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+
+/*!
+ \example widgets/tablet
+ \title Tablet Example
+
+ This example shows how to use a Wacom tablet in Qt applications.
+
+ \image tabletexample.png
+
+ When you use a tablet with Qt applications, \l{QTabletEvent}s are
+ generated. You need to reimplement the
+ \l{QWidget::}{tabletEvent()} event handler if you want to handle
+ tablet events. Events are generated when the device used for
+ drawing enters and leaves the proximity of the tablet (i.e., when
+ it is close but not pressed down on it), when a device is pushed
+ down and released from it, and when a device is moved on the
+ tablet.
+
+ The information available in QTabletEvent depends on the device
+ used. The tablet in this example has two different devices for
+ drawing: a stylus and an airbrush. For both devices the event
+ contains the position of the device, pressure on the tablet,
+ vertical tilt, and horizontal tilt (i.e, the angle between the
+ device and the perpendicular of the tablet). The airbrush has a
+ finger wheel; the position of this is also available in the tablet
+ event.
+
+ In this example we implement a drawing program. You can use the
+ stylus to draw on the tablet as you use a pencil on paper. When
+ you draw with the airbrush you get a spray of paint; the finger
+ wheel is used to change the density of the spray. The pressure and
+ tilt can change the alpha and saturation values of the QColor and the
+ width of the QPen used for drawing.
+
+ The example consists of the following:
+
+ \list
+ \o The \c MainWindow class inherits QMainWindow and creates
+ the examples menus and connect their slots and signals.
+ \o The \c TabletCanvas class inherits QWidget and
+ receives tablet events. It uses the events to paint on a
+ offscreen pixmap, which it draws onto itself.
+ \o The \c TabletApplication class inherits QApplication. This
+ class handles tablet events that are not sent to \c tabletEvent().
+ We will look at this later.
+ \o The \c main() function creates a \c MainWindow and shows it
+ as a top level window.
+ \endlist
+
+
+ \section1 MainWindow Class Definition
+
+ The \c MainWindow creates a \c TabletCanvas and sets it as its
+ center widget.
+
+ \snippet examples/widgets/tablet/mainwindow.h 0
+
+ The QActions let the user select if the tablets pressure and
+ tilt should change the pen width, color alpha component and color
+ saturation. \c createActions() creates all actions, and \c
+ createMenus() sets up the menus with the actions. We have one
+ QActionGroup for the actions that alter the alpha channel, color
+ saturation and line width respectively. The action groups are
+ connected to the \c alphaActionTriggered(), \c
+ colorSaturationActiontriggered(), and \c
+ lineWidthActionTriggered() slots, which calls functions in \c
+ myCanvas.
+
+
+ \section1 MainWindow Class Implementation
+
+ We start width a look at the constructor \c MainWindow():
+
+ \snippet examples/widgets/tablet/mainwindow.cpp 0
+
+ In the constructor we create the canvas, actions, and menus.
+ We set the canvas as the center widget. We also initialize the
+ canvas to match the state of our menus and start drawing with a
+ red color.
+
+ Here is the implementation of \c brushColorAct():
+
+ \snippet examples/widgets/tablet/mainwindow.cpp 1
+
+ We let the user pick a color with a QColorDialog. If it is valid,
+ we set a new drawing color with \c setColor().
+
+ Here is the implementation of \c alphaActionTriggered():
+
+ \snippet examples/widgets/tablet/mainwindow.cpp 2
+
+ The \c TabletCanvas class supports two ways by which the alpha
+ channel of the drawing color can be changed: tablet pressure and
+ tilt. We have one action for each and an action if the alpha
+ channel should not be changed.
+
+ Here is the implementation of \c lineWidthActionTriggered():
+
+ \snippet examples/widgets/tablet/mainwindow.cpp 3
+
+ We check which action is selected in \c lineWidthGroup, and set
+ how the canvas should change the drawing line width.
+
+ Here is the implementation of \c saturationActionTriggered():
+
+ \snippet examples/widgets/tablet/mainwindow.cpp 4
+
+ We check which action is selected in \c colorSaturationGroup, and
+ set how the canvas should change the color saturation of the
+ drawing color.
+
+ Here is the implementation of \c saveAct():
+
+ \snippet examples/widgets/tablet/mainwindow.cpp 5
+
+ We use the QFileDialog to let the user select a file to save the
+ drawing in. It is the \c TabletCanvas that save the drawing, so we
+ call its \c saveImage() function.
+
+ Here is the implementation of \c loadAct():
+
+ \snippet examples/widgets/tablet/mainwindow.cpp 6
+
+ We let the user select the image file to be opened with
+ a QFileDialog; we then ask the canvas to load the image with \c
+ loadImage().
+
+ Here is the implementation of \c aboutAct():
+
+ \snippet examples/widgets/tablet/mainwindow.cpp 7
+
+ We show a message box with a short description of the example.
+
+ \c createActions() creates all actions and action groups of
+ the example. We look at the creation of one action group and its
+ actions. See the \l{Application Example}{application example} if
+ you want a high-level introduction to QActions.
+
+ Here is the implementation of \c createActions:
+
+ \snippet examples/widgets/tablet/mainwindow.cpp 8
+ \dots
+ \snippet examples/widgets/tablet/mainwindow.cpp 9
+
+ We want the user to be able to choose if the drawing color's
+ alpha component should be changed by the tablet pressure or tilt.
+ We have one action for each choice and an action if the alpha
+ channel is not to be changed, i.e, the color is opaque. We make
+ the actions checkable; the \c alphaChannelGroup will then ensure
+ that only one of the actions are checked at any time. The \c
+ triggered() signal is emitted when an action is checked.
+
+ \dots
+ \snippet examples/widgets/tablet/mainwindow.cpp 10
+
+ Here is the implementation of \c createMenus():
+
+ \snippet examples/widgets/tablet/mainwindow.cpp 11
+
+ We create the menus of the example and add the actions to them.
+
+
+ \section1 TabletCanvas Class Definition
+
+ The \c TabletCanvas class provides a surface on which the
+ user can draw with a tablet.
+
+ \snippet examples/widgets/tablet/tabletcanvas.h 0
+
+ The canvas can change the alpha channel, color saturation,
+ and line width of the drawing. We have one enum for each of
+ these; their values decide if it is the tablet pressure or tilt
+ that will alter them. We keep a private variable for each, the \c
+ alphaChannelType, \c colorSturationType, and \c penWidthType,
+ which we provide access functions for.
+
+ We draw on a QPixmap with \c myPen and \c myBrush using \c
+ myColor. The \c saveImage() and \c loadImage() saves and loads
+ the QPixmap to disk. The pixmap is drawn on the widget in \c
+ paintEvent(). The \c pointerType and \c deviceType keeps the type
+ of pointer, which is either a pen or an eraser, and device
+ currently used on the tablet, which is either a stylus or an
+ airbrush.
+
+ The interpretation of events from the tablet is done in \c
+ tabletEvent(); \c paintPixmap(), \c updateBrush(), and \c
+ brushPattern() are helper functions used by \c tabletEvent().
+
+
+ \section1 TabletCanvas Class Implementation
+
+ We start with a look at the constructor:
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 0
+
+ In the constructor we initialize our class variables. We need
+ to draw the background of our pixmap, as the default is gray.
+
+ Here is the implementation of \c saveImage():
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 1
+
+ QPixmap implements functionality to save itself to disk, so we
+ simply call \l{QPixmap::}{save()}.
+
+ Here is the implementation of \c loadImage():
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 2
+
+ We simply call \l{QPixmap::}{load()}, which loads the image in \a
+ file.
+
+ Here is the implementation of \c tabletEvent():
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 3
+
+ We get three kind of events to this function: TabletPress,
+ TabletRelease, and TabletMove, which is generated when a device
+ is pressed down on, leaves, or moves on the tablet. We set the \c
+ deviceDown to true when a device is pressed down on the tablet;
+ we then know when we should draw when we receive move events. We
+ have implemented the \c updateBrush() and \c paintPixmap() helper
+ functions to update \c myBrush and \c myPen after the state of \c
+ alphaChannelType, \c colorSaturationType, and \c lineWidthType.
+
+ Here is the implementation of \c paintEvent():
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 4
+
+ We simply draw the pixmap to the top left of the widget.
+
+ Here is the implementation of \c paintPixmap():
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 5
+
+ In this function we draw on the pixmap based on the movement of the
+ device. If the device used on the tablet is a stylus we want to draw a
+ line between the positions of the stylus recorded in \c polyLine. We
+ also assume that this is a reasonable handling of any unknown device,
+ but update the statusbar with a warning so that the user can see that
+ for his tablet he might have to implement special handling.
+ If it is an airbrush we want to draw a circle of points with a
+ point density based on the tangential pressure, which is the position
+ of the finger wheel on the airbrush. We use the Qt::BrushStyle to
+ draw the points as it has styles that draw points with different
+ density; we select the style based on the tangential pressure in
+ \c brushPattern().
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 6
+
+ We return a brush style with a point density that increases with
+ the tangential pressure.
+
+ In \c updateBrush() we set the pen and brush used for drawing
+ to match \c alphaChannelType, \c lineWidthType, \c
+ colorSaturationType, and \c myColor. We will examine the code to
+ set up \c myBrush and \c myPen for each of these variables:
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 7
+
+ We fetch the current drawingcolor's hue, saturation, value,
+ and alpha values. \c hValue and \c vValue are set to the
+ horizontal and vertical tilt as a number from 0 to 255. The
+ original values are in degrees from -60 to 60, i.e., 0 equals
+ -60, 127 equals 0, and 255 equals 60 degrees. The angle measured
+ is between the device and the perpendicular of the tablet (see
+ QTabletEvent for an illustration).
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 8
+
+ The alpha channel of QColor is given as a number between 0
+ and 255 where 0 is transparent and 255 is opaque.
+ \l{QTabletEvent::}{pressure()} returns the pressure as a qreal
+ between 0.0 and 1.0. By subtracting 127 from the tilt values and
+ taking the absolute value we get the smallest alpha values (i.e.,
+ the color is most transparent) when the pen is perpendicular to
+ the tablet. We select the largest of the vertical and horizontal
+ tilt value.
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 9
+
+ The colorsaturation is given as a number between 0 and 255. It is
+ set with \l{QColor::}{setHsv()}. We can set the tilt values
+ directly, but must multiply the pressure to a number between 0 and
+ 255.
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 10
+
+ The width of the pen increases with the pressure. When the pen
+ width is controlled with the tilt we let the width increse with
+ the angle between the device and the perpendicular of the tablet.
+
+ \snippet examples/widgets/tablet/tabletcanvas.cpp 11
+
+ We finally check wether the pointer is the stylus or the eraser.
+ If it is the eraser, we set the color to the background color of
+ the pixmap an let the pressure decide the pen width, else we set
+ the colors we have set up previously in the function.
+
+
+ \section1 TabletApplication Class Definition
+
+ We inherit QApplication in this class because we want to
+ reimplement the \l{QApplication::}{event()} function.
+
+ \snippet examples/widgets/tablet/tabletapplication.h 0
+
+ We keep a \c TabletCanvas we send the device type of the events we
+ handle in the \c event() function to. The TabletEnterProximity
+ and TabletLeaveProximity events are not sendt to the QApplication
+ object, while other tablet events are sendt to the QWidget's
+ \c event(), which sends them on to \l{QWidget::}{tabletEvent()}.
+ Since we want to handle these events we have implemented \c
+ TabletApplication.
+
+
+ \section1 TabletApplication Class Implementation
+
+ Here is the implementation of \c event():
+
+ \snippet examples/widgets/tablet/tabletapplication.cpp 0
+
+ We use this function to handle the TabletEnterProximity and
+ TabletLeaveProximity events, which is generated when a device
+ enters and leaves the proximity of the tablet. The intended use of these
+ events is to do work that is dependent on what kind of device is
+ used on the tablet. This way, you don't have to do this work
+ when other events are generated, which is more frequently than the
+ leave and enter proximity events. We call \c setTabletDevice() in
+ \c TabletCanvas.
+
+ \section1 The \c main() function
+
+ Here is the examples \c main() function:
+
+ \snippet examples/widgets/tablet/main.cpp 0
+
+ In the \c main() function we create a \c MainWinow and display it
+ as a top level window. We use the \c TabletApplication class. We
+ need to set the canvas after the application is created. We cannot
+ use classes that implement event handling before an QApplication
+ object is instantiated.
+*/