QPainter drawRect to existing image draws rounded edges instead of sharp edges. How to draw sharp edges?

Question

My goal is to draw figures onto images and save to file. In this simplified example, I prepare an image for QPainter and then draw a rectangle via drawRect to the existing image and save to a file. The resultant image shows the combined image but the rectangle shows rounded edges instead of sharp edges. How to fix it? Your help is appreciated.

include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QPainter>
#include <QImage>
#include <QPen>
void paint_Object(QPainter *painter)
{
    QPen pen;
    pen.setWidth(40);
    pen.setColor(Qt::red);
    QRect rect;

    painter->setPen(pen);
    rect.setTopLeft(QPoint(100,100));
    rect.setWidth(100);
    rect.setHeight(200);
    painter->drawRect(rect);
}

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    QString filename_in = "C:/Users/seany/VideoAd/temp/test.png";
    QString filename_out = "C:/Users/seany/VideoAd/temp/test_out.png";

    //Setup initial
    QImage test_image(filename_in);

    //Setup Painter with initial Image
    QPainter painter(&test_image);

    //Paint an overlaid image onto initial image
    paint_Object(&painter);

    //Write resultant image to file
    test_image.save(filename_out);
}

MainWindow::~MainWindow()
{
    delete ui;
}

Answer 1

The default value for QPen join style is Qt::BevelJoin. That's way its coming round edges. Use pen.setJoinStyle(Qt::MiterJoin) for extended or sharp edges. For more information read this.