C++ 我如何用 QPainter 绘画？
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How do I paint with QPainter?
提问by chester89
I have started learning Qt recently.
I did not get quite clear how can I paint using QPainterclass. Let`s say I want just to place a few points in the window:
我最近开始学习Qt。
我不太清楚如何使用QPainter类进行绘画。假设我只想在窗口中放置几个点：
class PointDrawer: public QWidget {
Q_OBJECT
private:
QPainter p;
public:
PointDrawer(QWidget* obj=0): QWidget(obj), p(this) {}
virtual void paintEvent(QPaintEvent*) {
p.setPen(QPen(Qt::black, 3));
int n = 8;
while(...) {
qreal fAngle = 2 * 3.14 * i / n;
qreal x = 50 + cos(fAngle) * 40;
qreal y = 50 + sin(fAngle) * 40;
p.drawPoint(QPointF(x, y));
i++;
}
}
}
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
PointDrawer drawer;
drawer.resize(200, 200);
drawer.show();
return app.exec();
}
And after that, I got nothing!
Can you please tell me where I am wrong?
在那之后，我什么都没有！
你能告诉我我错在哪里吗？
采纳答案by Caotic
I think the problem is your QPainterinitialization.
我认为问题在于您的QPainter初始化。
You could just create the QPainterlike in hydroes' answer, it would look like this then:
您可以QPainter在 Hydroes 的回答中创建类似的内容，然后看起来像这样：
class PointDrawer: public QWidget {
Q_OBJECT
public:
PointDrawer(QWidget* obj=0): QWidget(obj) {}
virtual void paintEvent(QPaintEvent*) {
QPainter p(this)
p.setPen(QPen(Qt::black, 3));
int n = 8;
while(...) {
qreal fAngle = 2 * 3.14 * i / n;
qreal x = 50 + cos(fAngle) * 40;
qreal y = 50 + sin(fAngle) * 40;
p.drawPoint(QPointF(x, y));
i++;
}
}
}
It could also use something like this, but I don't really recommend it (I just prefer the other solution):
它也可以使用这样的东西，但我并不真正推荐它（我只是更喜欢其他解决方案）：
class PointDrawer: public QWidget {
Q_OBJECT
private:
QPainter p;
public:
PointDrawer(QWidget* obj=0): QWidget(obj) {}
virtual void paintEvent(QPaintEvent*) {
p.begin(this);
p.setPen(QPen(Qt::black, 3));
int n = 8;
while(...) {
qreal fAngle = 2 * 3.14 * i / n;
qreal x = 50 + cos(fAngle) * 40;
qreal y = 50 + sin(fAngle) * 40;
p.drawPoint(QPointF(x, y));
i++;
}
p.end();
}
}
The QPainter::begin(this)and QPainter::end()calls are essential in the second example. In the first example, you can think of QPainter::begin(this)being called in the constructor and QPainter::end()in the destructor
该QPainter::begin(this)和QPainter::end()电话是在第二个例子中是必不可少的。在第一个例子中，你可以想到QPainter::begin(this)在构造函数和QPainter::end()析构函数中被调用
For the reason, I'm guessing:
As QPaintDevices are usually double buffered in QT4, QPainter::end()might be where the image is transferred to the graphic memory.
出于这个原因，我猜测：由于QPaintDevices 通常在 QT4 中进行双缓冲，因此QPainter::end()可能是图像传输到图形内存的地方。
回答by Ronny Brendel
void SimpleExampleWidget::paintEvent(QPaintEvent *)
{
QPainter painter(this);
painter.setPen(Qt::blue);
painter.setFont(QFont("Arial", 30));
painter.drawText(rect(), Qt::AlignCenter, "Qt");
}
回答by shoosh
You need to initialize the painter with the widget you want to paint on.
Usually this is done using the constructor which takes a QPaintDevicebut you can also do it by calling begin().
您需要使用要在其上绘画的小部件来初始化画家。
通常这是使用带有 a 的构造函数完成的，QPaintDevice但您也可以通过调用begin().
