/**************************************************************************** ** ** Copyright (C) 2011 Nokia Corporation and/or its subsidiary(-ies). ** All rights reserved. ** Contact: Nokia Corporation (qt-info@nokia.com) ** ** This file is part of the documentation of the Qt Toolkit. ** ** $QT_BEGIN_LICENSE:FDL$ ** GNU Free Documentation License ** 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. ** ** Other Usage ** Alternatively, this file may be used in accordance with the terms ** and conditions contained in a signed written agreement between you ** and Nokia. ** ** ** ** ** $QT_END_LICENSE$ ** ****************************************************************************/ /*! \example opengl/hellogl \title Hello GL Example The Hello GL example demonstrates the basic use of the OpenGL-related classes provided with Qt. \image hellogl-example.png Qt provides the QGLWidget class to enable OpenGL graphics to be rendered within a standard application user interface. By subclassing this class, and providing reimplementations of event handler functions, 3D scenes can be displayed on widgets that can be placed in layouts, connected to other objects using signals and slots, and manipulated like any other widget. \tableofcontents \section1 GLWidget Class Definition The \c GLWidget class contains some standard public definitions for the constructor, destructor, \l{QWidget::sizeHint()}{sizeHint()}, and \l{QWidget::minimumSizeHint()}{minimumSizeHint()} functions: \snippet examples/opengl/hellogl/glwidget.h 0 We use a destructor to ensure that any OpenGL-specific data structures are deleted when the widget is no longer needed (although in this case nothing needs cleaning up). \snippet examples/opengl/hellogl/glwidget.h 1 The signals and slots are used to allow other objects to interact with the 3D scene. \snippet examples/opengl/hellogl/glwidget.h 2 OpenGL initialization, viewport resizing, and painting are handled by reimplementing the QGLWidget::initializeGL(), QGLWidget::resizeGL(), and QGLWidget::paintGL() handler functions. To enable the user to interact directly with the scene using the mouse, we reimplement QWidget::mousePressEvent() and QWidget::mouseMoveEvent(). \snippet examples/opengl/hellogl/glwidget.h 3 The rest of the class contains utility functions and variables that are used to construct and hold orientation information for the scene. The \c logo variable will be used to hold a pointer to the QtLogo object which contains all the geometry. \section1 GLWidget Class Implementation In this example, we split the class into groups of functions and describe them separately. This helps to illustrate the differences between subclasses of native widgets (such as QWidget and QFrame) and QGLWidget subclasses. \section2 Widget Construction and Sizing The constructor provides default rotation angles for the scene, sets the pointer to the QtLogo object to null, and sets up some colors for later use. \snippet examples/opengl/hellogl/glwidget.cpp 0 We also implement a destructor to release OpenGL-related resources when the widget is deleted: \snippet examples/opengl/hellogl/glwidget.cpp 1 In this case nothing requires cleaning up. We provide size hint functions to ensure that the widget is shown at a reasonable size: \snippet examples/opengl/hellogl/glwidget.cpp 2 \codeline \snippet examples/opengl/hellogl/glwidget.cpp 3 \snippet examples/opengl/hellogl/glwidget.cpp 4 The widget provides three slots that enable other components in the example to change the orientation of the scene: \snippet examples/opengl/hellogl/glwidget.cpp 5 In the above slot, the \c xRot variable is updated only if the new angle is different to the old one, the \c xRotationChanged() signal is emitted to allow other components to be updated, and the widget's \l{QGLWidget::updateGL()}{updateGL()} handler function is called. The \c setYRotation() and \c setZRotation() slots perform the same task for rotations measured by the \c yRot and \c zRot variables. \section2 OpenGL Initialization The \l{QGLWidget::initializeGL()}{initializeGL()} function is used to perform useful initialization tasks that are needed to render the 3D scene. These often involve defining colors and materials, enabling and disabling certain rendering flags, and setting other properties used to customize the rendering process. \snippet examples/opengl/hellogl/glwidget.cpp 6 In this example, we reimplement the function to set the background color, create a QtLogo object instance which will contain all the geometry to display, and set up the rendering process to use a particular shading model and rendering flags. \section2 Resizing the Viewport The \l{QGLWidget::resizeGL()}{resizeGL()} function is used to ensure that the OpenGL implementation renders the scene onto a viewport that matches the size of the widget, using the correct transformation from 3D coordinates to 2D viewport coordinates. The function is called whenever the widget's dimensions change, and is supplied with the new width and height. Here, we define a square viewport based on the length of the smallest side of the widget to ensure that the scene is not distorted if the widget has sides of unequal length: \snippet examples/opengl/hellogl/glwidget.cpp 8 A discussion of the projection transformation used is outside the scope of this example. Please consult the OpenGL reference documentation for an explanation of projection matrices. \section2 Painting the Scene The \l{QGLWidget::paintGL()}{paintGL()} function is used to paint the contents of the scene onto the widget. For widgets that only need to be decorated with pure OpenGL content, we reimplement QGLWidget::paintGL() \e instead of reimplementing QWidget::paintEvent(): \snippet examples/opengl/hellogl/glwidget.cpp 7 In this example, we clear the widget using the background color that we defined in the \l{QGLWidget::initializeGL()}{initializeGL()} function, set up the frame of reference for the geometry we want to display, and call the draw method of the QtLogo object to render the scene. \section2 Mouse Handling Just as in subclasses of native widgets, mouse events are handled by reimplementing functions such as QWidget::mousePressEvent() and QWidget::mouseMoveEvent(). The \l{QWidget::mousePressEvent()}{mousePressEvent()} function simply records the position of the mouse when a button is initially pressed: \snippet examples/opengl/hellogl/glwidget.cpp 9 The \l{QWidget::mouseMoveEvent()}{mouseMoveEvent()} function uses the previous location of the mouse cursor to determine how much the object in the scene should be rotated, and in which direction: \snippet examples/opengl/hellogl/glwidget.cpp 10 Since the user is expected to hold down the mouse button and drag the cursor to rotate the object, the cursor's position is updated every time a move event is received. \section1 QtLogo Class This class encapsulates the OpenGL geometry data which will be rendered in the basic 3D scene. \snippet examples/opengl/shared/qtlogo.h 0 The geometry is divided into a list of parts which may be rendered in different ways. The data itself is contained in a Geometry structure that includes the vertices, their lighting normals and index values which point into the vertices, grouping them into faces. \snippet examples/opengl/shared/qtlogo.cpp 0 The data in the Geometry class is stored in QVector members which are convenient for use with OpenGL because they expose raw contiguous floating point values via the constData() method. Methods are included for adding new vertex data, either with smooth normals, or facetted normals; and for enabling the geometry ready for rendering. \snippet examples/opengl/shared/qtlogo.cpp 1 The higher level Patch class has methods for accumulating the geometry one face at a time, and treating collections of faces or "patches" with transformations, applying different colors or smoothing. Although faces may be added as triangles or quads, at the OpenGL level all data is treated as triangles for compatibility with OpenGL/ES. \snippet examples/opengl/shared/qtlogo.cpp 2 Drawing a Patch is simply acheived by applying any transformation, and material effect, then drawing the data using the index range for the patch. The model-view matrix is saved and then restored so that any transformation does not affect other parts of the scene. \snippet examples/opengl/shared/qtlogo.cpp 3 The geometry is built once on construction of the QtLogo, and it is paramaterized on a number of divisions - which controls how "chunky" the curved section of the logo looks - and on a scale, so larger and smaller QtLogo objects can be created without having to use OpenGL scaling (which would force normal recalculation). The building process is done by helper classes (read the source for full details) which only exist during the build phase, to assemble the parts of the scene. \snippet examples/opengl/shared/qtlogo.cpp 4 Finally the complete QtLogo scene is simply drawn by enabling the data arrays and then iterating over the parts, calling draw() on each one. \section1 Window Class Definition The \c Window class is used as a container for the \c GLWidget used to display the scene: \snippet examples/opengl/hellogl/window.h 0 In addition, it contains sliders that are used to change the orientation of the object in the scene. \section1 Window Class Implementation The constructor constructs an instance of the \c GLWidget class and some sliders to manipulate its contents. \snippet examples/opengl/hellogl/window.cpp 0 We connect the \l{QAbstractSlider::valueChanged()}{valueChanged()} signal from each of the sliders to the appropriate slots in \c{glWidget}. This allows the user to change the orientation of the object by dragging the sliders. We also connect the \c xRotationChanged(), \c yRotationChanged(), and \c zRotationChanged() signals from \c glWidget to the \l{QAbstractSlider::setValue()}{setValue()} slots in the corresponding sliders. \snippet examples/opengl/hellogl/window.cpp 1 The sliders are placed horizontally in a layout alongside the \c GLWidget, and initialized with suitable default values. The \c createSlider() utility function constructs a QSlider, and ensures that it is set up with a suitable range, step value, tick interval, and page step value before returning it to the calling function: \snippet examples/opengl/hellogl/window.cpp 2 \section1 Summary The \c GLWidget class implementation shows how to subclass QGLWidget for the purposes of rendering a 3D scene using OpenGL calls. Since QGLWidget is a subclass of QWidget, subclasses of QGLWidget can be placed in layouts and provided with interactive features just like normal custom widgets. We ensure that the widget is able to correctly render the scene using OpenGL by reimplementing the following functions: \list \o QGLWidget::initializeGL() sets up resources needed by the OpenGL implementation to render the scene. \o QGLWidget::resizeGL() resizes the viewport so that the rendered scene fits onto the widget, and sets up a projection matrix to map 3D coordinates to 2D viewport coordinates. \o QGLWidget::paintGL() performs painting operations using OpenGL calls. \endlist Since QGLWidget is a subclass of QWidget, it can also be used as a normal paint device, allowing 2D graphics to be drawn with QPainter. This use of QGLWidget is discussed in the \l{2D Painting Example}{2D Painting} example. More advanced users may want to paint over parts of a scene rendered using OpenGL. QGLWidget allows pure OpenGL rendering to be mixed with QPainter calls, but care must be taken to maintain the state of the OpenGL implementation. See the \l{Overpainting Example}{Overpainting} example for more information. */