Overpainting Example

GLWidget Class Implementation

Again, we only show the parts of the GLWidget implementation that are relevant to this example. In the constructor, we initialize a QTimer to control the animation:

 GLWidget::GLWidget(QWidget *parent)
     : QGLWidget(QGLFormat(QGL::SampleBuffers), parent)
 {
     QTime midnight(0, 0, 0);
     qsrand(midnight.secsTo(QTime::currentTime()));

     object = 0;
     xRot = 0;
     yRot = 0;
     zRot = 0;

     trolltechGreen = QColor::fromCmykF(0.40, 0.0, 1.0, 0.0);
     trolltechPurple = QColor::fromCmykF(0.39, 0.39, 0.0, 0.0);

     animationTimer.setSingleShot(false);
     connect(&animationTimer, SIGNAL(timeout()), this, SLOT(animate()));
     animationTimer.start(25);

     setAttribute(Qt::WA_NoSystemBackground);
     setMinimumSize(200, 200);
     setWindowTitle(tr("Overpainting a Scene"));
 }

For a small performance improvement, we set the widget's Qt::WA_NoSystemBackground attribute to instruct the underlying window system not to paint a background for the widget.

As in the Hello GL example, the destructor is responsible for freeing any OpenGL-related resources:

 GLWidget::~GLWidget()
 {
     makeCurrent();
     glDeleteLists(object, 1);
 }

The initializeGL() function is fairly minimal, only setting up the display list used in the scene. To cooperate fully with QPainter, we defer matrix stack operations and attribute initialization until the widget needs to be updated.

In this example, we implement paintEvent() rather than paintGL() to render our scene. When drawing on a QGLWidget, the paint engine used by QPainter performs certain operations that change the states of the OpenGL implementation's matrix and property stacks. Therefore, it is necessary to construct a QPainter for use on the widget before making any OpenGL calls.

 void GLWidget::paintEvent(QPaintEvent *event)
 {
     QPainter painter;
     painter.begin(this);
     painter.setRenderHint(QPainter::Antialiasing);

After these calls, the OpenGL implementation is in the correct state for drawing 2D graphics. However, we first need to render a 3D scene by setting up model and projection transformations and other attributes. We use OpenGL stack operations to preserve the original state, allowing us to recover it later:

     glPushAttrib(GL_ALL_ATTRIB_BITS);
     glMatrixMode(GL_PROJECTION);
     glPushMatrix();
     glMatrixMode(GL_MODELVIEW);
     glPushMatrix();

We define a color to use for the widget's background, and set up various attributes that define how the scene will be rendered.

     qglClearColor(trolltechPurple.dark());
     glShadeModel(GL_SMOOTH);
     glEnable(GL_DEPTH_TEST);
     glEnable(GL_CULL_FACE);
     glEnable(GL_LIGHTING);
     glEnable(GL_LIGHT0);
     glEnable(GL_MULTISAMPLE);
     static GLfloat lightPosition[4] = { 0.5, 5.0, 7.0, 1.0 };
     glLightfv(GL_LIGHT0, GL_POSITION, lightPosition);

     setupViewport(width(), height());

We call the setupViewport() private function to set up the projection used for the scene. This is unnecessary in OpenGL examples that implement the paintGL() function because the matrix stacks are usually unmodified between calls to resizeGL() and paintGL().

Since the widget's background is not drawn by the system or by Qt, we use an OpenGL call to paint it before positioning the object defined earlier in the scene:

     glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
     glLoadIdentity();
     glTranslated(0.0, 0.0, -10.0);
     glRotated(xRot / 16.0, 1.0, 0.0, 0.0);
     glRotated(yRot / 16.0, 0.0, 1.0, 0.0);
     glRotated(zRot / 16.0, 0.0, 0.0, 1.0);
     glCallList(object);

Once the list containing the object has been executed, the matrix and attribute stacks need to be restored before we can begin overpainting:

     glPopAttrib();
     glMatrixMode(GL_MODELVIEW);
     glPopMatrix();
     glMatrixMode(GL_PROJECTION);
     glPopMatrix();

With the original state set by QPainter restored, we simply overpaint the widget with 2D graphics; in this case, using a helper class to draw a number of translucent bubbles onto the widget, and calling QPainter::drawImage() to overlay some instructions:

     foreach (Bubble *bubble, bubbles) {
         if (bubble->rect().intersects(event->rect()))
             bubble->drawBubble(&painter);
     }

     painter.drawImage((width() - image.width())/2, 0, image);
     painter.end();
 }

When QPainter::end() is called, suitable OpenGL-specific calls are made to write the scene, and its additional contents, onto the widget.

The implementation of the resizeGL() function sets up the dimensions of the viewport and defines a projection transformation:

 void GLWidget::resizeGL(int width, int height)
 {
     setupViewport(width, height);
     formatInstructions(width, height);
 }

Additionally, we take the opportunity to format the instructions to fit the width of the viewport.

Ideally, we want to arrange the 2D graphics to suit the widget's dimensions. To achieve this, we implement the showEvent() handler, creating new graphic elements (bubbles) if necessary at appropriate positions in the widget.

 void GLWidget::showEvent(QShowEvent *event)
 {
     Q_UNUSED(event);
     createBubbles(20 - bubbles.count());
 }

This function only has an effect if less than 20 bubbles have already been created.

The animate() slot is called every time the widget's animationTimer emits the timeout() signal. This keeps the bubbles moving around.

 void GLWidget::animate()
 {
     QMutableListIterator<Bubble*> iter(bubbles);

     while (iter.hasNext()) {
         Bubble *bubble = iter.next();
         bubble->move(rect());
     }
     update();
 }

We simply iterate over the bubbles in the bubbles list, updating the widget before and after each of them is moved.

The setupViewport() function is called from paintEvent() and resizeGL().

 void GLWidget::setupViewport(int width, int height)
 {
     int side = qMin(width, height);
     glViewport((width - side) / 2, (height - side) / 2, side, side);

     glMatrixMode(GL_PROJECTION);
     glLoadIdentity();
     glOrtho(-0.5, +0.5, +0.5, -0.5, 4.0, 15.0);
     glMatrixMode(GL_MODELVIEW);
 }

The formatInstructions() function is used to prepare some basic instructions that will be painted with the other 2D graphics over the 3D scene.

 void GLWidget::formatInstructions(int width, int height)
 {
     QString text = tr("Click and drag with the left mouse button "
                       "to rotate the Qt logo.");
     QFontMetrics metrics = QFontMetrics(font());
     int border = qMax(4, metrics.leading());

     QRect rect = metrics.boundingRect(0, 0, width - 2*border, int(height*0.125),
         Qt::AlignCenter | Qt::TextWordWrap, text);
     image = QImage(width, rect.height() + 2*border, QImage::Format_ARGB32_Premultiplied);
     image.fill(qRgba(0, 0, 0, 127));

     QPainter painter;
     painter.begin(&image);
     painter.setRenderHint(QPainter::TextAntialiasing);
     painter.setPen(Qt::white);
     painter.drawText((width - rect.width())/2, border,
                      rect.width(), rect.height(),
                      Qt::AlignCenter | Qt::TextWordWrap, text);
     painter.end();
 }

The instructions are painted onto a suitably-sized semi-transparent image that can be rendered using QPainter in the widget's paintEvent() handler function.