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BrailleBack is an accessibility service for Android that controls a
refreshable braille display.  It presents screen content on the braille
display and let's the user navigate the user interface.  It also provides an
input method that enables entering text using a braille keyboard on the
display.  BrailleBack is distributed on the Google Play Store where it can be
downloaded and installed.  This file contains information for developers.

In particular, you will find instructions for how to build the app from source
below.  Also, there are instructions on what to do, in addition to having
brltty drivers, to add support for a new braille display to BrailleBack.

Building BrailleBack From The Command Line

This section contains instructions for building BrailleBack from the command
line.  See the Android developer documentation at
for general instructions on how to build android packages.  These build
instructions apply to building on Linux.

NOTE: There's a build script ( that automates most of the process
for a clean build.  See below for how to use it.


BrailleBack has a few dependencies that need to be available before it can be

* Android SDK for JellyBean (API level 16) and NDK, revision 7a or later.
  These can be downloaded from
  You'll need the platform tools r20 or later for JellyBean.
  Make sure that the SDK and NDK tools (android, ndk-build, etc) are
  available in your $PATH.

* A checked out copy of the BrailleBack source code from GitHub

After making sure that the dependencies are in place (see Prerequisites
above), perform the following steps to build the BrailleBack apk.
Please run all commands from the root of the eyes-free source tree.

* Modify Specify your Sdk and Ndk path.

* If you are building on Linux, you might be able to use the shell script
  that automates the build process.  This script builds a debug apk.
  It performs a clean build, which takes longer than an incremental build.
  You might want to use some of the steps below while developing.

  $ ./

  If the script finishes with a success message, then you're done and
  can skip the rest of this section.  Use the adb command to install
  the apk on a device.  Otherwise, follow and adjust the steps below for your
  build environment.

* Make sure to have a copy of brltty, version 5.6 checked out and copy 
  Drivers, Headers, Programs folders to braille/service/jni/brlttywrapper/brltty.
  This can be checked out from Github :

  $ git clone --branch BRLTTY-5.6

* Make sure to have a copy of liblouis, version 3.4.0 checked out and copy
  liblouis and tables folders to braille/service/jni/liblouiswrapper/liblouis.  
  This can be checked out from Github as follows :

  $ git clone --branch v3.4.0

* Run ./autogen && ./configure for these two libraries.

* Make sure to have a copy of android-midi-lib and copy source code under java to 

  $ git clone

* Apply patches to the dependencies:
  $ patch -p1 < brltty.patch
  $ patch -p1 < liblouis.patch

* Build the BrailleBack apk:
  Under the braille directory, run:

  $ ./gradlew assembDebug

NOTE: If you try to install your own build on a device where a version of
BrailleBack is already installed from the Google Play store, you will need to
uninstall the package first.  The reason for that is that you are using a
different key to sign your debug version than was used to sign the installed
release build.  The command to uninstall BrailleBack is:
  $ adb uninstall com.googlecode.eyesfree.brailleback

Adding Support for A New Braille Display

This section details how to add support for a new braille display.
On a high level, the following steps need to be performed:

* Make sure that the display meets the requirements listed in the
  subsection on Display Requirements below.

* Include the brltty driver in the native library build.

* Add the display in the braille display service source code
  so it gets automatically detected.

* Adjust keyboard maps for Android.

* Test the display with actual hardware.

Support for hardware braille displays is handled by a service that is separate
from BrailleBack.  The source code for the display service is located in the
directory braille/service in the Eyes-Free code repository.

The instructions below assume that you've already built BrailleBack and
that you have an actual device to test on.

Display Requirements

The braille display service currently supports displays connected via
bluetooth.  Adding support for other ways of connecting such as USB would be
useful, but is out of scope for this description.  Further, the display must
support rfcomm (serial) bluetooth connections.

The display service uses hardware drivers from brltty.  If there is no brltty
driver for the display, that needs to be implemented and contributed
to the brltty project (see  A stable version
of brltty is being used (see the build instructions above for which
version is currently imported).

Including The Driver In The Build

The brltty native library is compiled using the Android NDK.  The source files
for a new driver need to be added in the file:
If not already present, add the base name of the driver directory to the list
in the build-braille-drivers function call close to the top of this file.

The new driver also needs to be listed in the file
A brl_driver_XX declaration needs to be added and included in the
driverTable array for the brltty initialization code to be able to locate
the driver.

Now it is time to compile the new driver.  Use this command:
  $ (cd braille/service && ndk-build)
to make sure that the native library still builds.  Since only a subset of
brltty is used, you might encounter some unexpected compilation or linking
errors that you will need to address.

Detecting The Display

When the driver is compiled into the native brltty library, the display
service needs to know to use the driver for the specific display model
being added.  For this, some code needs to be added to the file

Towards the end of this file, there is a list of supported bluetooth displays.
Simply add an entry to the list and the service should detect the display when
it is paired to the Android device.  Note that the name prefixes are
the human-readable device names.  Ideally, they should be unique enough
to not have conflicts with other devices.  Please refer to the
next section for a discussion of friendly key names.

Keyboard Mappings

brltty comes with keyboard bindings for the supported displays.  These are
written primarily for the Linux text mode use case and to provide
a great user experience, they need to be customized for BrailleBack.
This is done by modifying a copy of the appropriate .ktb and .kti
files, which then get included with the BrailleBack as a .zip file
resource.  This is handled by a few scripts in the directory
See the README file in that directory for more information.

One important change that need to be made for each display is to include
the file
in the keymap.  This adds common mnemonic keyboard chords (space + braille
dot keys) that are typically common across devices.

To support translation of key names in the help screen and to support cases
where the brltty internal key names don't correspond to the names in user
documentation, user-friendly key name mappings should be added in the
DeviceFinder class (see the previous section).  brltty key names are mapped to
integer resource ids.  The resources for the friendly key name strings belong
in the file:

Testing And Debugging

If all the above steps were implemented correctly, BrailleBack should be able
to find and start using the display when it is paired in the bluetooth
settings screen.  If that's not the case, try some of the debugging tips below:

* Write a small app that uses the braille display service directly to not have
  to debug with the accessibility service.  BrailleBack uses a thin client
  library located in braille/client.  This library can be used to connect to
  the display service for testing.

* adb logcat is your friend.  Look for logs from the various classes in the
  display service (they use the simple class name as log tags).  The native
  C code uses log tags starting with Brltty.

* Issues in brltty (including the driver and keyboard maps) might be easier to
  debug on a development system than on a real device.

* Bluetooth connectivity issues: for certain displays, a bluetooth connection
  may be established and then disconnected within a few seconds.
  One thing to try in this case is to set the connectSecurely flag to false
  when constructing the NamePrefixSupportedDevice object for the display
  in DeviceFinder.  Note that it is preferable to set this parameter
  to true whenever possible.  See
  for more information.

In addition to using BrailleBack to make sure it works as expected, open the
keyboard help screen (space+l on the braille display) to make sure that keys
and commands are listed correctly.  Check that all commands that BrailleBack
supports are listed (see braille/brailleback/res/values/help.xml for a list of
supported commands).

If this works, you are done.  Please contribute the changes back to the
Eyes-Free project so that they can benefit Android braille users around the


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