(3) Color gradient style

1. Linear gradient QLinearGradient

// 第一种
void Widget::paintEvent(QPaintEvent *event)
{
    
    
    QPainter painter;

    painter.begin(this);
    QLinearGradient ling(QPointF(70, 70), QPoint( 140, 140 ) );  //从起点到终点的渐变
    ling.setColorAt(0, Qt::blue);  //起点到中心要显示的颜色
    ling.setColorAt(1, Qt::green);
    ling.setSpread( QGradient::PadSpread );   //默认显示模式

    QBrush brush(ling);
    painter.setBrush(brush);
    painter.drawRect(0, 0, 200, 200);

    painter.end();
}

// 第二种
void Widget::paintEvent(QPaintEvent *event)
{
    
    
    QPainter painter;

    painter.begin(this);
    QLinearGradient ling(QPointF(70, 70), QPoint( 140, 140 ) );  //从起点到终点的渐变
    ling.setColorAt(0, Qt::blue);  //起点到中心要显示的颜色
    ling.setColorAt(1, Qt::green);
    ling.setSpread( QGradient::RepeatSpread );   //默认显示模式

    QBrush brush(ling);
    painter.setBrush(brush);
    painter.drawRect(0, 0, 200, 200);

    painter.end();
}

// 第三种
void Widget::paintEvent(QPaintEvent *event)
{
    
    
    QPainter painter;

    painter.begin(this);
    QLinearGradient ling(QPointF(70, 70), QPoint( 140, 140 ) );  //从起点到终点的渐变
    ling.setColorAt(0, Qt::blue);  //起点到中心要显示的颜色
    ling.setColorAt(1, Qt::green);
    ling.setSpread( QGradient::ReflectSpread );   //默认显示模式

    QBrush brush(ling);
    painter.setBrush(brush);
    painter.drawRect(0, 0, 200, 200);

    painter.end();
}

2. Center gradient QRadialGradient

// 第一种
void Widget::paintEvent(QPaintEvent *event)
{
    
    
    QPainter painter;
    painter.begin(this);

    QRadialGradient radg(100, 100, 50, 120, 120);  //在圆心(100, 100)，半径50的圆上渐变。渐变起点是(120, 120)
    radg.setColorAt( 0, Qt::black );
    radg.setColorAt( 1, Qt::white );
    //radg.setSpread(QGradient::PadSpread);   //默认
    QBrush brush(radg);
    painter.setBrush(brush);
    painter.drawRect(0, 0, 200, 200);
    painter.end();
}

// 第二种
void Widget::paintEvent(QPaintEvent *event)
{
    
    
    QPainter painter;
    painter.begin(this);

    QRadialGradient radg(100, 100, 50, 120, 120);  //在圆心(100, 100)，半径50的圆上渐变。渐变起点是(120, 120)
    radg.setColorAt( 0, Qt::black );
    radg.setColorAt( 1, Qt::white );
    radg.setSpread(QGradient::RepeatSpread);
    QBrush brush(radg);
    painter.setBrush(brush);
    painter.drawRect(0, 0, 200, 200);
    painter.end();
}

// 第三种
void Widget::paintEvent(QPaintEvent *event)
{
    
    
    QPainter painter;
    painter.begin(this);

    QRadialGradient radg(100, 100, 50, 120, 120);  //在圆心(100, 100)，半径50的圆上渐变。渐变起点是(120, 120)
    radg.setColorAt( 0, Qt::black );
    radg.setColorAt( 1, Qt::white );
    radg.setSpread(QGradient::ReflectSpread);
    QBrush brush(radg);
    painter.setBrush(brush);
    painter.drawRect(0, 0, 200, 200);
    painter.end();
}

3. Coordinate angle gradient QConicalGradient

void Widget::paintEvent(QPaintEvent *event)
{
    
    
    QPainter painter;
    painter.begin(this);

    QConicalGradient cong( 100, 100, 45.0 );  //在(100, 100)坐标上以45°为中心渐变
    cong.setColorAt( 0, Qt::black );
    cong.setColorAt( 1, Qt::white );

    QBrush brush(cong);
    painter.setBrush(brush);
    painter.drawRect(0, 0, 200, 200);
    painter.end();
}