The Transformations example shows how transformations influence the way that QPainter renders graphics primitives.
The application allows the user to manipulate the rendering of a shape by changing the translation, rotation and scale of QPainter 's coordinate system.
The example consists of two classes and a global enum:
RenderArea
class controls the rendering of a given shape.
Window
class is the application's main window.
Operation
enum describes the various transformation operations available in the application.
First we will take a quick look at the
Operation
enum, then we will review the
RenderArea
class to see how a shape is rendered. Finally, we will take a look at the Transformations application's features implemented in the
Window
类。
通常， QPainter operates on the associated device's own coordinate system, but it also has good support for coordinate transformations.
The default coordinate system of a paint device has its origin at the top-left corner. The x values increase to the right and the y values increase downwards. You can scale the coordinate system by a given offset using the QPainter::scale () function, you can rotate it clockwise using the QPainter::rotate () function and you can translate it (i.e. adding a given offset to the points) using the QPainter::translate () function. You can also twist the coordinate system around the origin (called shearing) using the QPainter::shear () 函数。
All the tranformation operations operate on QPainter 's tranformation matrix that you can retrieve using the QPainter::worldTransform () function. A matrix transforms a point in the plane to another point. For more information about the transformation matrix, see the 坐标系 and QTransform 文档编制。
enum Operation { NoTransformation, Translate, Rotate, Scale };
全局
Operation
enum is declared in the
renderarea.h
file and describes the various transformation operations available in the Transformations application.
RenderArea
类继承
QWidget
, and controls the rendering of a given shape.
class RenderArea : public QWidget { Q_OBJECT public: RenderArea(QWidget *parent = nullptr); void setOperations(const QList<Operation> &operations); void setShape(const QPainterPath &shape); QSize minimumSizeHint() const override; QSize sizeHint() const override; protected: void paintEvent(QPaintEvent *event) override;
We declare two public functions,
setOperations()
and
setShape()
, to be able to specify the
RenderArea
widget's shape and to transform the coordinate system the shape is rendered within.
重实现
QWidget
's
minimumSizeHint()
and
sizeHint()
functions to give the
RenderArea
小部件在应用程序内的合理大小，和重实现
QWidget::paintEvent
() event handler to draw the render area's shape applying the user's transformation choices.
private: void drawCoordinates(QPainter &painter); void drawOutline(QPainter &painter); void drawShape(QPainter &painter); void transformPainter(QPainter &painter); QList<Operation> operations; QPainterPath shape; QRect xBoundingRect; QRect yBoundingRect; };
We also declare several convenience functions to draw the shape, the coordinate system's outline and the coordinates, and to transform the painter according to the chosen transformations.
此外，
RenderArea
widget keeps a list of the currently applied transformation operations, a reference to its shape, and a couple of convenience variables that we will use when rendering the coordinates.
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
小部件：
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 类。
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(); }
在
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); }
重实现
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);
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
() 或
QWidget::update
(), or because the widget was obscured and has now been uncovered, or for many other reasons.
First we create a
QPainter
为
RenderArea
小部件。
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
() 函数。
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 () 函数。
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()
函数。
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); }
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
基本绘制
范例。
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: ; } } }
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.
Window
class is the Transformations application's main window.
The application displays four
RenderArea
widgets. The left-most widget renders the shape in
QPainter
's default coordinate system, the others render the shape with the chosen transformation in addition to all the transformations applied to the
RenderArea
widgets to their left.
class Window : public QWidget { Q_OBJECT public: Window(); public slots: void operationChanged(); void shapeSelected(int index);
We declare two public slots to make the application able to respond to user interaction, updating the displayed
RenderArea
widgets according to the user's transformation choices.
operationChanged()
slot updates each of the
RenderArea
widgets applying the currently chosen transformation operations, and is called whenever the user changes the selected operations. The
shapeSelected()
slot updates the
RenderArea
widgets' shapes whenever the user changes the preferred shape.
private: void setupShapes(); enum { NumTransformedAreas = 3 }; RenderArea *originalRenderArea; RenderArea *transformedRenderAreas[NumTransformedAreas]; QComboBox *shapeComboBox; QComboBox *operationComboBoxes[NumTransformedAreas]; QList<QPainterPath> shapes; };
We also declare a private convenience function,
setupShapes()
, that is used when constructing the
Window
widget, and we declare pointers to the various components of the widget. We choose to keep the available shapes in a
QList
of
QPainterPath
s. In addition we declare a private enum counting the number of displayed
RenderArea
widgets except the widget that renders the shape in
QPainter
's default coordinate system.
In the constructor we create and initialize the application's components:
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);
First we create the
RenderArea
widget that will render the shape in the default coordinate system. We also create the associated
QComboBox
that allows the user to choose among four different shapes: A clock, a house, a text and a truck. The shapes themselves are created at the end of the constructor, using the
setupShapes()
convenience function.
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\xC2\xB0"));
operationComboBoxes[i]->addItem(tr("Scale to 75%"));
operationComboBoxes[i]->addItem(tr("Translate by (50, 50)"));
connect(operationComboBoxes[i], &QComboBox::activated,
this, &Window::operationChanged);
layout->addWidget(transformedRenderAreas[i], 0, i + 1);
layout->addWidget(operationComboBoxes[i], 1, i + 1);
}
Then we create the
RenderArea
widgets that will render their shapes with coordinate tranformations. By default the applied operation is
No Transformation
, i.e. the shapes are rendered within the default coordinate system. We create and initialize the associated
QComboBox
es with items corresponding to the various transformation operations decribed by the global
Operation
枚举。
We also connect the
QComboBox
es'
activated()
信号到
operationChanged()
slot to update the application whenever the user changes the selected transformation operations.
setLayout(layout);
setupShapes();
shapeSelected(0);
setWindowTitle(tr("Transformations"));
}
Finally, we set the layout for the application window using the
QWidget::setLayout
() function, construct the available shapes using the private
setupShapes()
convenience function, and make the application show the clock shape on startup using the public
shapeSelected()
slot before we set the window title.
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, &QComboBox::activated, this, &Window::shapeSelected); }
setupShapes()
function is called from the constructor and create the
QPainterPath
objects representing the shapes that are used in the application. For construction details, see the
painting/transformations/window.cpp
example file. The shapes are stored in a
QList
。
QList::append
() function inserts the given shape at the end of the list.
We also connect the associated
QComboBox
's
activated()
信号到
shapeSelected()
slot to update the application when the user changes the preferred shape.
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); } }
公共
operationChanged()
slot is called whenever the user changes the selected operations.
We retrieve the chosen transformation operation for each of the transformed
RenderArea
widgets by querying the associated
QComboBoxes
. The transformed
RenderArea
widgets are supposed to render the shape with the transformation specified by its associated combobox
in addition to
all the transformations applied to the
RenderArea
widgets to its left. For that reason, for each widget we query, we append the associated operation to a
QList
of transformations which we apply to the widget before proceeding to the next.
void Window::shapeSelected(int index) { QPainterPath shape = shapes[index]; originalRenderArea->setShape(shape); for (int i = 0; i < NumTransformedAreas; ++i) transformedRenderAreas[i]->setShape(shape); }
shapeSelected()
slot is called whenever the user changes the preferred shape, updating the
RenderArea
widgets using their public
setShape()
函数。
The Transformations example shows how transformations influence the way that QPainter renders graphics primitives. Normally, the QPainter operates on the device's own coordinate system, but it also has good support for coordinate transformations. With the Transformations application you can scale, rotate and translate QPainter 's coordinate system. The order in which these tranformations are applied is essential for the result.
All the tranformation operations operate on QPainter 's tranformation matrix. For more information about the transformation matrix, see the 坐标系 and QTransform 文档编制。
The Qt reference documentation provides several painting examples. Among these is the 仿射变换 example that shows Qt's ability to perform transformations on painting operations. The example also allows the user to experiment with the various transformation operations.