QPainter画箭头

画箭头需要注意：

计算箭头两个线的位置和长度与直线或弧线间的位置关系。

1.画直线箭头

关键代码

    const int length = 10;//箭头斜着的投影到线上的长度
    QVector<QLineF> lines;
    lines.append(QLineF(20,height()/2,width()/2,height()/2));
    lines.append(QLineF(width()/2-length,height()/2+length,width()/2,height()/2));
    lines.append(QLineF(width()/2-length,height()/2-length,width()/2,height()/2));
    QPen pen;
    pen.setColor(Qt::blue);
    pen.setWidthF(3.5);
    painter.setPen(pen);
    painter.drawLines(lines);

 

2.画弧线箭头

关键代码

    painter.translate(width()/2,height()/2);
    painter.setRenderHint(QPainter::Antialiasing);
    const int radius = 150;
    const int startAngle = 45;
    const int spanAngle = 90;
    const float PI = 3.14;
    const float ratio = 0.5;
    QPen pen;
    pen.setWidth(3);
    pen.setColor(Qt::red);
    pen.setCapStyle(Qt::RoundCap);
    painter.setPen(pen);
    painter.drawArc(QRectF(-radius,-radius,radius*2,radius*2),startAngle*16,spanAngle*16);
    QVector<QLineF> lines;
    lines.append(QLineF(radius*cos(startAngle*PI/180),-radius*sin(startAngle*PI/180),radius*cos(startAngle*PI/180)-10,-radius*sin(startAngle*PI/180)));
    lines.append(QLineF(radius*cos(startAngle*PI/180),-radius*sin(startAngle*PI/180),radius*cos(startAngle*PI/180),-radius*sin(startAngle*PI/180)-10));

    painter.drawLines(lines);

painter.translate(width()/2,height()/2);
    painter.setRenderHint(QPainter::Antialiasing);
    const int radius = 150;
    const int startAngle = 45;
    const int spanAngle = 90;
    const float PI = 3.14;
    const float ratio = 0.5;
    QPen pen;
    pen.setWidth(3);
    pen.setColor(Qt::red);
    pen.setCapStyle(Qt::RoundCap);
    painter.setPen(pen);
    painter.drawArc(QRectF(-radius,-radius,radius*2,radius*2),startAngle*16,spanAngle*16);
    QVector<QLineF> lines;
//    lines.append(QLineF(radius*cos(startAngle*PI/180),-radius*sin(startAngle*PI/180),radius*cos(startAngle*PI/180)-10,-radius*sin(startAngle*PI/180)));
//    lines.append(QLineF(radius*cos(startAngle*PI/180),-radius*sin(startAngle*PI/180),radius*cos(startAngle*PI/180),-radius*sin(startAngle*PI/180)-10));
    lines.append(QLineF(radius*cos((startAngle+spanAngle)*PI/180),-radius*sin((startAngle+spanAngle)*PI/180),radius*cos((startAngle+spanAngle)*PI/180)+10,-radius*sin((startAngle+spanAngle)*PI/180)));
    lines.append(QLineF(radius*cos((startAngle+spanAngle)*PI/180),-radius*sin((startAngle+spanAngle)*PI/180),radius*cos((startAngle+spanAngle)*PI/180),-radius*sin((startAngle+spanAngle)*PI/180)-10));


    painter.drawLines(lines);