c++ - 围绕其中心旋转矩形

Question

我需要围绕它的中心点旋转一个矩形并将其显示在 QWidget 的中心。你能完成这个特定的代码吗？如果可能，您是否也可以简化解释或提供指向最简单解释的链接？

请注意:我已经阅读了 Qt 文档，编译了处理旋转的示例/演示，但我仍然无法理解它!

void Canvas::paintEvent(QPaintEvent *event)
{
    QPainter paint(this);

    paint.setBrush(Qt::transparent);
    paint.setPen(Qt::black);
    paint.drawLine(this->width()/2, 0, this->width()/2, this->height());
    paint.drawLine(0, this->height()/2, this->width(), this->height()/2);

    paint.setBrush(Qt::white);
    paint.setPen(Qt::blue);

    // Draw a 13x17 rectangle rotated to 45 degrees around its center-point
    // in the center of the canvas.

    paint.drawRect(QRect(0,0, 13, 17));

}

Answer 1

 void paintEvent(QPaintEvent* event){
    QPainter painter(this);

    // xc and yc are the center of the widget's rect.
    qreal xc = width() * 0.5;
    qreal yc = height() * 0.5;

    painter.setPen(Qt::black);

    // draw the cross lines.
    painter.drawLine(xc, rect().top(), xc, rect().bottom());
    painter.drawLine(rect().left(), yc, rect().right(), yc);

    painter.setBrush(Qt::white);
    painter.setPen(Qt::blue);

    // Draw a 13x17 rectangle rotated to 45 degrees around its center-point
    // in the center of the canvas.

    // translates the coordinate system by xc and yc
    painter.translate(xc, yc);

    // then rotate the coordinate system by 45 degrees
    painter.rotate(45);

    // we need to move the rectangle that we draw by rx and ry so it's in the center.
    qreal rx = -(13 * 0.5);
    qreal ry = -(17 * 0.5);
    painter.drawRect(QRect(rx, ry, 13, 17));
  }

你在画家的坐标系中。当您调用 drawRect(x, y, 13, 17) 时，它的左上角位于 (x,y)。如果你想让 (x, y) 成为矩形的中心，那么你需要将矩形移动一半，因此 rx 和 ry.

您可以调用 resetTransform() 来重置由 translate() 和 rotate() 进行的转换。