test.cpp

#include <QPainter>
#include <QGuiApplication>
#include <QTextLayout>
#include <QElapsedTimer>
#include <QWindow>
#include <QScreen>
#include <QImage>
#include <QOpenGLContext>
#include <QOpenGLPaintDevice>
#include <QDebug>

class Window : public QWindow
{
public:
    Window(QScreen *screen = 0);

protected:
    void exposeEvent(QExposeEvent *);

private:
    void render();

    QImage m_buffer;
    QTextLayout m_layout;
    QOpenGLContext *m_context;
};

Window::Window(QScreen *screen)
    : QWindow(screen)
{
    setSurfaceType(QSurface::OpenGLSurface);
    setGeometry(QRect(0, 0, 400, 400));
    create();
    m_context = new QOpenGLContext(this);
    m_context->create();
    m_buffer = QImage(geometry().size(), QImage::Format_ARGB32_Premultiplied);

    QFont font = QFont("Monospace");
    font.setPixelSize(12);
    m_layout.setFont(font);
    QTextOption option;
    m_layout.setTextOption(option);
    m_layout.setText(QString::fromUtf8("Hello. Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua."));
    m_layout.beginLayout();
    qreal height = 0;
    while (1) {
        QTextLine line = m_layout.createLine();
        if (!line.isValid())
            break;
        line.setLineWidth(geometry().size().width());
        line.setPosition(QPointF(0, height));
        height += line.height();
    }
    m_layout.endLayout();
}

void Window::exposeEvent(QExposeEvent *)
{
    render();
}

void Window::render()
{
    {
        m_buffer.fill(Qt::white);
        QPainter p(&m_buffer);
        QElapsedTimer t;
        t.start();
        m_layout.draw(&p, QPointF(0, 0));
    }

    QRect rect(QPoint(), geometry().size());

    m_context->makeCurrent(this);
    QOpenGLPaintDevice device(rect.size());

    QPainter p(&device);
    p.drawImage(0, 0, m_buffer);
    p.end();
    m_context->swapBuffers(this);
}

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

    Window a;
    a.show();

    return app.exec();
}