't thinking of duplicating the code, but of rethinking the naming
>>> a bit.  I think what we meant by "pdf->png" is actually the process that
>>> extracts pages (which just happened to use the PNG format and now can
>>> also use the SVG format).
>
>> Indeed, it is a misleading name.  This change will have to go to master,
>> I believe?  I have to look around a bit more to see where the function
>> is being used.
>> There's also the fact that there's more than one more program that can
>> generate SVG files (as Gregory pointed out in this thread) so it might
>> be nice to have pdf->png and pdf->svg "function variables" and a "super
>> function" that actually does the job.  Hopefully, this will allow to
>> fall back gracefully to PNG if SVG generation is faulty.
>
> Indeed, I think there's some cleanup/orthogonalization in order here.

When I have more time and familiarity, I will try to give this a shot.

>         Stefan