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Thumbnailing...
(Gnome Environment)

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Thumbnail Cache filename

To convert a filename into the thumbnail cache filename you
need to convert the md5 checksum of its URI, then append ".png"

For example..
  file="/home/anthony/t.txt"
  file_uri="file://$file"
  iconname=`echo -n "file://$file" | md5sum - | sed 's/ .*/.png/'`
  iconfile="$HOME/.cache/thumbnails/normal/$icon"

If the icon generation filed a special file is placed in
  $HOME/.cache/thumbnails/fail/{factory}/$icon

For gnome nautilus the factory is "gnome-thumbnail-factory" which is why
clearing the ".thumbnails" directory and restarting nautilus completely
between each text can be important.

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Nautilus (Gnome-3) Thumbnailers

export XDG_DATA_HOME="$HOME/.local/share/"    # default value
export XDG_DATA_DIRS="$XDG_DATA_HOME:/usr/local/share/:/usr/share/"

Create a file "$XDG_DATA_HOME/thumbnailers/text.thumbnailer"

See XDG Base Directory Specification
  https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

=======8<--------
[Thumbnailer Entry]
MimeType=text/plain;text/x-log;text/x-log;
Exec=bin/thumbnailers/text %i %o
=======8<--------

The command is executed from the launch directory of nautilus (your home)
as such can be made relative to that home.

The "[Thumbnailer Entry]" and the ".thumbnailer" suffix IS REQUIRED.

Note using %i and %o escapes you get plain and simple filenames for the
thumbnail conversion making it much simplier to create your own thumbnailers.

%o typically points to Hashed image filenames in the directory...
    "$XDG_CACHE_HOME/thumbnails/normal"
where  XDG_CACHE_HOME  defaults to  "$HOME/.cache"
so the path is normally..
    "$HOME/.cache/thumbnails/normal"

See above if you want to know the hashing method, though not normally needed.

For example this "image.thumbnailer" uses the ImageMagick "convert" command
directly to generate a fairly sophisticated image resized thumbnail with
a border and padding.

=======8<--------
[Thumbnailer Entry]
MimeType=MimeType=image/gif;image/jpeg;image/png;image/x-xpixmap;image/x-xbitmap;image/vnd.microsoft.icon;
Exec=/bin/convert %i'[0]' -thumbnail 46x46\> -border 1x1 -gravity center -background white -extent 48x48  png:%o
=======8<--------

Remember "convert" can basically handle just about ANY image file format!

But as you can see a dedicated script would probably work better due to the
complexity of a thumbnail generator.  In fact my own image thumbnailer (based
on the above) is rather more complex with better handling of transparency.

For more information onusing the ImageMagick convert command to create
different styles of thumbnails see...
   http://www.imagemagick.org/Usage/thumbnails/


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Nautilus (Gnome-2) Thumbnailing...

Install schema for the thumbnailer...

  gconftool-2 --install-schema-file \
     $HOME/bin/gnome_thumbnails/text-thumbs.schemas

This installs multiple mime entries like...
=======8<--------CUT HERE----------
<!--
   Declare my personal text/plain mimetype thumbnailer to the gnome system,
   so that nautilus can thumbnail these files.

   text-thumbs {file-uri} {thumbnail-uri}
-->
<gconfschemafile>
<!-- mimetype: text/plain -->
  <schemalist>
    <schema>
      <key>/schemas/desktop/gnome/thumbnailers/text@plain/enable</key>
      <applyto>/desktop/gnome/thumbnailers/text@plain/enable</applyto>
      <owner>text-thumbs</owner>
      <type>bool</type>
      <default>true</default>
      <locale name="C">
        <short></short>
        <long></long>
      </locale>
    </schema>
    <schema>
      <key>/schemas/desktop/gnome/thumbnailers/text@plain/command</key>
      <applyto>/desktop/gnome/thumbnailers/text@plain/command</applyto>
      <owner>text-thumbs</owner>
      <type>string</type>
      <default>/home/anthony/bin/gnome_thumbnails/text-thumbs %u %o</default>
      <locale name="C">
        <short></short>
        <long></long>
      </locale>
    </schema>
  </schemalist>
</gconfschemafile>

=======8<--------CUT HERE----------

The "text-thumbs" program then generates PNG thumbnail using the given URIs.
For this script decodes the URI's and calls a "text" thumbnailer script.
Note the output file is PNG but may not have a 'png' suffix.

See also  "Gnome.hints" on the Thumbnail cache

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Thunar 1.2 (DBus) Thumbnailer

For standard non generated icons the theme can be set by creating a
"~/.gtkrc-2.0" file with the line giving the primary theme name.
  gtk-icon-theme-name="Tango"
Note the theme will still fall bacj to "hicolor" theme regardless.

For generated thumbnails you need a DBus thumbnailer
in  XDG_DATA_DIRS/dbus-1/services a file needs to have something like...

=======8<--------
[D-BUS Service]
Name=org.freedesktop.thumbnails.Thumbnailer1
Exec=your-thumbnailerd
=======8<--------

Adding a cache interface is optional: "org.freedesktop.thumbnails.Cache1"

The application Tumler provides this.
See  http://live.gnome.org/ThumbnailerSpec

It points the "Exec=" entry to the executable...
  /usr/lib/tumbler-1/tumblerd


User defined thumbnailers are currently not available.

Is there some other DBus thumbnailer that can be used instead of "tumbler"?
One that allows extra or overriding, user defined thumbnail generators,
just as Gnome allows?

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