The QPainter class is used to draw 2D graphics primitives onto paint devices provided by QPaintDevice subclasses, such as QWidget and QImage .
Since QGLWidget is a subclass of QWidget , it is possible to reimplement its paintEvent() and use QPainter to draw on the device, just as you would with a QWidget . The only difference is that the painting operations will be accelerated in hardware if it is supported by your system's OpenGL drivers.
In this example, we perform the same painting operations on a QWidget and a QGLWidget. The QWidget is shown with anti-aliasing enabled, and the QGLWidget will also use anti-aliasing if the required extensions are supported by your system's OpenGL driver.
To be able to compare the results of painting onto a QGLWidget subclass with native drawing in a
QWidget
subclass, we want to show both kinds of widget side by side. To do this, we derive subclasses of
QWidget
and QGLWidget, using a separate
Helper
class to perform the same painting operations for each, and lay them out in a top-level widget, itself provided a the
Window
类。
In this example, the painting operations are performed by a helper class. We do this because we want the same painting operations to be performed for both our QWidget subclass and the QGLWidget subclass.
The
Helper
class is minimal:
class Helper { public: Helper(); public: void paint(QPainter *painter, QPaintEvent *event, int elapsed); private: QBrush background; QBrush circleBrush; QFont textFont; QPen circlePen; QPen textPen; };
Apart from the constructor, it only provides a
paint()
function to paint using a painter supplied by one of our widget subclasses.
The constructor of the class sets up the resources it needs to paint content onto a widget:
Helper::Helper() { QLinearGradient gradient(QPointF(50, -20), QPointF(80, 20)); gradient.setColorAt(0.0, Qt::white); gradient.setColorAt(1.0, QColor(0xa6, 0xce, 0x39)); background = QBrush(QColor(64, 32, 64)); circleBrush = QBrush(gradient); circlePen = QPen(Qt::black); circlePen.setWidth(1); textPen = QPen(Qt::white); textFont.setPixelSize(50); }
The actual painting is performed in the
paint()
function. This takes a
QPainter
that has already been set up to paint onto a paint device (either a
QWidget
or a QGLWidget), a
QPaintEvent
that provides information about the region to be painted, and a measure of the elapsed time (in milliseconds) since the paint device was last updated.
void Helper::paint(QPainter *painter, QPaintEvent *event, int elapsed) { painter->fillRect(event->rect(), background); painter->translate(100, 100);
We begin painting by filling in the region contained in the paint event before translating the origin of the coordinate system so that the rest of the painting operations will be displaced towards the center of the paint device.
We draw a spiral pattern of circles, using the elapsed time specified to animate them so that they appear to move outward and around the coordinate system's origin:
painter->save();
painter->setBrush(circleBrush);
painter->setPen(circlePen);
painter->rotate(elapsed * 0.030);
qreal r = elapsed / 1000.0;
int n = 30;
for (int i = 0; i < n; ++i) {
painter->rotate(30);
qreal factor = (i + r) / n;
qreal radius = 0 + 120.0 * factor;
qreal circleRadius = 1 + factor * 20;
painter->drawEllipse(QRectF(radius, -circleRadius,
circleRadius * 2, circleRadius * 2));
}
painter->restore();
Since the coordinate system is rotated many times during this process, we save() the QPainter 's state beforehand and restore() it afterwards.
painter->setPen(textPen);
painter->setFont(textFont);
painter->drawText(QRect(-50, -50, 100, 100), Qt::AlignCenter, QStringLiteral("Qt"));
}
We draw some text at the origin to complete the effect.
The
Widget
class provides a basic custom widget that we use to display the simple animation painted by the
Helper
类。
class Helper; class Widget : public QWidget { Q_OBJECT public: Widget(Helper *helper, QWidget *parent); public slots: void animate(); protected: void paintEvent(QPaintEvent *event) override; private: Helper *helper; int elapsed; };
Apart from the constructor, it only contains a
paintEvent()
function, that lets us draw customized content, and a slot that is used to animate its contents. One member variable keeps track of the
Helper
that the widget uses to paint its contents, and the other records the elapsed time since it was last updated.
The constructor only initializes the member variables, storing the
Helper
object supplied and calling the base class's constructor, and enforces a fixed size for the widget:
Widget::Widget(Helper *helper, QWidget *parent) : QWidget(parent), helper(helper) { elapsed = 0; setFixedSize(200, 200); }
The
animate()
slot is called whenever a timer, which we define later, times out:
void Widget::animate() { elapsed = (elapsed + qobject_cast<QTimer*>(sender())->interval()) % 1000; update(); }
Here, we determine the interval that has elapsed since the timer last timed out, and we add it to any existing value before repainting the widget. Since the animation used in the
Helper
class loops every second, we can use the modulo operator to ensure that the
elapsed
variable is always less than 1000.
由于
Helper
class does all of the actual painting, we only have to implement a paint event that sets up a
QPainter
for the widget and calls the helper's
paint()
函数：
void Widget::paintEvent(QPaintEvent *event) { QPainter painter; painter.begin(this); painter.setRenderHint(QPainter::Antialiasing); helper->paint(&painter, event, elapsed); painter.end(); }
The
GLWidget
class definition is basically the same as the
Widget
class except that it is derived from QGLWidget.
class Helper; class GLWidget : public QOpenGLWidget { Q_OBJECT public: GLWidget(Helper *helper, QWidget *parent); public slots: void animate(); protected: void paintEvent(QPaintEvent *event) override; private: Helper *helper; int elapsed; };
Again, the member variables record the
Helper
used to paint the widget and the elapsed time since the previous update.
The constructor differs a little from the
Widget
class's constructor:
GLWidget::GLWidget(Helper *helper, QWidget *parent) : QOpenGLWidget(parent), helper(helper) { elapsed = 0; setFixedSize(200, 200); setAutoFillBackground(false); }
As well as initializing the
elapsed
member variable and storing the
Helper
object used to paint the widget, the base class's constructor is called with the format that specifies the
QGL::SampleBuffers
flag. This enables anti-aliasing if it is supported by your system's OpenGL driver.
The
animate()
slot is exactly the same as that provided by the
Widget
类：
void GLWidget::animate() { elapsed = (elapsed + qobject_cast<QTimer*>(sender())->interval()) % 1000; update(); }
The
paintEvent()
is almost the same as that found in the
Widget
类：
void GLWidget::paintEvent(QPaintEvent *event) { QPainter painter; painter.begin(this); painter.setRenderHint(QPainter::Antialiasing); helper->paint(&painter, event, elapsed); painter.end(); }
Since anti-aliasing will be enabled if available, we only need to set up a
QPainter
on the widget and call the helper's
paint()
function to display the widget's contents.
The
Window
class has a basic, minimal definition:
class Window : public QWidget { Q_OBJECT public: Window(); private: Helper helper; };
It contains a single
Helper
object that will be shared between all widgets.
The constructor does all the work, creating a widget of each type and inserting them with labels into a layout:
Window::Window() { setWindowTitle(tr("2D Painting on Native and OpenGL Widgets")); Widget *native = new Widget(&helper, this); GLWidget *openGL = new GLWidget(&helper, this); QLabel *nativeLabel = new QLabel(tr("Native")); nativeLabel->setAlignment(Qt::AlignHCenter); QLabel *openGLLabel = new QLabel(tr("OpenGL")); openGLLabel->setAlignment(Qt::AlignHCenter); QGridLayout *layout = new QGridLayout; layout->addWidget(native, 0, 0); layout->addWidget(openGL, 0, 1); layout->addWidget(nativeLabel, 1, 0); layout->addWidget(openGLLabel, 1, 1); setLayout(layout); QTimer *timer = new QTimer(this); connect(timer, &QTimer::timeout, native, &Widget::animate); connect(timer, &QTimer::timeout, openGL, &GLWidget::animate); timer->start(50); }
A timer with a 50 millisecond time out is constructed for animation purposes, and connected to the
animate()
slots of the
Widget
and
GLWidget
objects. Once started, the widgets should be updated at around 20 frames per second.
The example shows the same painting operations performed at the same time in a
Widget
和
GLWidget
. The quality and speed of rendering in the
GLWidget
depends on the level of support for multisampling and hardware acceleration that your system's OpenGL driver provides. If support for either of these is lacking, the driver may fall back on a software renderer that may trade quality for speed.
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