Transformations Example

RenderArea Class Implementation

The RenderArea widget controls the rendering of a given shape, including the transformations of the coordinate system, by reimplementing the QWidget::paintEvent() event handler. But first we will take a quick look at the constructor and at the functions that provides access to the RenderArea widget:

 RenderArea::RenderArea(QWidget *parent)
     : QWidget(parent)
 {
     QFont newFont = font();
     newFont.setPixelSize(12);
     setFont(newFont);

     QFontMetrics fontMetrics(newFont);
     xBoundingRect = fontMetrics.boundingRect(tr("x"));
     yBoundingRect = fontMetrics.boundingRect(tr("y"));
 }

In the constructor we pass the parent parameter on to the base class, and customize the font that we will use to render the coordinates. The QWidget::font() function returns the font currently set for the widget. As long as no special font has been set, or after QWidget::setFont() is called, this is either a special font for the widget class, the parent's font or (if this widget is a top level widget) the default application font.

After ensuring that the font's size is 12 points, we extract the rectangles enclosing the coordinate letters, 'x' and 'y', using the QFontMetrics class.

QFontMetrics provides functions to access the individual metrics of the font, its characters, and for strings rendered in the font. The QFontMetrics::boundingRect() function returns the bounding rectangle of the given character relative to the left-most point on the base line.

 void RenderArea::setOperations(const QList<Operation> &operations)
 {
     this->operations = operations;
     update();
 }

 void RenderArea::setShape(const QPainterPath &shape)
 {
     this->shape = shape;
     update();
 }

In the setShape() and setOperations() functions we update the RenderArea widget by storing the new value or values followed by a call to the QWidget::update() slot which schedules a paint event for processing when Qt returns to the main event loop.

 QSize RenderArea::minimumSizeHint() const
 {
     return QSize(182, 182);
 }

 QSize RenderArea::sizeHint() const
 {
     return QSize(232, 232);
 }

We reimplement the QWidget's minimumSizeHint() and sizeHint() functions to give the RenderArea widget a reasonable size within our application. The default implementations of these functions returns an invalid size if there is no layout for this widget, and returns the layout's minimum size or preferred size, respectively, otherwise.

 void RenderArea::paintEvent(QPaintEvent *event)
 {
     QPainter painter(this);
     painter.setRenderHint(QPainter::Antialiasing);
     painter.fillRect(event->rect(), QBrush(Qt::white));

     painter.translate(66, 66);

The paintEvent() event handler receives the RenderArea widget's paint events. A paint event is a request to repaint all or part of the widget. It can happen as a result of QWidget::repaint() or QWidget::update(), or because the widget was obscured and has now been uncovered, or for many other reasons.

First we create a QPainter for the RenderArea widget. The QPainter::Antialiasing render hint indicates that the engine should antialias edges of primitives if possible. Then we erase the area that needs to be repainted using the QPainter::fillRect() function.

We also translate the coordinate system with an constant offset to ensure that the original shape is renderend with a suitable margin.

     painter.save();
     transformPainter(painter);
     drawShape(painter);
     painter.restore();

Before we start to render the shape, we call the QPainter::save() function.

QPainter::save() saves the current painter state (i.e. pushes the state onto a stack) including the current coordinate system. The rationale for saving the painter state is that the following call to the transformPainter() function will transform the coordinate system depending on the currently chosen transformation operations, and we need a way to get back to the original state to draw the outline.

After transforming the coordinate system, we draw the RenderArea's shape, and then we restore the painter state using the QPainter::restore() function (i.e. popping the saved state off the stack).

     drawOutline(painter);

Then we draw the square outline.

     transformPainter(painter);
     drawCoordinates(painter);
 }

Since we want the coordinates to correspond with the coordinate system the shape is rendered within, we must make another call to the transformPainter() function.

The order of the painting operations is essential with respect to the shared pixels. The reason why we don't render the coordinates when the coordinate system already is transformed to render the shape, but instead defer their rendering to the end, is that we want the coordinates to appear on top of the shape and its outline.

There is no need to save the QPainter state this time since drawing the coordinates is the last painting operation.

 void RenderArea::drawCoordinates(QPainter &painter)
 {
     painter.setPen(Qt::red);

     painter.drawLine(0, 0, 50, 0);
     painter.drawLine(48, -2, 50, 0);
     painter.drawLine(48, 2, 50, 0);
     painter.drawText(60 - xBoundingRect.width() / 2,
                      0 + xBoundingRect.height() / 2, tr("x"));

     painter.drawLine(0, 0, 0, 50);
     painter.drawLine(-2, 48, 0, 50);
     painter.drawLine(2, 48, 0, 50);
     painter.drawText(0 - yBoundingRect.width() / 2,
                      60 + yBoundingRect.height() / 2, tr("y"));
 }

 void RenderArea::drawOutline(QPainter &painter)
 {
     painter.setPen(Qt::darkGreen);
     painter.setPen(Qt::DashLine);
     painter.setBrush(Qt::NoBrush);
     painter.drawRect(0, 0, 100, 100);
 }

 void RenderArea::drawShape(QPainter &painter)
 {
     painter.fillPath(shape, Qt::blue);
 }

The drawCoordinates(), drawOutline() and drawShape() are convenience functions called from the paintEvent() event handler. For more information about QPainter's basic drawing operations and how to display basic graphics primitives, see the Basic Drawing example.

 void RenderArea::transformPainter(QPainter &painter)
 {
     for (int i = 0; i < operations.size(); ++i) {
         switch (operations[i]) {
         case Translate:
             painter.translate(50, 50);
             break;
         case Scale:
             painter.scale(0.75, 0.75);
             break;
         case Rotate:
             painter.rotate(60);
             break;
         case NoTransformation:
         default:
             ;
         }
     }
 }

The transformPainter() convenience function is also called from the paintEvent() event handler, and transforms the given QPainter's coordinate system according to the user's transformation choices.

 class Window : public QWidget
 {
     Q_OBJECT

 public:
     Window();

 public slots:
     void operationChanged();
     void shapeSelected(int index);

 private:
     void setupShapes();

     enum { NumTransformedAreas = 3 };
     RenderArea *originalRenderArea;
     RenderArea *transformedRenderAreas[NumTransformedAreas];
     QComboBox *shapeComboBox;
     QComboBox *operationComboBoxes[NumTransformedAreas];
     QList<QPainterPath> shapes;
 };

 Window::Window()
 {
     originalRenderArea = new RenderArea;

     shapeComboBox = new QComboBox;
     shapeComboBox->addItem(tr("Clock"));
     shapeComboBox->addItem(tr("House"));
     shapeComboBox->addItem(tr("Text"));
     shapeComboBox->addItem(tr("Truck"));

     QGridLayout *layout = new QGridLayout;
     layout->addWidget(originalRenderArea, 0, 0);
     layout->addWidget(shapeComboBox, 1, 0);

     for (int i = 0; i < NumTransformedAreas; ++i) {
         transformedRenderAreas[i] = new RenderArea;

         operationComboBoxes[i] = new QComboBox;
         operationComboBoxes[i]->addItem(tr("No transformation"));
         operationComboBoxes[i]->addItem(tr("Rotate by 60\xB0"));
         operationComboBoxes[i]->addItem(tr("Scale to 75%"));
         operationComboBoxes[i]->addItem(tr("Translate by (50, 50)"));

         connect(operationComboBoxes[i], SIGNAL(activated(int)),
                 this, SLOT(operationChanged()));

         layout->addWidget(transformedRenderAreas[i], 0, i + 1);
         layout->addWidget(operationComboBoxes[i], 1, i + 1);
     }

     setLayout(layout);
     setupShapes();
     shapeSelected(0);

     setWindowTitle(tr("Transformations"));
 }

 void Window::setupShapes()
 {
     QPainterPath truck;
     QPainterPath clock;
     QPainterPath house;
     QPainterPath text;
     ...
     shapes.append(clock);
     shapes.append(house);
     shapes.append(text);
     shapes.append(truck);

     connect(shapeComboBox, SIGNAL(activated(int)),
             this, SLOT(shapeSelected(int)));
 }

 void Window::operationChanged()
 {
     static const Operation operationTable[] = {
         NoTransformation, Rotate, Scale, Translate
     };

     QList<Operation> operations;
     for (int i = 0; i < NumTransformedAreas; ++i) {
         int index = operationComboBoxes[i]->currentIndex();
         operations.append(operationTable[index]);
         transformedRenderAreas[i]->setOperations(operations);
     }
 }

 void Window::shapeSelected(int index)
 {
     QPainterPath shape = shapes[index];
     originalRenderArea->setShape(shape);
     for (int i = 0; i < NumTransformedAreas; ++i)
         transformedRenderAreas[i]->setShape(shape);
 }