сб, 23 февр. 2019 г. в 10:45, Eli Zaretskii <eliz@gnu.org>:

> > From: Evgeny Zajcev <lg.zevlg@gmail.com>
> > Date: Sat, 23 Feb 2019 01:01:02 +0300
> >
> > Currently, when SVG image is loaded with `svg_load` it first examines
> :file value and only if :file is missing it
> > looks for :data, using current buffer's filename (if any) as base_uri
> >
> > Would not it be better to check for :data first and if present, then use
> :file as value for base_uri?  Falling back
> > to current buffer filename in case :file is missing
> >
> > Unfortunately embedding raster images into svg with base64 encoding,
> using `svg-embed` is slow.
>
> I don't think I understand what you are saying here.  The test whether
> an image spec specifies a file or not is just 2 lines of C:
>
>
Ah, sorry, let me explain in more details.  SVG has ability embedding
raster images using "image" node with "xlink:href" attribute pointing to
the filename to embed.  rsvg library disallows absolute path names in
"xlin:href" due to security reasons.  To refer the filename with rsvg you
need to setup base_filename (aka base_uri) and then point to the file
relatively to this base_filename.  If you load .svg from filename, then
this filename is used as base_filename, but if you load SVG from memory you
need to specify base_uri explicitly.

`svg-embed` from svg.el does not bother with base_uri stuff and just embeds
raster images using base64 trick: it reads content of the file, encode it
with base64 and inserts encoded data into "image" node.  This is relative
slow thing to do.

To suppress this base64 trick, I want "xlink:href" to point directly to
filename, so I need some way to explicitly specify base_uri for rsvg to
use.  svg_load() from "image.c" uses `:file` as base_uri if loading from
filename, and current buffer's filename if loading from memory via `:data`.

I want to load svg `:data` from memory and specify base_uri, using image's
`:file` attribute.  This is not possible right now because `:file`
attribute is examined first in svg_load() function


  /* If IMG->spec specifies a file name, create a non-file spec from it.  */
>   file_name = image_spec_value (img->spec, QCfile, NULL);
>   if (STRINGP (file_name))
>     {
>       [load an image from file...]
>     }
>   /* Else its not a file, it's a lisp object.  Load the image from a
>      lisp object rather than a file.  */
>   else
>     {
>       [load an image from data...]
>     }
>
> And image_spec_value just walks a Lisp list looking for ':file':
>
>   static Lisp_Object
>   image_spec_value (Lisp_Object spec, Lisp_Object key, bool *found)
>   {
>     Lisp_Object tail;
>
>     eassert (valid_image_p (spec));
>
>     for (tail = XCDR (spec);
>          CONSP (tail) && CONSP (XCDR (tail));
>          tail = XCDR (XCDR (tail)))
>       {
>         if (EQ (XCAR (tail), key))
>           {
>             if (found)
>               *found = 1;
>             return XCAR (XCDR (tail));
>           }
>       }
>
> So I don't see how reversing the order would speed up SVG loading.
> What did I miss?
>


-- 
lg