扩展的辐射渐变构造函数:

QRadialGradient(const QPointF &center, qreal centerRadius, const QPointF &focalPoint, qreal focalRadius)

---Constructs an extended radial gradient with the given center, centerRadius, focalPoint, and focalRadius.

【译文：构造一个扩展的辐射渐变，中心圆的圆点是center，半径是centerRadius；焦点圆的圆心focalPoint，半径focalRadius。】

--- Extended radial gradients interpolate colors between a focal circle and a center circle. Points outside the cone defined by the two circles will be transparent.

【译文：扩展的辐射渐变，在焦点圆和中心圆之间插入颜色。由两个圆定义的锥形之外的点变成透明的，即不会被填充。】

在“扩展的辐射渐变”这种模式中，有“中心圆”、“焦点圆”、“锥形区域”，文档中的这些名词，到底如何理解？为了搞清楚这些细节，笔者经过研究，构造了一个辐射渐变对象，根据要求提供了中心圆（图中左侧的大圆）、焦点圆（图中右侧的小圆），设置了四种插值的颜色，在此基础上，绘制了窗体的边框，这样，就可以直观见到文档中所说的锥形区域了。程序运行的效果见下图所示，笔者的测试代码附在其后：

扩展的辐射渐变，由焦点圆和中心圆构成的锥形区域内渐变

void Widget::paintEvent(QPaintEvent *event)

{

const int r = 400;

QPainter painter(this);

//move origin to the center

painter.translate(width()/2.0,height()/2.0);

painter.setPen(QPen(Qt::black));

/*creat a radial grdient object by given center circle and focal circle.

center circle: its center is QPointF(0,0), radius is r/2.0

focal circle: its center is QPointF(r,0), radius is r/4.0. */

QRadialGradient radialGradient(QPointF(0,0),r/2.0,QPointF(r,0),r/4.0);

//interpolate colors from focal circle to center circle

radialGradient.setColorAt(0.0, Qt::red);

radialGradient.setColorAt(0.3, Qt::yellow);

radialGradient.setColorAt(0.6, Qt::blue);

radialGradient.setColorAt(1.0, Qt::magenta);

//set radial gradient to brush's style

QBrush brush(radialGradient);

painter.setBrush(brush);

/*draw a border of the widget with the brush of specified style

and you will see the entire cone defined by center circle and focal circle*/

painter.drawRect(-width()/2,-height()/2,width(),height());

/*redefine the brush to Qt::Nobrsh,

so as to draw center circle and focal circle. */

painter.setBrush(QBrush(Qt::NoBrush));

painter.drawEllipse(QPoint(0, 0), r/2.0, r/2.0);

painter.drawEllipse(QPoint(r, 0), r/4.0, r/4.0);

//Set font's family and pointSize to painter

painter.setFont(QFont("Times New Roman",20));

//draw the tabs of center circle and focal circle

painter.drawText(-60,0,"Center circle");

painter.drawText(0.77*r,0,"Focal circle");

}

小结：通过本文的测试，搞清楚了辐射渐变中的"扩展的辐射渐变“的用法细节。