In gnome_desktop_thumbnail_factory_generate_no_script, we have:

  /* Use a registered thumbnail generator */
  pixbuf = gth_hook_invoke_get ("generate-thumbnail", (char *) uri, mime_type, size);

  /* Fall back to gdk-pixbuf */
  if (pixbuf == NULL)
    pixbuf = _gdk_pixbuf_new_from_uri_at_scale (uri, size, size, TRUE);


_gdk_pixbuf_new_from_uri_at_scale checks for gvfs preview::icons, and then reads the whole file. This function should be split into two, so we have:

pixbuf = _gdk_pixbuf_new_from_preview_icon_at_scale
if (pixbuf == NULL) pixbuf = gth_hook_invoke_get
if (pixbuf == NULL) _gdk_pixbuf_new_from_full_file_at_scale

In other words, check for preview::icon first (from the gphoto backend, for example), then use exiv2 to read the embedded thumbnail, and lastly try the whole file.

This is important for the importer - the gphoto gvfs backend provides the fastest access to the thumbnails (much faster than exiv2).

- Mike