如何使用QPainter绘制？ [英] How do I paint with QPainter?

问题描述

我最近才开始学习Qt。

我不太清楚如何使用 QPainter 类绘制。让我们说我只想在窗口中放几个点：

  
 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抽屉; 
 
 drawer.resize（200，200）; 
 drawer.show（）; 
 
 return app.exec（）; 
} 
  

然后，我什么都没有！
请你告诉我我想问题是你的 QPainter 初始化。

你可以创建 QPainter 就像在hydroes的回答，它看起来像这样： / p>

  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 ++; 
} 
} 
} 
  

这个，但我不真的推荐它（我只是喜欢其他解决方案）：

  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（）; 
} 
} 
  

QPainter :: begin （this）和 QPainter :: end（）调用在第二个例子中是必不可少的。在第一个例子中，你可以认为 QPainter :: begin（this）在构造函数和 QPainter :: end（）在析构函数

原因，我猜猜：
As QPaintDevice 通常在QT4中双缓冲， QPainter :: end（）可能是图像传输到图形内存的地方。

I have started learning Qt recently.
I did not get quite clear how can I paint using QPainter class. Let`s say I want just to place a few points in the window:

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?

解决方案

I think the problem is your QPainter initialization.

You could just create the QPainter like in hydroes' answer, it would look like this then:

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):

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

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.

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