QPainter draws text on QPixmap, but the text is ugly

I have code that draws a console onto a QMainWindow. If I use the paintEvent, everything looks beautiful. If I paint the text to a QPixmap, and then in my paintEvent copy it over, then the text looks ugly.

#include "mainwindow.hxx"
#include "./ui_mainwindow.h"

#include <QPainter>
#include <QString>
#include <QStaticText>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    auto f = font();
    f.setPixelSize(70);
    setFont(f);
}

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

void MainWindow::paintEvent(QPaintEvent *event)
{
    {
    QPixmap img(width(), height());
    QPainter painter(&img);
    painter.setFont(font());
    painter.setBrush(Qt::black);
    painter.setPen(Qt::NoPen);
    painter.drawRect(rect());
    painter.setRenderHint(QPainter::HighQualityAntialiasing);


    painter.setPen(Qt::blue);
    painter.drawText(0, 80, "/// lorem ipsum dolor sit amet");
    // Create a QPainter to draw on the window
    QPainter painter2(this);
    painter2.setRenderHint(QPainter::HighQualityAntialiasing);
    painter2.drawPixmap(rect(), img);
    }

    {
    // Create a QPainter to draw on the window
    QPainter painter(this);
    painter.setFont(font());
    painter.setBrush(Qt::black);
    painter.setPen(Qt::NoPen);
    painter.setRenderHint(QPainter::HighQualityAntialiasing);

    painter.setPen(Qt::blue);
    painter.drawText(0, 300, "/// lorem ipsum dolor sit amet");
    }

}

main.cxx

#include "mainwindow.hxx"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

mainwindow.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>800</width>
    <height>600</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralwidget"/>
 </widget>
 <resources/>
 <connections/>
</ui>

I have enabled anti-aliasing on the QPainter, it does not make any difference.

In both cases, the same font is used (the Window's default font()). What is going on here? Notice in particular '/' looks pretty bad.

Just a couple of thoughts:

Did you try setting the QPainter::TextAntialiasing render hint?

Also, sometimes when using HDPI (high res screen), the reported window width & height are not actual pixels; you can check this with devicePixelRatio() and use the value to scale the size of your pixmap.