如何获得翻译后的QPolygon的端点

Question

我试图绘制一个箭头，所以我刚才引用了示例代码，在这里我们可以绘制箭头：

http://doc.qt.io/qt-5/qtwidgets-graphicsview-elasticnodes-edge-cpp.html

我决定使用同样的公式绘制，并尝试如下：

theCurrentLine->setP1(QPointF(0, 0)  );
theCurrentLine->setP2((theLineVector));
p->drawLine(*theCurrentLine);

double angle = ::acos(theCurrentLine->dx() / theCurrentLine->length());
if (theCurrentLine->dy() >= 0)
    angle = TwoPi - angle;

QPointF sourcePoint = QPointF(0,0);

QPointF sourceArrowP1 = sourcePoint + QPointF(sin(angle + Pi / 3) * theArrowSize,
                                              cos(angle + Pi / 3) * theArrowSize);
QPointF sourceArrowP2 = sourcePoint + QPointF(sin(angle + Pi - Pi / 3) * theArrowSize,
                                              cos(angle + Pi - Pi / 3) * theArrowSize);

p->drawPolygon(QPolygonF() << theCurrentLine->p1() << sourceArrowP1 << sourceArrowP2);

但是现在我想在箭头多边形画出后画出这条线。

如何更改theCurrentLine的theCurrentLine值，该值可以在多边形之后开始，如当前的polygon(arrowHead)和行开始于同一点？我需要在箭头画好后开始画线。原因是，有时，如果钢笔宽度增加，箭头头看起来比线小。

Answer 1

您可以在QPolygon中获得索引点。

 QPoint QPolygon::point ( int index ) const

当你知道有多少点的时候，这是很容易的。Qt文档是您的朋友。

您可以使用count()，例如：

 QPoint lastPoint = myPolygon.point(myPolygon.count() - 1);

只要给你的多边形起个名字，你就没事了。

编辑：最新版本的这些代码应该可以解决你的问题。，我想我需要举个例子。您需要按以下顺序添加点数：

 QPolygon myPolygon;
 myPolygon.setPoint(0, sourceArrowP1);
 myPolygon.setPoint(1, theCurrentLine->p1());
 myPolygon.setPoint(2, sourceArrowP2);
 p->drawPolygon(myPolygon);
 QPoint lastPoint;
 lastPoint = myPolygon.point(myPolygon.count() - 1);

您需要在最后一点和第一个点之间划定界限。在此：

 p->drawLine(myPolygon.point(0, myPolygon.point(myPolygon.count() - 1));

如果希望箭头头部充满颜色，则需要使用QPainterPath而不是QPolygon：

 QPen pen(Qt::black); //Or whatever color you want
 pen.setWidthF(10); //Or whatever size you want your lines to be
 QBrush brush(Qt::red); //Or whatever color you want to fill the arrow head
 p->setPen(pen);
 p->setBrush(brush);
 QPainterPath arrow(sourceArrowP1);
 arrow.lineTo(theCurrentLine->p1());
 arrow.lineTo(sourceArrowP2);
 arrow.lineTo(sourceArrowP1); //This returns to the first point. You might eliminate this line if you want to.
 p->drawPath(arrow);

Answer 2

我的实际执行情况：

theCurrentLine->setP1(QPointF(0, 0)  );  // arrow line coordinates
theCurrentLine->setP2((theLineVector));
double angle = ::acos(theCurrentLine->dx() / theCurrentLine->length());  // angle of the current Line
if (theCurrentLine->dy() >= 0)
    angle = TwoPi - angle;

// getting arrow head points to be drawn
QPointF sourcePoint = QPointF(0,0);

QPointF sourceArrowP1 = sourcePoint + QPointF(sin(angle + Pi / 3) * theArrowSize,
                                              cos(angle + Pi / 3) * theArrowSize);
QPointF sourceArrowP2 = sourcePoint + QPointF(sin(angle + Pi - Pi / 3) * theArrowSize,
                                              cos(angle + Pi - Pi / 3) * theArrowSize);

p->drawPolygon(QPolygonF() << theCurrentLine->p1() << sourceArrowP1 << sourceArrowP2);

// to find the center point in the arrow head right
QLineF perpLine = QLineF(theCurrentLine->p1(), theCurrentLine->p2());
QLineF arrowheadWidth = QLineF(sourceArrowP1, sourceArrowP2);

QPointF originPoint;
QLineF::IntersectType res = perpLine.intersect(arrowheadWidth, &originPoint);

theCurrentLine->setP1(originPoint);
p->drawLine(*theCurrentLine);

如果有人知道更好的方法来实现这一点(我肯定会有)，请纠正我。