Add engine specific documentation

Without going too much into implementation details, document which
engines are available, which dependencies they have, and which
engine-specific configuration parameters they support.

Change-Id: I2e787444a5841e1eaefc4ba156f1b71303b6dcab
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

4 files changed, 156 insertions, 38 deletions

diff --git a/src/tts/doc/src/qt6-changes.qdoc b/src/tts/doc/src/qt6-changes.qdoc
index cd4a37f..99b30fa 100644
--- a/src/tts/doc/src/qt6-changes.qdoc
+++ b/src/tts/doc/src/qt6-changes.qdoc
@@ -43,42 +43,12 @@
 
     \section1 Available Engines
 
-    The following engines are available in Qt 6.4:
+    All engines that were available in Qt 5 are also available in Qt 6.4. For a
+    complete list of available engines in Qt 6.4, see \l{Qt TextToSpeech Engines}.
 
-    \table
-        \header
-            \li Engine name
-            \li Platform
-            \li Dependency
-        \row
-            \li sapi
-            \li Windows
-            \li Windows SDK (SAPI 5.3)
-        \row
-            \li winrt
-            \li Windows
-            \li Windows SDK (not available with MinGW)
-        \row
-            \li android
-            \li Android
-            \li TextToSpeech package
-        \row
-            \li ios
-            \li macOS 10.15+, iOS
-            \li macOS 10.15 SDK
-        \row
-            \li macos
-            \li macOS 10.14+
-            \li macOS 10.15 SDK
-        \row
-            \li speechd
-            \li Linux
-            \li libspeechd 0.9 and speech-dispatcher daemon
-        \row
-            \li flite
-            \li Linux
-            \li flite 2.2
-    \endtable
+    Engines can now receive additional configuration parameters. When constructing
+    a QTextToSpeech instance, pass a QVariantMap with engine-specific key/value
+    pairs.
 
     \section1 API changes
 
@@ -87,8 +57,7 @@
 
     \section2 The QTextToSpeech class
 
-    Existing code using the QTextToSpeech class continues to work as before.
-    The new {QTextToSpeech::engine} property allows changing of the engine on an
+    The new \l{QTextToSpeech::}{engine} property allows changing of the engine on an
     initialized QTextToSpeech object.
 
     \section2 The QVoice class
diff --git a/src/tts/doc/src/qttexttospeech-engines.qdoc b/src/tts/doc/src/qttexttospeech-engines.qdoc
new file mode 100644
index 0000000..a45c3aa
--- /dev/null
+++ b/src/tts/doc/src/qttexttospeech-engines.qdoc
@@ -0,0 +1,146 @@
+/****************************************************************************
+**
+** Copyright (C) 2022 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:FDL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Free Documentation License Usage
+** Alternatively, this file may be used under the terms of the GNU Free
+** Documentation License version 1.3 as published by the Free Software
+** Foundation and appearing in the file included in the packaging of
+** this file. Please review the following information to ensure
+** the GNU Free Documentation License version 1.3 requirements
+** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+/*!
+    \page qttexttospeech-engines.html
+    \title Qt TextToSpeech Engines
+    \brief Engine specific documentation
+
+    Qt TextToSpeech supports several engine implementations for different platforms, and
+    with different tradeoffs between capabilities, availability, and runtime cost.
+
+    An engine can be selected when a QTextToSpeech instance is constructed, or on an already
+    instantiated object by setting the \l {QTextToSpeech::}{engine} property. If no engine
+    is specified, the preferred engine for the current platform will be used. Call
+    \l{QTextToSpeech::}{availableEngines()} to get the list of engines that can be used on
+    the current system. This will include a "mock" engine that is used for testing, and that
+    should not be deployed to target systems.
+
+    When setting the engine, applications can pass a QVariantMap with key/value pairs of
+    engine specific parameters through to the engine. The following sections list the
+    configuration parameters supported for each engine. Parameters that are not supported
+    by the engine will be silently ignored.
+
+    \section1 WinRT
+
+    The "winrt" engine uses the APIs from the \l{https://docs.microsoft.com/en-us/uwp/api/windows.media.speechsynthesis}
+    {Windows.Media.SpeechSynthesis Namespace}. It requires a recent Windows SDK, and
+    is not available when using the MinGW compiler. The engine provides access to all
+    voices available for installation on current Windows versions.
+
+    The implementation uses QAudioSink from \l{Qt Multimedia} to play the PCM data
+    stream generated by the synthesizer to an audio device.
+
+    \table
+        \header
+            \li Name
+            \li Type
+            \li Remarks
+        \row
+            \li audioDevice
+            \li QAudioDevice
+            \li
+    \endtable
+
+    \section1 SAPI
+
+    The "sapi" engine uses the \l{https://docs.microsoft.com/en-us/previous-versions/windows/desktop/ms723627(v=vs.85)}
+    {SAPI 5.3} framework that is included in the Windows SDK. It provides a limited selection
+    of voices, with reduced quality compared to the "winrt" engine.
+
+    The SAPI engine does not support any engine specific parameters.
+
+    \section1 iOS
+
+    The "ios" engine uses the \l{https://developer.apple.com/documentation/avfoundation/speech_synthesis?language=objc}
+    {AVFoundation} framework that is available on iOS devices and on macOS 10.15. The
+    documentation of the framework states support for macOS 10.14 as well, but by default
+    no voices are available on that platform.
+
+    The iOS engine does not support any engine specific parameters.
+
+    \section1 macOS
+
+    The "macos" engine uses the \l{https://developer.apple.com/documentation/appkit/nsspeechsynthesizer?language=objc}
+    {NSSpeechDispatcher} framework from AppKit. It is only available on macOS systems, but
+    works out of the box on macOS 10.14. The number of available voices is limited compared
+    to the iOS engine.
+
+    The macOS engine does not support any engine specific parameters.
+
+    \section1 Android
+
+    The "android" engine is the only engine available on the Android platform. It uses the
+    \l{https://developer.android.com/reference/android/speech/tts/TextToSpeech}{TextToSpeech}
+    package, which in turn supports multiple engine backends.
+
+    \table
+        \header
+            \li Name
+            \li Type
+            \li Remarks
+        \row
+            \li androidEngine
+            \li QString
+            \li There is no API in Qt to get the list of installed engines.
+    \endtable
+
+
+    \section1 Flite
+
+    The "flite" engine uses the \l{https://github.com/festvox/flite}{flite} synthesizer.
+    The engine's small footprint makes it particularly useful for embedded environments.
+    The plugin requires at least Flite 2.2, and uses \l QAudioSink from \l{Qt Multimedia}
+    to render the generated PCM data stream.
+
+    The engine plugin searches for voice libraries in the directories listed in the
+    \c LD_LIBRARY_PATH environment variable, and falls back to search common library
+    locations such as \c {/usr/lib}, \c {/usr/lib64}, and \c {/usr/lib/x86_64-linux-gnu}.
+
+    If Flite is used as a static library, then the desired voice libraries also need to
+    be statically linked into the engine plugin. There is currently not build system API
+    implemented for selecting such voice libraries when configuring Qt.
+
+    \table
+        \header
+            \li Name
+            \li Type
+            \li Remarks
+        \row
+            \li audioDevice
+            \li QAudioDevice
+            \li
+    \endtable
+
+    \section1 speech-dispatcher
+
+    The "speechd" engine communicates with the
+    \l{https://htmlpreview.github.io/?https://github.com/brailcom/speechd/blob/master/doc/speech-dispatcher.html#Top}
+    {speech-dispatcher} daemon, and requires at least libspeechd 0.9.
+
+    The speech-dispatcher engine does not support any engine specific parameters.
+*/
diff --git a/src/tts/doc/src/qttexttospeech-index.qdoc b/src/tts/doc/src/qttexttospeech-index.qdoc
index 94693c7..6f63a1d 100644
--- a/src/tts/doc/src/qttexttospeech-index.qdoc
+++ b/src/tts/doc/src/qttexttospeech-index.qdoc
@@ -80,5 +80,6 @@
       \li \l{Qt TextToSpeech QML Types}{QML Types}
       \li \l{Qt TextToSpeech C++ Classes}{C++ Classes}
       \li \l{Qt TextToSpeech Examples}{Examples}
+      \li \l{Qt TextToSpeech Engines}{Engine Documentation}
    \endlist
 */
diff --git a/src/tts/qtexttospeech.cpp b/src/tts/qtexttospeech.cpp
index ef41e8e..a8b1721 100644
--- a/src/tts/qtexttospeech.cpp
+++ b/src/tts/qtexttospeech.cpp
@@ -289,7 +289,8 @@ QTextToSpeech::QTextToSpeech(const QString &engine, QObject *parent)
 
     If \a engine is empty, the default engine plug-in is used. The default
     engine is platform-specific. Which key/value pairs in \a params are supported
-    depends on the engine. Unsupported entries will be ignored.
+    depends on the engine. See \l{Qt TextToSpeech Engines}{the engine documentation}
+    for details. Unsupported entries will be ignored.
 
     If the engine initializes correctly, the \l state of the engine will be set
     to QTextToSpeech::Ready. If the plugin fails to load, or if the engine fails to
@@ -331,6 +332,7 @@ QTextToSpeech::~QTextToSpeech()
     \return whether \a engine could be set successfully.
 
     Which key/value pairs in \a params are supported depends on the engine.
+    See \l{Qt TextToSpeech Engines}{the engine documentation} for details.
     Unsupported entries will be ignored.
 */
 bool QTextToSpeech::setEngine(const QString &engine, const QVariantMap &params)