Mozc

From the project's home page:

Mozc is a Japanese input method editor (IME) designed for multi-platform such as Android OS, Apple OS X, Chromium OS, GNU/Linux and Microsoft Windows. This OpenSource project originates from Google Japanese Input.

The differences between Mozc and Google Japanese Input are described in detail at the project's About Branding page, but in short, Mozc's open-source code does not include Google's extensive word conversion tables (so called "dictionaries"), so its conversion quality is not equivalent to that of Google's branded product. This can be mostly mitigated with custom dictionaries though (see below).

Installation

Split vs. integrated packages

As an IME, Mozc has two distinct parts: a server, which does all the work of the conversion, and a set of modules which allow the server to communicate with the system's input method framework and output the result on the screen. A separate module is required for each of IBus, Fcitx5, Fcitx and Emacs.

Some Mozc packages contain only the IMF-specific modules and split the server into a separate mozc^AUR package; this allows for the different modules to be installed side by side, independently from the server itself (which can be useful in a multi-user setup where different users use different IMFs, or when using Emacs).

Other Mozc packages forego this split and instead integrate the server together with the modules. This does have its advantages, but one very notable disadvantage is that it makes it impossible to install additional modules due to file conflicts (because more than one package are competing to provide the server's files).

If for any reason you will be needing more than one IMF module installed at the same time, the solution is to either build the non-split packages yourself and manually resolve the file conflicts, or to only choose from among the packages that split the server from the modules.

The UT Dictionary

As already mentioned, Mozc's conversion quality is not quite as good as that of Google Japanese Input because it does not include Google's extensive word conversion tables (so called "dictionaries"). A solution exists for that in the form of the UT dictionary, which is a third-party dictionary that enhances Mozc's conversion quality and brings it closer to Google Japanese Input. It achieves that by including thousands of additional words aggregated from several popular online sources (based on page rankings from Google, Yahoo and Wikipedia) and also by integrating a variety of other specialized sources such as the NEologd dictionary, which in the words of its creator contains "neologisms (new words), which are extracted from many language resources on the Web".

Tip: Use of the UT dictionary is optional but highly recommended.

Summary

Install one or more of fcitx5-mozc-ut^AUR, fcitx-mozc-ut^AUR, ibus-mozc^AUR and emacs-mozc^AUR.

When you are asked which flavor of Mozc you wish to use, select either mozc^AUR for the vanilla Mozc or mozc-ut^AUR for the UT-enhanced Mozc.

Tip: If you wish to use the integrated packages you can instead install fcitx5-mozc or fcitx-mozc (both of them are based on the vanilla Mozc).

Note: Once Mozc is installed, you may need to restart X or your input method framework before you can use it.

Configuration

IBus

See IBus for more information.

Tip: When used with IBus, by default Mozc will launch in Direct (Eisu) input mode, which can be problematic on keyboards that lack the Eisu key. To change this behavior and make Mozc launch in Hiragana input mode instead, edit ~/.config/mozc/ibus_config.textproto and append the following line:

~/.config/mozc/ibus_config.textproto

...
}
active_on_launch: True

Tip: On Gnome, IBus is integrated with the System Settings so there is no need to run a separate app. Simply go to System Settings → Keyboard and add the Mozc layout there. You can then disregard the following instructions.

Open the IBus configuration tool by running:

$ ibus-setup

In the Input Method tab, click on 'Add', and then search for and add the Mozc layout.

You can switch to the new Mozc layout with Alt+Shift_L (as per the IBus default).

Fcitx5

See Fcitx5 for more information.

Tip: On KDE, Fcitx5 is integrated with the System Settings so there is no need to run a separate app. Simply go to System Settings → Regional Settings → Input Method

Open the Fcitx5 configuration tool by running:

$ fcitx5-configtool

In the Input Method tab, find Mozc on the list of available layouts to the right and add it to the list of active layouts by clicking the leftward arrow. You can then switch to the Mozc layout with the standard keyboard shortcuts for switching between layouts.

Emacs

You can use mozc.el (mozc-mode) to input Japanese via LEIM (Library of Emacs Input Method). To use mozc-mode, write the following into your .emacs.d/init.el or some other file for Emacs customizing:

(require 'mozc)  ; or (load-file "/path/to/mozc.el")
(setq default-input-method "japanese-mozc")

mozc.el provides "overlay" mode in the styles of showing candidates (from mozc r77) which shows a candidate window in box style close to the point. If you want to use it by default, add the following:

(setq mozc-candidate-style 'overlay)

C-\ (toggle-input-method) enables and disables use of mozc-mode.

Disabling XIM

When you are using input method on your desktop and assigning activation/deactivation of input method to C-SPC, you will be not able to use C-SPC/C-@ as set-mark-command on Emacs. To avoid this problem, add the following into your ~/.Xresources or ~/.Xdefaults. xim will be disabled on Emacs.

Emacs*UseXIM: false

Tips and tricks

Confirming Mozc version which you are using now

Type "ばーじょん" ("version") and convert it while activating Mozc. The version number of Mozc will be shown in the candidate list like follows:

ばーじょん

バージョン
ヴァージョン
ばーじょん
Mozc-1.6.1187.102  ⇐ Current version of Mozc
...

Launching Mozc tools from command line

The followings are commands to launch Mozc tools.

Mozc Settings: $ /usr/lib/mozc/mozc_tool --mode=config_dialog
Mozc Dictionary Tool: $ /usr/lib/mozc/mozc_tool --mode=dictionary_tool
Mozc Word Register: $ /usr/lib/mozc/mozc_tool --mode=word_register_dialog

Use CapsLock as Eisu_toggle key on ASCII layout keyboard

In all of the preset keymap styles of Mozc, the command Toggle alphanumeric mode on Composition mode is assigned to the Eisu (Eisu_toggle), Hiragana/Katakana or Muhenkan key, but the ASCII keyboard has none of them.

One solution for it is to use Caps Lock key as Eisu_toggle (Mozc does not recognize the Caps Lock key as of r124). The following is a way to assign Eisu_toggle to Caps Lock (without any modifier keys) and Caps_Lock to Shift+CapsLock, as on the OADG keyboard layout.

Warning: This will affect all applications.

Edit ~/.Xmodmap as follows:

keycode 66 = Eisu_toggle Caps_Lock
clear Lock

Then, restart X or run xmodmap to apply the changes immediately:

$ xmodmap ~/.Xmodmap

Use underlying Japanese keymap on Non-English/Non-Japanese systems

If you are primarily using a Non-English or Non-Japanese system with a keyboard layout different than QWERTY (e.g. German) and only want to use Japanese as a secondary input language, the key mapping might be based on this main keyboard language. This might be odd, especially if migrating from Windows or MacOS or directly from a Japanese computer.

To change the underlying keyboard layout on Ibus, change the default keymap in ~/.config/mozc/ibus_config.textproto by setting the layout key to Japanese (or some other keyboard with QWERTY):

~/.config/mozc/ibus_config.textproto

engines {
...
  layout : "jp"
...
}

Troubleshooting

Building Mozc fails (process is killed)

If the build process fails with an error message like the following:

...
/bin/sh: line 1:  xxxx killed
...
make: *** [xxx/xxx...] error 137
...

Make sure you have not run out of memory.

New version of Mozc does not appear though I upgraded Mozc and restarted X or Input Method Framework (not rebooted)

The old version of Mozc may be still on your memory. Try to kill the existing mozc_server process:

$ killall mozc_server

mozc_server becomes defunct

Mozc cannot run in root. Start X in normal user.

mozc_emacs_helper not found in emacs

When installing mozc.el you need to install a helper program called mozc_emacs_helper.

You need to install emacs-mozc^AUR for this helper program.

mozc tool(s) does not launch

It is also possible to launch Mozc tools with log sent directly to console, which is useful for debugging. To enable this, simply append --logtostderr like this for example:

$ /usr/lib/mozc/mozc_tool --mode=config_dialog --logtostderr