How to create an Arrow in QT

mrjj

Hi
You could show how you defined "paint" as that could help.

If you call it from the painter function of the sample
where paint is a pointer you need
DrawLineWithArrow( * paint,QPoint(0,0), QPoint(330,330));

Or simply change it to take a pointer and not a ref (&)

If doubt plese show the complete code from where u call it.

ManiRon

This post is deleted!

ManiRon

@mrjj I Found the error i didnt define the paint correctly

ManiRon

@mrjj

                #include "mainwindow.h"
                
                #include "ui_mainwindow.h"
                
                #include "qpainter.h"
                
                #include "math.h"
                
                #include "cmath"
                
                #include "QPointF"
                
                #include "QPolygonF"
                
                
                
                MainWindow::MainWindow(QWidget *parent) :
                
                    QMainWindow(parent),
                
                    ui(new Ui::MainWindow)
                
                {
                
                    ui->setupUi(this);
                
                
                
                    QPainter paint;
                
                
                
                    DrawLineWithArrow(paint,QPoint(0,0), QPoint(0,0));
                
                }
                
                
                
                MainWindow::~MainWindow()
                
                {
                
                    delete ui;
                
                }
                
                void MainWindow::DrawLineWithArrow(QPainter& painter, QPoint start, QPoint end)
                
                {
                
                
                
                  painter.setRenderHint(QPainter::Antialiasing, true);
                
                
                
                  qreal arrowSize = 40; // size of head
                
                  painter.setPen(Qt::black);
                
                  painter.setBrush(Qt::black);
                
                
                
                  QLineF line(end, start);
                
                
                
                  double angle = std::atan2(-line.dy(), line.dx());
                
                  QPointF arrowP1 = line.p1() + QPointF(sin(angle + M_PI / 3) * arrowSize,
                
                                                        cos(angle + M_PI / 3) * arrowSize);
                
                  QPointF arrowP2 = line.p1() + QPointF(sin(angle + M_PI - M_PI / 3) * arrowSize,
                
                                                        cos(angle + M_PI - M_PI / 3) * arrowSize);
                
                
                
                  QPolygonF arrowHead;
                
                  arrowHead.clear();
                
                  arrowHead << line.p1() << arrowP1 << arrowP2;
                
                  painter.drawLine(line);
                
                  painter.drawPolygon(arrowHead);
                
                
                
                }

This is my whole code , Now my doubt is where i will be able to see this arrow

jsulm

@ManiRon You need to overwrite paintEvent to be able to paint! https://doc.qt.io/qt-5/qwidget.html#paintEvent
"Now my doubt is where i will be able to see this arrow" - on the widget where you overwrite paintEvent.

ManiRon

@jsulm I tried it on my Mainwindow and i was not able to see

ManiRon

@jsulm said in How to create an Arrow in QT:

You need to overwrite paintEvent to be able to paint!

You need to overwrite paintEvent to be able to paint!

I am new to this thats why , can you please explain How this can be done ?

jsulm

@ManiRon Take a look at this example: https://doc.qt.io/qt-5/qtwidgets-painting-basicdrawing-example.html

ManiRon

@jsulm
Thanks Sir, I got it and made it, Here i provide my working code so that it will be useful for others

         #include "mainwindow.h"
         
         #include "ui_mainwindow.h"
         
         #include "qpainter.h"
         
         #include "math.h"
         
         #include "cmath"
         
         #include "QPointF"
         
         #include "QPolygonF"
         
         #include "QPaintEvent"
         
         
         
         MainWindow::MainWindow(QWidget *parent) :
         
             QMainWindow(parent),
         
             ui(new Ui::MainWindow)
         
         {
         
             ui->setupUi(this);
         
         
         
             setBackgroundRole(QPalette::Base);
         
             setAutoFillBackground(true);
         
         
         
         
         
         }
         
         
         
         MainWindow::~MainWindow()
         
         {
         
             delete ui;
         
         }
         
         void MainWindow::DrawLineWithArrow(QPainter& painter, QPoint start, QPoint end)
         
         {
         
         
         
           painter.setRenderHint(QPainter::Antialiasing, true);
         
         
         
           qreal arrowSize = 40; // size of head
         
           painter.setPen(Qt::black);
         
           painter.setBrush(Qt::black);
         
         
         
           QLineF line(end, start);
         
         
         
           double angle = std::atan2(-line.dy(), line.dx());
         
           QPointF arrowP1 = line.p1() + QPointF(sin(angle + M_PI / 3) * arrowSize,
         
                                                 cos(angle + M_PI / 3) * arrowSize);
         
           QPointF arrowP2 = line.p1() + QPointF(sin(angle + M_PI - M_PI / 3) * arrowSize,
         
                                                 cos(angle + M_PI - M_PI / 3) * arrowSize);
         
         
         
           QPolygonF arrowHead;
         
           arrowHead.clear();
         
           arrowHead << line.p1() << arrowP1 << arrowP2;
         
           painter.drawLine(line);
         
           painter.drawPolygon(arrowHead);
         
         
         
         
         
         
         
         }
         
         void MainWindow::paintEvent(QPaintEvent *)
         
         {
         
             static const QPoint points[2] = {
         
                 QPoint(50, 100),
         
                 QPoint(20, 20),
         
             };
         
                 QPainter painter(this);
         
                 painter.setPen(pen);
         
                 painter.setBrush(brush);
         
         
         
             painter.setRenderHint(QPainter::Antialiasing, true);
         
         
         
             qreal arrowSize = 40; // size of head
         
             painter.setPen(Qt::black);
         
             painter.setBrush(Qt::black);
         
         
         
             QLineF line(points[1], points[0]);
         
         
         
             double angle = std::atan2(-line.dy(), line.dx());
         
             QPointF arrowP1 = line.p1() + QPointF(sin(angle + M_PI / 3) * arrowSize,
         
                                                   cos(angle + M_PI / 3) * arrowSize);
         
             QPointF arrowP2 = line.p1() + QPointF(sin(angle + M_PI - M_PI / 3) * arrowSize,
         
                                                   cos(angle + M_PI - M_PI / 3) * arrowSize);
         
         
         
         
         
         
         
             QPolygonF arrowHead;
         
             arrowHead.clear();
         
             arrowHead << line.p1() << arrowP1 << arrowP2;
         
             painter.drawLine(line);
         
             painter.drawPolygon(arrowHead);
         
         
         
         
         
         
         
         
         
         
         
         //    painter.setRenderHint(QPainter::Antialiasing, false);
         
         //    painter.setPen(palette().dark().color());
         
         //    painter.setBrush(Qt::NoBrush);
         
         //    painter.drawRect(QRect(0, 0, width() - 1, height() - 1));
         
         
         
         //    painter.restore();
         
         
         
         }

ManiRon

@jsulm One more doubt if i want to use QPainterpath to draw the same arrow how this can be done

Why I ask the same in QPainterPath is that i want to use it in QCustomplot

mrjj

@ManiRon said in How to create an Arrow in QT:

QPainterPath

Yes i think you can convert the code to use QPainterPath .
it has a line function
https://doc.qt.io/qt-5/qpainterpath.html#lineTo
and a
https://doc.qt.io/qt-5/qpainterpath.html#addPolygon
for the QPolygonF arrowHead;

ManiRon

@mrjj Thanks but i used QPixmap and it worked thanks all for helping me out