Only reset the backend pointer after we're done with it

[qt-netbsd.git] / doc / src / paintsystem.qdoc

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/*!
    \page paintsystem.html

    \title The Paint System

    Qt's paint system enables painting on screen and print devices
    using the same API, and is primarily based on the QPainter,
    QPaintDevice, and QPaintEngine classes.

    QPainter is used to perform drawing operations, QPaintDevice is an
    abstraction of a two-dimensional space that can be painted on
    using a QPainter, and QPaintEngine provides the interface that the
    painter uses to draw onto different types of devices. The
    QPaintEngine class is used internally by QPainter and
    QPaintDevice, and is hidden from application programmers unless
    they create their own device type.

    \image paintsystem-core.png

    The main benefit of this approach is that all painting follows the
    same painting pipeline making it easy to add support for new
    features and providing default implementations for unsupported
    ones.

    Alternatively, Qt provides the QtOpenGL module, offering classes
    that makes it easy to use OpenGL in Qt applications. Among others,
    the module provides an OpenGL widget class that can be used just
    like any other Qt widget, except that it opens an OpenGL display
    buffer where the OpenGL API can be used to render the contents.

    \tableofcontents section1

    \section1 Drawing

    QPainter provides highly optimized functions to do most of the
    drawing GUI programs require. It can draw everything from simple
    graphical primitives (represented by the QPoint, QLine, QRect,
    QRegion and QPolygon classes) to complex shapes like vector
    paths. In Qt vector paths are represented by the QPainterPath
    class. QPainterPath provides a container for painting operations,
    enabling graphical shapes to be constructed and reused.

    \table 100%
    \row
    \o \image paintsystem-painterpath.png
    \o \bold QPainterPath

    A painter path is an object composed of lines and curves. For
    example, a rectangle is composed by lines and an ellipse is
    composed by curves.

    The main advantage of painter paths over normal drawing operations
    is that complex shapes only need to be created once; then they can
    be drawn many times using only calls to the QPainter::drawPath()
    function.

    A QPainterPath object can be used for filling, outlining, and
    clipping. To generate fillable outlines for a given painter path,
    use the QPainterPathStroker class.

    \endtable

    Lines and outlines are drawn using the QPen class. A pen is
    defined by its style (i.e. its line-type), width, brush, how the
    endpoints are drawn (cap-style) and how joins between two
    connected lines are drawn (join-style). The pen's brush is a
    QBrush object used to fill strokes generated with the pen,
    i.e. the QBrush class defines the fill pattern.

    QPainter can also draw aligned text and pixmaps.

    When drawing text, the font is specified using the QFont class. Qt
    will use the font with the specified attributes, or if no matching
    font exists, Qt will use the closest matching installed font. The
    attributes of the font that is actually used can be retrieved
    using the QFontInfo class. In addition, the QFontMetrics class
    provides the font measurements, and the QFontDatabase class
    provides information about the fonts available in the underlying
    window system.

    Normally, QPainter draws in a "natural" coordinate system, but it
    is able to perform view and world transformations using the
    QMatrix class. For more information, see \l {The Coordinate
    System} documentation which also describes the rendering process,
    i.e. the relation between the logical representation and the
    rendered pixels, and the benefits of anti-aliased painting.

    \table 100%
    \row \o
    \bold {Anti-Aliased Painting}

    When drawing, the pixel rendering is controlled by the
    QPainter::Antialiasing render hint. The QPainter::RenderHint enum
    is used to specify flags to QPainter that may or may not be
    respected by any given engine.

    The QPainter::Antialiasing value indicates that the engine should
    antialias edges of primitives if possible, i.e. smoothing the
    edges by using different color intensities.

    \o \image paintsystem-antialiasing.png

    \endtable

    \section1 Filling

    Shapes are filled using the QBrush class. A brush is defined
    by its color and its style (i.e. its fill pattern).

    Any color in Qt is represented by the QColor class which supports
    the RGB, HSV and CMYK color models. QColor also support
    alpha-blended outlining and filling (specifying the transparency
    effect), and the class is platform and device independent (the
    colors are mapped to hardware using the QColormap class). For more
    information, see the QColor class documentation.

    When creating a new widget, it is recommend to use the colors in
    the widget's palette rather than hard-coding specific colors. All
    widgets in Qt contain a palette and use their palette to draw
    themselves. A widget's palette is represented by the QPalette
    class which contains color groups for each widget state.

    The available fill patterns are described by the Qt::BrushStyle
    enum. These include basic patterns spanning from uniform color to
    very sparse pattern, various line combinations, gradient fills and
    textures. Qt provides the QGradient class to define custom
    gradient fills, while texture patterns are specified using the
    QPixmap class.

    \table 100%
    \row
    \o \image paintsystem-fancygradient.png
    \o \bold QGradient

    The QGradient class is used in combination with QBrush to specify
    gradient fills.

    \image paintsystem-gradients.png

    Qt currently supports three types of gradient fills: Linear
    gradients interpolate colors between start and end points, radial
    gradients interpolate colors between a focal point and end points
    on a circle surrounding it, and conical gradients interpolate
    colors around a center point.

    \endtable

    \section1 Creating a Paint Device

    The QPaintDevice class is the base class of objects that can be
    painted, i.e. QPainter can draw on any QPaintDevice
    subclass. QPaintDevice's drawing capabilities are currently
    implemented by the QWidget, QImage, QPixmap, QGLWidget,
    QGLPixelBuffer, QPicture and QPrinter subclasses.

    \image paintsystem-devices.png

    \table 100%
    \row \o \bold {Custom Backends}

    Support for a new backend can be implemented by deriving from the
    QPaintDevice class and reimplementing the virtual
    QPaintDevice::paintEngine() function to tell QPainter which paint
    engine should be used to draw on this particular device. To
    actually be able to draw on the device, this paint engine must be
    a custom paint engine created by deriving from the QPaintEngine
    class.

    \endtable

    \section2 Widget

    The QWidget class is the base class of all user interface
    objects. The widget is the atom of the user interface: it receives
    mouse, keyboard and other events from the window system, and
    paints a representation of itself on the screen.

    \section2 Image

    The QImage class provides a hardware-independent image
    representation which is designed and optimized for I/O, and for
    direct pixel access and manipulation. QImage supports several
    image formats including monochrome, 8-bit, 32-bit and
    alpha-blended images.

    One advantage of using QImage as a paint device is that it is
    possible to guarantee the pixel exactness of any drawing operation
    in a platform-independent way. Another benefit is that the
    painting can be performed in another thread than the current GUI
    thread.

    \section2 Pixmap

    The QPixmap class is an off-screen image representation which is
    designed and optimized for showing images on screen. Unlike
    QImage, the pixel data in a pixmap is internal and is managed by
    the underlying window system, i.e. pixels can only be accessed
    through QPainter functions or by converting the QPixmap to a
    QImage.

    To optimize drawing with QPixmap, Qt provides the QPixmapCache
    class which can be used to store temporary pixmaps that are
    expensive to generate without using more storage space than the
    cache limit.

    Qt also provides the QBitmap convenience class, inheriting
    QPixmap. QBitmap guarantees monochrome (1-bit depth) pixmaps, and
    is mainly used for creating custom QCursor and QBrush objects,
    constructing QRegion objects, and for setting masks for pixmaps
    and widgets.

    \section2 OpenGL Widget

    As mentioned above, Qt provides the QtOpenGL module offering
    classes that makes it easy to use OpenGL in Qt applications. For
    example, the QGLWidget enables the OpenGL API for
    rendering.

    But QGLWidget is also a QWidget subclass, and can be used by
    QPainter as any other paint device. One huge benefit from this is
    that it enables Qt to utilize the high performance of OpenGL for
    most drawing operations, such as transformations and pixmap
    drawing.

    \section2 Pixel Buffer

    The QtOpenGL module also provides the QGLPixelBuffer class which
    inherits QPaintDevice directly.

    QGLPixelBuffer encapsulates an OpenGL pbuffer. Rendering into a
    pbuffer is normally done using full hardware acceleration which
    can be significantly faster than rendering into a QPixmap.

    \section2 Framebuffer Object

    The QtOpenGL module also provides the QGLFramebufferObject class
    which inherits QPaintDevice directly.

    QGLFramebufferObject encapsulates an OpenGL framebuffer object.
    Framebuffer objects can also be used for off-screen rendering, and
    offer several advantages over pixel buffers for this purpose.
    These are described in the QGLFramebufferObject class documentation.

    \section2 Picture

    The QPicture class is a paint device that records and replays
    QPainter commands. A picture serializes painter commands to an IO
    device in a platform-independent format. QPicture is also
    resolution independent, i.e. a QPicture can be displayed on
    different devices (for example svg, pdf, ps, printer and screen)
    looking the same.

    Qt provides the QPicture::load() and QPicture::save() functions
    for loading and saving pictures. But in addition the QPictureIO
    class is provided to enable the programmer to install new picture
    file formats in addition to those that Qt provides.

    \section2 Printer

    The QPrinter class is a paint device that paints on a printer. On
    Windows or Mac OS X, QPrinter uses the built-in printer
    drivers. On X11, QPrinter generates postscript and sends that to
    lpr, lp, or another print program. QPrinter can also print to any
    other QPrintEngine object.

    The QPrintEngine class defines an interface for how QPrinter
    interacts with a given printing subsystem. The common case when
    creating your own print engine, is to derive from both
    QPaintEngine and QPrintEngine.

    The output format is by default determined by the platform the
    printer is running on, but by explicitly setting the output format
    to QPrinter::PdfFormat, QPrinter will generate its output as a PDF
    file.

    \section1 Reading and Writing Image Files

    The most common way to read images is through QImage and QPixmap's
    constructors, or by calling the QImage::load() and QPixmap::load()
    functions. In addition, Qt provides the QImageReader class which
    gives more control over the process. Depending on the underlying
    support in the image format, the functions provided by the class
    can save memory and speed up loading of images.

    Likewise, Qt provides the QImageWriter class which supports
    setting format specific options, such as the gamma level,
    compression level and quality, prior to storing the image. If you
    do not need such options, you can use QImage::save() or
    QPixmap::save() instead.

    \table 100%
    \row
    \o \bold QMovie

    QMovie is a convenience class for displaying animations, using the
    QImageReader class internally.  Once created, the QMovie class
    provides various functions for both running and controlling the
    given animation.

    \o \image paintsystem-movie.png
    \endtable

    The QImageReader and QImageWriter classes rely on the
    QImageIOHandler class which is the common image I/O interface for
    all image formats in Qt. QImageIOHandler objects are used
    internally by QImageReader and QImageWriter to add support for
    different image formats to Qt.

    A list of the supported file formats are available through the
    QImageReader::supportedImageFormats() and
    QImageWriter::supportedImageFormats() functions. Qt supports
    several file formats by default, and in addition new formats can
    be added as plugins. The currently supported formats are listed in
    the QImageReader and QImageWriter class documentation.

    Qt's plugin mechanism can also be used to write a custom image
    format handler. This is done by deriving from the QImageIOHandler
    class, and creating a QImageIOPlugin object which is a factory for
    creating QImageIOHandler objects. When the plugin is installed,
    QImageReader and QImageWriter will automatically load the plugin
    and start using it.

    \table 100%
    \row
    \o \image paintsystem-svg.png
    \o \bold {SVG Rendering}

    Scalable Vector Graphics (SVG) is an language for describing both
    static and animated two-dimensional vector graphics. Qt includes
    support for the static features of SVG 1.2 Tiny.

    SVG drawings can be rendered onto any QPaintDevice subclass.  This
    approach gives developers the flexibility to experiment, in order
    to find the best solution for each application.

    The easiest way to render SVG files is to construct a QSvgWidget
    and load an SVG file using one of the QSvgWidget::load()
    functions. The rendering is performed by the QSvgRenderer class
    which also can be used directly to provide SVG support for custom
    widgets.

    For more information, see the QtSvg module documentation.

    \endtable

    \section1 Styling

    Qt's built-in widgets use the QStyle class to perform nearly all
    of their drawing.  QStyle is an abstract base class that
    encapsulates the look and feel of a GUI, and can be used to make
    the widgets look exactly like the equivalent native widgets or to
    give the widgets a custom look.

    Qt provides a set of QStyle subclasses that emulate the native
    look of the different platforms supported by Qt (QWindowsStyle,
    QMacStyle, QMotifStyle, etc.). These styles are built into the
    QtGui library, other styles can be made available using Qt's
    plugin mechansim.

    Most functions for drawing style elements take four arguments:

    \list
    \o an enum value specifying which graphical element to draw
    \o a QStyleOption object specifying how and where to render that element
    \o a QPainter object that should be used to draw the element
    \o a QWidget object on which the drawing is performed (optional)
    \endlist

    The style gets all the information it needs to render the
    graphical element from the QStyleOption class. The widget is
    passed as the last argument in case the style needs it to perform
    special effects (such as animated default buttons on Mac OS X),
    but it isn't mandatory. In fact, QStyle can be used to draw on any
    paint device (not just widgets), in which case the widget argument
    is a zero pointer.

    \image paintsystem-stylepainter.png

    The paint system also provides the QStylePainter class inheriting
    from QPainter.  QStylePainter is a convenience class for drawing
    QStyle elements inside a widget, and extends QPainter with a set
    of high-level drawing functions implemented on top of QStyle's
    API. The advantage of using QStylePainter is that the parameter
    lists get considerably shorter.

    \table 100%
    \row
    \o \inlineimage paintsystem-icon.png
    \o \bold QIcon

    The QIcon class provides scalable icons in different modes and states.

    QIcon can generate pixmaps reflecting an icon's state, mode and
    size. These pixmaps are generated from the set of pixmaps
    made available to the icon, and are used by Qt widgets to show an
    icon representing a particular action.

    The rendering of a QIcon object is handled by the QIconEngine
    class. Each icon has a corresponding icon engine that is
    responsible for drawing the icon with a requested size, mode and
    state.

    \endtable

    \section1 Selecting the Painting Backend

    Since Qt 4.5, it is possible to replace the paint engines and paint
    devices used for widgets, pixmaps and the offscreen double buffer. By
    default the backends are:

    \table
        \row
            \o Windows
            \o Software Rasterizer
        \row
            \o X11
            \o X11
        \row
            \o Mac OS X
            \o CoreGraphics
        \row
            \o Embedded
            \o Software Rasterizer
    \endtable

    Passing a command line parameter to the application, such as,
    \c{-graphicssystem raster}, specifies that Qt should use the software
    rasterizer for this application. The Software rasterizer is fully
    supported on all platforms.

    \code
      > analogclock -graphicssystem raster
    \endcode

    There is also a \c{-graphicssystem opengl} mode that uses OpenGL for
    all drawing. Currently, this engine is experimental as it does not draw
    everything correctly.

    Qt also supports being configured using \c {-graphicssystem
    raster|opengl} in which case all applications will use the
    specified graphics system for its graphics.

 */