This is a test case of tablet support for Qt 5.

main.cpp

#include "widget.h"
#include <QApplication>
#include <QScrollArea>


int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QScrollArea area;
    Widget w;
    area.setWidget(&w);
    area.setBackgroundRole(QPalette::Dark);
    area.show();
    area.ensureWidgetVisible(&w);
    
    return a.exec();
}

PainterTest.pro

QT       += core gui concurrent

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = PainterTest
TEMPLATE = app
CONFIG += c++11


SOURCES += main.cpp\
        widget.cpp

HEADERS  += widget.h

widget.cpp

#include "widget.h"
#include <QTabletEvent>
#include <QMouseEvent>
#include <QDebug>

Widget::Widget(QWidget *parent) :
    QWidget(parent),
    pic(new QPixmap(1366, 768))
{
    pic->fill(Qt::white);
    drawing = false;
    this->grabMouse();
}

Widget::~Widget()
{
    delete pic;
}

QSize Widget::sizeHint() const
{
    return QSize(720,480);
}

void Widget::paintEvent(QPaintEvent *)
{
    QPainter painter(this);
    painter.drawLine(0,0,100,100);
    painter.drawPixmap(0,0,*pic);
}

void Widget::tabletEvent(QTabletEvent *ev)
{
    QPainter painter(pic);
    painter.setRenderHint(QPainter::Antialiasing);
    QPen pen;
    int maxWidth = 5;
    pen.setWidth(maxWidth);
    pen.setCapStyle(Qt::RoundCap);
    pen.setJoinStyle(Qt::RoundJoin);
    pen.setColor(Qt::blue);
    painter.setPen(pen);
    switch(ev->type()){
    case QEvent::TabletPress:
        if(!drawing){
            lastPoint = ev->pos();
            drawing = true;
            pen.setWidthF(maxWidth * ev->pressure());
            painter.setPen(pen);
            painter.drawPoint(ev->pos());
        }
        break;
    case QEvent::TabletMove:
        if(drawing && lastPoint != ev->pos()){
            pen.setWidthF(maxWidth * ev->pressure());
            painter.setPen(pen);
            painter.drawLine(lastPoint, ev->pos());
            lastPoint = ev->pos();
        }
        break;
    case QEvent::TabletRelease:
        if(drawing){
            drawing = false;
        }
        break;
    default:
        break;
    }
    ev->accept();
    update();
}

widget.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>
#include <QPainter>
#include <QPen>
#include <QBrush>

class Widget : public QWidget
{
    Q_OBJECT
    
public:
    explicit Widget(QWidget *parent = 0);
    ~Widget();
    QSize sizeHint() const;
protected:
    void paintEvent(QPaintEvent *ev);
    void tabletEvent(QTabletEvent *ev);
private:
    QPixmap *pic;
    bool drawing = false;
    QPoint lastPoint;
};

#endif // WIDGET_H