#include <QtGui>

class MyGraphicsScene : public QGraphicsScene
{
public:
    MyGraphicsScene(QWidget *parent = 0) : QGraphicsScene(parent), pressed(false) {}

    void inputPress(QPointF pos)
    {
        pressed = true;
        path = new QPainterPath(pos);
    }

    void inputMove(QPointF pos, qreal pressure = 1.0)
    {
        if (pressed) {
            path->lineTo(pos);
            QGraphicsPathItem *pathItem = new QGraphicsPathItem(*path);
            pathItem->setPen(QPen(Qt::SolidPattern, 3 * pressure, Qt::SolidLine, Qt::RoundCap, Qt::RoundJoin));
            addItem(pathItem);

            delete path;
            path = new QPainterPath(pos);
        }
    }

    void inputRelease(QPointF pos)
    {
        Q_UNUSED(pos);
        delete path;
        pressed = false;
    }

private:
    QPainterPath *path;
    bool pressed;
};


class MyGraphicsView : public QGraphicsView
{
public:
    MyGraphicsView(MyGraphicsScene *scene, QWidget * parent = 0) : QGraphicsView(scene, parent) {}

    MyGraphicsScene* scene()
    {
        return static_cast<MyGraphicsScene*>(QGraphicsView::scene());
    }

    void keyPressEvent(QKeyEvent *event)
    {
        if (event->key() == Qt::Key_K) {
            scene()->clear();
            event->accept();
        } else {
            event->ignore();
        }
    }

    void tabletEvent(QTabletEvent *event)
    {
        bool accepted = true;
        switch (event->type())
        {
            case QEvent::TabletPress:
                scene()->inputPress(event->hiResGlobalPos() + mapFromGlobal(QPoint(0, 0)));
                break;
            case QEvent::TabletMove:
                scene()->inputMove(event->hiResGlobalPos() + mapFromGlobal(QPoint(0, 0)), event->pressure());
                break;
            case QEvent::TabletRelease:
                scene()->inputRelease(event->hiResGlobalPos() + mapFromGlobal(QPoint(0, 0)));
                break;
            default:
                accepted = false;
                break;
        }
        event->setAccepted(accepted);
    }

    void mousePressEvent(QMouseEvent *event)
    {
        scene()->inputPress(mapToScene(event->pos()));
    }

    void mouseMoveEvent(QMouseEvent *event)
    {
        scene()->inputMove(mapToScene(event->pos()));
    }

    void mouseReleaseEvent(QMouseEvent *event)
    {
        scene()->inputRelease(mapToScene(event->pos()));
    }
};


class OverlayWidget : public QWidget
{
public:
    OverlayWidget(QWidget *parent = 0) : QWidget(parent)
    {
        setAttribute(Qt::WA_NoMousePropagation);
    }

    void paintEvent(QPaintEvent *)
    {
        QPainter painter(this);
        painter.setPen(Qt::NoPen);
        painter.setBrush(QColor(255, 0, 0, 127));
        painter.drawRect(0, 0, width(), height());
    }
};


int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    const int width = 800;
    const int height = 600;

    MyGraphicsScene scene;
    MyGraphicsView view(&scene);
    QRect screenRect = app.desktop()->availableGeometry();
    view.setGeometry((screenRect.width() - width) / 2, (screenRect.height() - height) / 2, width, height);
    view.setSceneRect(0, 0, width, height);
    view.setRenderHints(QPainter::Antialiasing);
    view.show();

    QWidget *overlay = new OverlayWidget(&view);
    overlay->setGeometry(view.width() / 2, 0, view.width() / 2, view.height());
    overlay->show();

    return app.exec();
}