Ubuntu – How to generate thumbnails in advance on KDE / Dolphin

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Revisiting this Q&A about pre-generating thumbnails on Nautilus/GNOME I recently had to discover that none of the scripts posted there would work on KDE Plasma 5 / in the Dolphin file manager.

Is there any way I can generate thumbnails in advance on KDE, without having to manually browse through each and every folder?

Best Answer

Introduction

So it turns out that KDE and GNOME now follow slightly different thumbnail naming and metadata conventions. This is quite unfortunate since issues like these were supposed to be eliminated with the work of the Free Standards Group.

I've filed a bug report with KDE that will hopefully get this addressed eventually, but for now thumbnails generated by KDE and GNOME are sadly incompatible with each other.

Thumbnailer script to bridge the KDE/GNOME gap

In order to work around this incompatibility, I ended up modifying the Python script that James Henstridge posted in the Q&A linked to above. The main change I implemented is a function that updates the generated thumbnails to be recognized by KDE (by renaming them and updating the PNG metadata chunk).

Here is the aforementioned script in its current revision:

#!/usr/bin/python3

"""
Thumbnail generator for KDE/GNOME

Largely based on a script by James Henstridge
(https://askubuntu.com/a/201997)

Unfortunately there seems to be some disagreement between GNOME and KDE
towards how to follow the XDG specs for saving thumbnails. This script
is meant as a workaround to that issue, generating thumbnails that follow
both specifications.

Dependencies: python3 gir1.2-gnomedesktop-3.0 python-pillow

pillow can be installed with `pip install pillow`

You will also need to have the corresponding thumbnailers installed (e.g.
evince-thumbnailer). KDE thumbnailers are not supported. All previews are
generated through GNOME's thumbnail factory and then made compatible with KDE.

Further references:

Thumbnail specifications in KDE/GNOME:

- https://bugs.kde.org/show_bug.cgi?id=393015
- https://api.kde.org/frameworks/kio/html/previewjob_8cpp_source.html
- https://lazka.github.io/pgi-docs/GnomeDesktop-3.0/classes/DesktopThumbnailFactory.html

Setting PNG metadata:

- http://pillow.readthedocs.io/en/5.1.x/PIL.html#PIL.PngImagePlugin.PngInfo
- https://stackoverflow.com/a/10552742/1708932

Copyright: (c) 2012 James Henstridge <https://launchpad.net/~jamesh>
           (c) 2018 Glutanimate <https://glutanimate.com/>
License: MIT license
"""

import os
import sys
from hashlib import md5

from PIL import Image
from PIL import PngImagePlugin

import gi
gi.require_version('GnomeDesktop', '3.0')
from gi.repository import Gio, GnomeDesktop

# FIXME: Hardcoding the Thumbnailer to a generic name
#        regardless of MIME type might not always work
KDE_THUMBNAILER = "KDE Thumbnail Generator"


def update_name_and_meta(thumb_path, filename, mtime, mime_type, size):
    print("Making thumb compatible with KDE...")
    abs_path = os.path.abspath(filename)
    # The spaces in our URI are not escaped. This is not in accordance
    # with the URI RFC2396 which is listed in the freedesktop specs,
    # but it's what KDE currently uses 
    # (cf.: https://bugs.kde.org/show_bug.cgi?id=393015)
    kde_uri = "file://" + abs_path  
    kde_md5 = md5(kde_uri.encode("utf-8")).hexdigest()
    thumb_dir = os.path.dirname(thumb_path)
    kde_thumb_path = os.path.join(thumb_dir, kde_md5 + ".png")

    if os.path.exists(kde_thumb_path):
        print("KDE thumb already exists. Skipping")
        return

    im = Image.open(thumb_path)

    # Set PNG metadata chunk
    meta = PngImagePlugin.PngInfo()
    meta.add_itxt("Software", KDE_THUMBNAILER)
    meta.add_text("Thumb::MTime", str(int(mtime)))
    meta.add_text("Thumb::Mimetype", mime_type)
    meta.add_text("Thumb::Size", str(size))
    meta.add_itxt("Thumb::URI", kde_uri)

    im.save(kde_thumb_path, "png", pnginfo=meta)

    # uncomment this to remove GNOME thumbnails:
    # os.remove(thumb_path)


def make_thumbnail(factory, filename):
    mtime = os.path.getmtime(filename)
    # Use Gio to determine the URI and mime type
    f = Gio.file_new_for_path(filename)
    uri = f.get_uri()
    info = f.query_info(
        'standard::content-type', Gio.FileQueryInfoFlags.NONE, None)
    mime_type = info.get_content_type()
    size = info.get_size()

    if factory.lookup(uri, mtime) is not None:
        print("FRESH       %s" % uri)
        return False

    if not factory.can_thumbnail(uri, mime_type, mtime):
        print("UNSUPPORTED %s" % uri)
        return False

    thumbnail = factory.generate_thumbnail(uri, mime_type)
    if thumbnail is None:
        print("ERROR       %s" % uri)
        return False

    factory.save_thumbnail(thumbnail, uri, mtime)

    thumb_path = factory.lookup(uri, mtime)
    update_name_and_meta(thumb_path, filename, mtime, mime_type, size)

    print("OK          %s" % uri)

    return True


def thumbnail_folder(factory, folder):
    for dirpath, dirnames, filenames in os.walk(folder):
        for filename in filenames:
            make_thumbnail(factory, os.path.join(dirpath, filename))


def main(argv):
    factory = GnomeDesktop.DesktopThumbnailFactory()
    for filename in argv[1:]:
        if os.path.isdir(filename):
            thumbnail_folder(factory, filename)
        else:
            make_thumbnail(factory, filename)


if __name__ == '__main__':
    sys.exit(main(sys.argv))

Installation

Copy and paste the code section above into a new file, choose a fitting name for it (e.g. thumbnailer), and mark it executable.

Dependencies

For the script to work properly you will need to have the python bindings for GNOME installed. The script also depends on Python's pillow library which may be installed via pip.

The following commands should take care of all dependencies:

sudo apt install gir1.2-gnomedesktop-3.0 python3-pip
pip3 install pillow

Thumbnails are first generated through GNOME's thumbnail factory and then made compatible with KDE. So you will still need to have all the corresponding GNOME thumbnailer modules installed. KDE's own thumbnailers are not supported. E.g.: for the script to support generating PDF thumbnails you would have to install evince.

(I would have loved to use KDE's python bindings directly, but it looks like both pykde4 and pykde5 have been abandoned for years).

Usage

The general use is the same as with any other thumbnailing script. Just invoke it with the files or folders where you want to generate thumbnails as arguments, e.g.:

thumbnailer /home/Documents

References

Thumbnail specifications in KDE/GNOME:

Setting PNG metadata:

Related Solutions

Ubuntu – How to run Dolphin instead of Nautilus

There are several caveats in what you tried to do. I already mentioned the danger introduced by your approach:

Next time nautilus is going to be updated, your dolphin gets overwritten (as your link points there). Same goes for gnome-terminal.

So we figured, this was not a good idea :)

But there are some ways to try working around, so "x" gets run when "z" was requested -- but I'm not aware of any as soon not "z", but "/full/path/to/z" gets called. As long as it is just "z":

creating an alias for z, like alias z=x (works on a per-user-level -- or globally, depending on where it was defined)
creating a "replacement" for z in a location mentioned in the PATH before the location the real z resides in

A little more details on the second approach. Taking your original problem, you want to have dolphin executed whenever nautilus is called upon. You already found nautilus at /usr/bin/nautilus. Now let's (probably correctly) assume your $PATH contains (in this order) /usr/local/bin:/usr/bin -- so you see /usr/local/bin would be searched before /usr/bin. So we simply create a shell script /usr/local/bin/nautilus with the following content:

#!/bin/bash
/usr/bin/dolphin %$@

So what will happen? If you (or some script/program/daemon/...) invokes nautilus, this will execute /usr/local/bin/nautilus (as this is the first "nautilus" found in the PATH), which simply starts /usr/bin/dolphin -- voila! But if the "whatever" uses the full path, this won't work.

So you say: Hey, why didn't Izzy say "just do a ln -s /usr/bin/dolphin /usr/local/bin/nautilus?" Sure you can do that -- and it will work the same. But using a script as shown may come in handy if you need to introduce additional parameters which are not passed with the original call. With above script, dolphin simply gets passed the same parameters the original call used (%$@). But you can play around with things in the script, replace parameters, etc. As for your current problem, the link would be enough (as long as nautilus doesn't get called with the full path).

Ubuntu – How to instruct Nautilus to pre-generate thumbnails

Nautilus's thumbnailing routines actually come from the libgnome-desktop library, so it is possible to run the same thumbnailers outside of the file manager.

The API is a little complex, but the following Python script should help:

#!/usr/bin/python
import os
import sys

from gi.repository import Gio, GnomeDesktop

def make_thumbnail(factory, filename):
    mtime = os.path.getmtime(filename)
    # Use Gio to determine the URI and mime type
    f = Gio.file_new_for_path(filename)
    uri = f.get_uri()
    info = f.query_info(
        'standard::content-type', Gio.FileQueryInfoFlags.NONE, None)
    mime_type = info.get_content_type()

    if factory.lookup(uri, mtime) is not None:
        print "FRESH       %s" % uri
        return False

    if not factory.can_thumbnail(uri, mime_type, mtime):
        print "UNSUPPORTED %s" % uri
        return False

    thumbnail = factory.generate_thumbnail(uri, mime_type)
    if thumbnail is None:
        print "ERROR       %s" % uri
        return False

    print "OK          %s" % uri
    factory.save_thumbnail(thumbnail, uri, mtime)
    return True

def thumbnail_folder(factory, folder):
    for dirpath, dirnames, filenames in os.walk(folder):
        for filename in filenames:
            make_thumbnail(factory, os.path.join(dirpath, filename))

def main(argv):
    factory = GnomeDesktop.DesktopThumbnailFactory()
    for filename in argv[1:]:
        if os.path.isdir(filename):
            thumbnail_folder(factory, filename)
        else:
            make_thumbnail(factory, filename)

if __name__ == '__main__':
    sys.exit(main(sys.argv))

Save this to a file and mark it executable. You may also need to install the gir1.2-gnomedesktop-3.0 package if it is not already installed.

After that, simply invoke the script with the files or folders you want to thumbnail as arguments. Thumbnails will be saved to ~/.thumbnails where applications like Nautilus expect to find them.