Accessibility stack¶
Software interfaces for assistive technologies¶
This section provides details on the components that make up Ubuntu Desktop’s accessibility stack, their roles, interdependencies, and how they all work together. There are also recommendations for developers and integrators who want to create applications that will work with Desktop’s default assistive solutions as well as developers who want to create their own assitive solutions that would work on Desktop.
AT-SPI¶
The Assistive Technology Service Provider Interface (AT-SPI) is a platform-neutral framework for communication between assistive technologies (AT) and applications.
AT-SPI defines a set of interfaces for assistive technologies which allow to get names, roles, and states of interface objects, listen for events, or perform actions.
AT-SPI uses D-Bus method calls and signals. D-Bus is an inter-process communication system in Linux, which makes AT-SPI a default accessibility framework for Linux.
AT-SPI is made of two components:
at-spi2-coreA collection of XML interfaces for accessibility
at-spi2-atkA library that allows applications made with GTK 3 to register accessible objects with AT-SPI
If you are building custom assistive technology tools or want to test the accessibility of custom applications that were not built with GTK on Ubuntu Desktop, you must use AT-SPI DBus XML interfaces.
GTK4 and Gtk.Accessible¶
GTK is a cross-platform widget toolkit for creating graphical user interfaces. It offers a set of user interface elements that allow developers to build apps quickly and in a consistent manner. The majority of applications within GNOME desktop environment are built with GTK.
GTK4 is a newer version of GTK that implements its own Gtk.Accessible interface instead of the previously used ATK.
If you are building an application for Ubuntu Desktop and want to make it accessible for Ubuntu Desktop’s default assistive technologies such as Orca, it is recommended that you use GTK. Desktop’s assistive solutions are able to access information about the apps built with both GTK3 and GTK4.
If you are using a different UI toolkit, you must ensure it implements the AT-SPI D-Bus interfaces; otherwise, Desktop’s assistive technologies wouldn’t be able to interact with it.
GTK3 and ATK¶
GTK3 version of the GTK toolkit that integrates with Accessibility Toolkit (ATK). Applications built with GTK3 expose information about UI objects using the ATK. ATK is an Application Programming Interface (API) that allows developers to avoid working with AT-SPI API directly, as it provides a set of special input methods.
Flutter¶
A selection of applications on Ubuntu Desktop such as App Center are built with Flutter. Flutter engine has compatibility issues with AT-SPI. To learn how to make Flutter applications that would be compatible with Ubuntu Desktop’s default assistive solutions, reach out to the development team at Discourse.
Overview of Ubuntu Desktop applications and their interfaces¶
Application |
Built with |
API |
|---|---|---|
Gnome Initial Setup |
GTK4 |
|
Login and Lock screens |
GTK3 |
|
System Settings app |
GTK4 |
|
App Center |
Flutter |
|
Software and Updates |
GTK3 |
Screen reader and speech synthesis¶
Speech synthesizers converts text to human-sounding speech. Typically, they are offered as a backend for screen readers, but they can also be used as standalone programs. Orca is the default screen reader on Ubuntu Desktop. eSpeak and Speech Dispatcher are the speech synthesizers that Orca depends on. When Orca identifies the text, it passes it to the Speech Dispatcher for speech synthesis; the Speech Dispatcher then passes the synthesized speech to eSpeak, which plays the sound.
Orca¶
Orca is a screen reader for vision impaired and blind users.
Orca is preinstalled on Ubuntu Desktop and can be used out-of-box. To learn how to configure it, go to Read the screen aloud.
BRLTTY¶
The Orca screen reader can display the user interface on a refreshable Braille display. It uses the BRLTTY service, which provides access to the Linux console for a blind person using a refreshable Braille display.
As Orca, BRLTTY is preinstalled and can be enabled and used without any additional configurations, see Read the screen in Braille.
eSpeak¶
eSpeak is a speech synthesizer that supports over 100 languages and accents. eSpeak can be used as a standalalone tool or as a backend component for screen readers, see eSpeak website
Speech dispatcher¶
Speech dispatcher is a device-independent high-level interface for speech synthesis. It can be used by other software and hardware speech synthesizers as a backend, see Speech Dispatcher website.
Therefore, Speech Dispatcher is the layer that allows you to switch between text-to-speech (TTS) solutions.
On-screen keyboards¶
GNOME on-screen keyboard¶
Ubuntu Desktop comes with a GNOME on-screen keyboard preinstalled, it is the on-screen keyboard that we recommend for Desktop. To start using it, see Use an on-screen keyboard.
Hardware interfaces for assistive technologies¶
USB¶
Ubuntu Desktop supports USB out of the box.
In Ubuntu, kernel USB drivers handle communications with USB devices such as, for example, usbhid which provides support for USB Human Interface Devices (HID) class. Kernel supports all standard USB classes.
udev is a device manager in Ubuntu that detects, adds and removes devices. udev provides access to the detected USB devices in the /dev directory.
If your device conforms to a USB class, it will be assigned a /dev entry and recognized correctly.
USB device requirements¶
You can use USB to connect assistive devices for input/output such as Braille displays, sip-and-puff, foot pedals.
To be able to work on Ubuntu properly, a USB device must:
use a standard USB class
provide a proper USB descriptor so that Ubuntu can identify it
not require proprietary drivers
If the device uses a custom class, it must provide a udev .rules file instead so that Ubuntu can identify it correctly. See udev documentation
Bluetooth¶
While the Ubuntu Desktop team works continuously to enhance the Bluetooth experience, the Bluetooth stack involves a complex interplay of multiple components such as firmware, kernel modules, drivers, and userspace applications. This complex set of dependencies makes it difficult to guarantee consistent compatibility with all Bluetooth devices. For an in-depth explanation of Ubuntu’s hardware architecture, see Hardware stack.
Ubuntu uses BlueZ as its core Bluetooth stack.
To be able to work on Desktop, a device must:
be HCI-compliant
have a corresponding firmware provided by the manufacturer as part of the
linux-firmwarepackageuse the profiles supported by BlueZ.