qtbase/examples/corelib/threads/doc/src/mandelbrot.qdoc

// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \example threads/mandelbrot
    \title Mandelbrot Example
    \ingroup qtconcurrent-mtexamples

    \brief The Mandelbrot example demonstrates multi-thread programming
    using Qt. It shows how to use a worker thread to
    perform heavy computations without blocking the main thread's
    event loop.

    \image mandelbrot-example.png Screenshot of the Mandelbrot example

    The heavy computation here is the Mandelbrot set, probably the world's most
    famous fractal. These days, while sophisticated programs, such as
    \l{https://xaos-project.github.io/}{XaoS}, provide real-time zooming in
    the Mandelbrot set, the standard Mandelbrot algorithm is just slow enough
    for our purposes.

    In real life, the approach described here is applicable to a
    large set of problems, including synchronous network I/O and
    database access, where the user interface must remain responsive
    while some heavy operation is taking place. The \l
    {Blocking Fortune Client} example shows the same principle at
    work in a TCP client.

    The Mandelbrot application supports zooming and scrolling using
    the mouse or the keyboard. To avoid freezing the main thread's
    event loop (and, as a consequence, the application's user
    interface), we put all the fractal computation in a separate
    worker thread. The thread emits a signal when it is done
    rendering the fractal.

    During the time where the worker thread is recomputing the
    fractal to reflect the new zoom factor position, the main thread
    simply scales the previously rendered pixmap to provide immediate
    feedback. The result doesn't look as good as what the worker
    thread eventually ends up providing, but at least it makes the
    application more responsive. The sequence of screenshots below
    shows the original image, the scaled image, and the rerendered
    image.

    \table
    \row
    \li \inlineimage mandelbrot_zoom1.png
    \li \inlineimage mandelbrot_zoom2.png
    \li \inlineimage mandelbrot_zoom3.png
    \endtable

    Similarly, when the user scrolls, the previous pixmap is scrolled
    immediately, revealing unpainted areas beyond the edge of the
    pixmap, while the image is rendered by the worker thread.

    \table
    \row
    \li \inlineimage mandelbrot_scroll1.png
    \li \inlineimage mandelbrot_scroll2.png
    \li \inlineimage mandelbrot_scroll3.png
    \endtable

    The application consists of two classes:

    \list
    \li \c RenderThread is a QThread subclass that renders
       the Mandelbrot set.
    \li \c MandelbrotWidget is a QWidget subclass that shows the
       Mandelbrot set on screen and lets the user zoom and scroll.
    \endlist

    If you are not already familiar with Qt's thread support, we
    recommend that you start by reading the \l{Thread Support in Qt}
    overview.

    \section1 RenderThread Class Definition

    We'll start with the definition of the \c RenderThread class:

    \snippet threads/mandelbrot/renderthread.h 0

    The class inherits QThread so that it gains the ability to run in
    a separate thread. Apart from the constructor and destructor, \c
    render() is the only public function. Whenever the thread is done
    rendering an image, it emits the \c renderedImage() signal.

    The protected \c run() function is reimplemented from QThread. It
    is automatically called when the thread is started.

    In the \c private section, we have a QMutex, a QWaitCondition,
    and a few other data members. The mutex protects the other data
    member.

    \section1 RenderThread Class Implementation

    \snippet threads/mandelbrot/renderthread.cpp 0

    In the constructor, we initialize the \c restart and \c abort
    variables to \c false. These variables control the flow of the \c
    run() function.

    We also initialize the \c colormap array, which contains a series
    of RGB colors.

    \snippet threads/mandelbrot/renderthread.cpp 1

    The destructor can be called at any point while the thread is
    active. We set \c abort to \c true to tell \c run() to stop
    running as soon as possible. We also call
    QWaitCondition::wakeOne() to wake up the thread if it's sleeping.
    (As we will see when we review \c run(), the thread is put to
    sleep when it has nothing to do.)

    The important thing to notice here is that \c run() is executed
    in its own thread (the worker thread), whereas the \c
    RenderThread constructor and destructor (as well as the \c
    render() function) are called by the thread that created the
    worker thread. Therefore, we need a mutex to protect accesses to
    the \c abort and \c condition variables, which might be accessed
    at any time by \c run().

    At the end of the destructor, we call QThread::wait() to wait
    until \c run() has exited before the base class destructor is
    invoked.

    \snippet threads/mandelbrot/renderthread.cpp 2

    The \c render() function is called by the \c MandelbrotWidget
    whenever it needs to generate a new image of the Mandelbrot set.
    The \c centerX, \c centerY, and \c scaleFactor parameters specify
    the portion of the fractal to render; \c resultSize specifies the
    size of the resulting QImage.

    The function stores the parameters in member variables. If the
    thread isn't already running, it starts it; otherwise, it sets \c
    restart to \c true (telling \c run() to stop any unfinished
    computation and start again with the new parameters) and wakes up
    the thread, which might be sleeping.

    \snippet threads/mandelbrot/renderthread.cpp 3

    \c run() is quite a big function, so we'll break it down into
    parts.

    The function body is an infinite loop which starts by storing the
    rendering parameters in local variables. As usual, we protect
    accesses to the member variables using the class's mutex. Storing
    the member variables in local variables allows us to minimize the
    amount of code that needs to be protected by a mutex. This ensures
    that the main thread will never have to block for too long when
    it needs to access \c{RenderThread}'s member variables (e.g., in
    \c render()).

    The \c forever keyword is, like \c foreach, a Qt pseudo-keyword.

    \snippet threads/mandelbrot/renderthread.cpp 4
    \snippet threads/mandelbrot/renderthread.cpp 5
    \snippet threads/mandelbrot/renderthread.cpp 6
    \snippet threads/mandelbrot/renderthread.cpp 7

    Then comes the core of the algorithm. Instead of trying to create
    a perfect Mandelbrot set image, we do multiple passes and
    generate more and more precise (and computationally expensive)
    approximations of the fractal.

    We create a high resolution pixmap by applying the device
    pixel ratio to the target size (see
    \l{Drawing High Resolution Versions of Pixmaps and Images}).

    If we discover inside the loop that \c restart has been set to \c
    true (by \c render()), we break out of the loop immediately, so
    that the control quickly returns to the very top of the outer
    loop (the \c forever loop) and we fetch the new rendering
    parameters. Similarly, if we discover that \c abort has been set
    to \c true (by the \c RenderThread destructor), we return from
    the function immediately, terminating the thread.

    The core algorithm is beyond the scope of this tutorial.

    \snippet threads/mandelbrot/renderthread.cpp 8
    \snippet threads/mandelbrot/renderthread.cpp 9

    Once we're done with all the iterations, we call
    QWaitCondition::wait() to put the thread to sleep,
    unless \c restart is \c true. There's no use in keeping a worker
    thread looping indefinitely while there's nothing to do.

    \snippet threads/mandelbrot/renderthread.cpp 10

    The \c rgbFromWaveLength() function is a helper function that
    converts a wave length to a RGB value compatible with 32-bit
    \l{QImage}s. It is called from the constructor to initialize the
    \c colormap array with pleasing colors.

    \section1 MandelbrotWidget Class Definition

    The \c MandelbrotWidget class uses \c RenderThread to draw the
    Mandelbrot set on screen. Here's the class definition:

    \snippet threads/mandelbrot/mandelbrotwidget.h 0

    The widget reimplements many event handlers from QWidget. In
    addition, it has an \c updatePixmap() slot that we'll connect to
    the worker thread's \c renderedImage() signal to update the
    display whenever we receive new data from the thread.

    Among the private variables, we have \c thread of type \c
    RenderThread and \c pixmap, which contains the last rendered
    image.

    \section1 MandelbrotWidget Class Implementation

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 0

    The implementation starts with a few constants that we'll need
    later on.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 1

    The interesting part of the constructor is the
    QObject::connect() call.

    Although it looks like a standard signal-slot connection between
    two \l{QObject}s, because the signal is emitted in a different
    thread than the receiver lives in, the connection is effectively a
    \l{Qt::QueuedConnection}{queued connection}. These connections are
    asynchronous (i.e., non-blocking), meaning that the slot will be
    called at some point after the \c emit statement. What's more, the
    slot will be invoked in the thread in which the receiver lives.
    Here, the signal is emitted in the worker thread, and the slot is
    executed in the GUI thread when control returns to the event loop.

    With queued connections, Qt must store a copy of the arguments
    that were passed to the signal so that it can pass them to the
    slot later on. Qt knows how to take of copy of many C++ and Qt
    types, so, no further action is needed for QImage.
    If a custom type was used, a call to the template function
    qRegisterMetaType() would be required before the type
    could be used as a parameter in queued connections.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 2
    \snippet threads/mandelbrot/mandelbrotwidget.cpp 3
    \snippet threads/mandelbrot/mandelbrotwidget.cpp 4

    In \l{QWidget::paintEvent()}{paintEvent()}, we start by filling
    the background with black. If we have nothing to paint yet (\c
    pixmap is null), we display a message on the widget asking the user
    to be patient and return from the function immediately.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 5
    \snippet threads/mandelbrot/mandelbrotwidget.cpp 6
    \snippet threads/mandelbrot/mandelbrotwidget.cpp 7
    \snippet threads/mandelbrot/mandelbrotwidget.cpp 8

    If the pixmap has the right scale factor, we draw the pixmap directly onto
    the widget.

    Otherwise, we create a preview pixmap to be shown until the calculation
    finishes and translate the \l{Coordinate System}{coordinate system}
    accordingly.

    Since we are going to use transformations on the painter
    and use an overload of QPainter::drawPixmap() that does not support
    high resolution pixmaps in that case, we create a pixmap with device pixel
    ratio 1.

    By reverse mapping the widget's rectangle using the scaled painter matrix,
    we also make sure that only the exposed areas of the pixmap are drawn.
    The calls to QPainter::save() and QPainter::restore() make sure that any
    painting performed afterwards uses the standard coordinate system.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 9

    At the end of the paint event handler, we draw a text string and
    a semi-transparent rectangle on top of the fractal.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 10

    Whenever the user resizes the widget, we call \c render() to
    start generating a new image, with the same \c centerX, \c
    centerY, and \c curScale parameters but with the new widget size.

    Notice that we rely on \c resizeEvent() being automatically
    called by Qt when the widget is shown the first time to generate
    the initial image.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 11

    The key press event handler provides a few keyboard bindings for
    the benefit of users who don't have a mouse. The \c zoom() and \c
    scroll() functions will be covered later.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 12

    The wheel event handler is reimplemented to make the mouse wheel
    control the zoom level. QWheelEvent::angleDelta() returns the angle
    of the wheel mouse movement, in eighths of a degree. For most mice,
    one wheel step corresponds to 15 degrees. We find out how many
    mouse steps we have and determine the resulting zoom factor.
    For example, if we have two wheel steps in the positive direction
    (i.e., +30 degrees), the zoom factor becomes \c ZoomInFactor
    to the second power, i.e. 0.8 * 0.8 = 0.64.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 13

    Pinch to zoom has been implemented with QGesture as outlined in
    \l{Gestures in Widgets and Graphics View}.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp gesture1

    When the user presses the left mouse button, we store the mouse
    pointer position in \c lastDragPos.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 14

    When the user moves the mouse pointer while the left mouse button
    is pressed, we adjust \c pixmapOffset to paint the pixmap at a
    shifted position and call QWidget::update() to force a repaint.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 15

    When the left mouse button is released, we update \c pixmapOffset
    just like we did on a mouse move and we reset \c lastDragPos to a
    default value. Then, we call \c scroll() to render a new image
    for the new position. (Adjusting \c pixmapOffset isn't sufficient
    because areas revealed when dragging the pixmap are drawn in
    black.)

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 16

    The \c updatePixmap() slot is invoked when the worker thread has
    finished rendering an image. We start by checking whether a drag
    is in effect and do nothing in that case. In the normal case, we
    store the image in \c pixmap and reinitialize some of the other
    members. At the end, we call QWidget::update() to refresh the
    display.

    At this point, you might wonder why we use a QImage for the
    parameter and a QPixmap for the data member. Why not stick to one
    type? The reason is that QImage is the only class that supports
    direct pixel manipulation, which we need in the worker thread. On
    the other hand, before an image can be drawn on screen, it must
    be converted into a pixmap. It's better to do the conversion once
    and for all here, rather than in \c paintEvent().

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 17

    In \c zoom(), we recompute \c curScale. Then we call
    QWidget::update() to draw a scaled pixmap, and we ask the worker
    thread to render a new image corresponding to the new \c curScale
    value.

    \snippet threads/mandelbrot/mandelbrotwidget.cpp 18

    \c scroll() is similar to \c zoom(), except that the affected
    parameters are \c centerX and \c centerY.

    \section1 The main() Function

    The application's multithreaded nature has no impact on its \c
    main() function, which is as simple as usual:

    \snippet threads/mandelbrot/main.cpp 0
*/