paint device returned engine == 0, type: 3

[MarkoSan]

What does this debugger warning means????

paint device returned engine == 0, type: 3

paint device returned engine == 0, type: 3

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And I installed 4.3.3. Are there any bugs regarding qpainter, becuase SAME CODE worked on 4.3.2.???

[high_flyer]

becuase SAME CODE worked on 4.3.2.???

Is this a warning or an error?

[marcel]

Probably because you are trying to paint on a null QPaintDevice...
Can you post some code?

[MarkoSan]

I compiled same code under 4.3.3. and my app crashes. Then I went into debug mode with gdb and I get that warning (at runtime - after app crashes). And I do not have any clue what to do with it. I have been seeking error for 14 hours and I am desperate.

[MarkoSan]

Here is some code, I am "playing" with the code of "Cover browser demo". Here is code:

void CMerchandizeBrowser::drawItemAt(QPainter *p, const QPoint &loc,
                                     const QImage &img,
                                     qint16 w, qint16 h,
                                     qint16 angle)
{
    qDebug() << "Entered CMerchandizeBrowser::drawItemsAt ...";    // debug
    Q_ASSERT(p!=0);
    p->save();
    qDebug() << "After first p->save()";    // debug 
    if (!m_MirrorCache.contains(img.cacheKey())) {
        QImage mirror=mirrorImage(img);
        m_MirrorCache.insert(img.cacheKey(), mirror);
    }
 
    QPointF pt(-img.height()/2, -img.height()/2);
    QTransform trans;
    trans.scale(qreal(w)/img.width(), qreal(h)/img.height());
    trans.rotate(angle, Qt::YAxis);
    p->setTransform(trans * QTransform().translate(loc.x(), loc.y()), true);
 
    const QImage &mirror=m_MirrorCache[img.cacheKey()];
    QPointF pt2(pt.x(), img.height()/2);
    p->save();
    qDebug() << "After second p->save()";    // debug
    Q_ASSERT(p!=0);
    p->setCompositionMode(QPainter::CompositionMode_Source);
    p->drawImage(pt2, mirror);
    p->restore();
 
    p->drawImage(pt, img);
 
    p->restore();
    qDebug() << "Exiting CMerchandizeBrowser::drawItemsAt ...";    // debug
}

void CMerchandizeBrowser::drawItemAt(QPainter *p, const QPoint &loc,
                                     const QImage &img,
                                     qint16 w, qint16 h,
                                     qint16 angle)
{
    qDebug() << "Entered CMerchandizeBrowser::drawItemsAt ...";    // debug
    Q_ASSERT(p!=0);
    p->save();
    qDebug() << "After first p->save()";    // debug 
    if (!m_MirrorCache.contains(img.cacheKey())) {
        QImage mirror=mirrorImage(img);
        m_MirrorCache.insert(img.cacheKey(), mirror);
    }

    QPointF pt(-img.height()/2, -img.height()/2);
    QTransform trans;
    trans.scale(qreal(w)/img.width(), qreal(h)/img.height());
    trans.rotate(angle, Qt::YAxis);
    p->setTransform(trans * QTransform().translate(loc.x(), loc.y()), true);

    const QImage &mirror=m_MirrorCache[img.cacheKey()];
    QPointF pt2(pt.x(), img.height()/2);
    p->save();
    qDebug() << "After second p->save()";    // debug
    Q_ASSERT(p!=0);
    p->setCompositionMode(QPainter::CompositionMode_Source);
    p->drawImage(pt2, mirror);
    p->restore();

    p->drawImage(pt, img);

    p->restore();
    qDebug() << "Exiting CMerchandizeBrowser::drawItemsAt ...";    // debug
}

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That warning is written only ONCE and "After first p->save()". And under 4.3.2 this exact code worked perfectly.

[marcel]

Chill dude...
Post some code and someone will figure it out

[marcel]

On what paint device did you use the painter passed to the function before entering this function? Maybe that paint device has been deallocated.

[MarkoSan]

The warning is laucnhed after call DrawItem at from:

void CMerchandizeBrowser::drawItemsBeforeSelected(QPainter *p)
{
    qDebug() << "Entered CMerchandizeBrowser::drawItemsBeforeSelected ...";    // debug
    Q_ASSERT(p!=0);
    QPoint selPt(width()/2-m_ImagesList[m_iSelected].width()/2,
                 height()/2-m_ImagesList[m_iSelected].height()/4);
    qint16 widthDiff = selPt.x();
    qint16 startIdx = m_iSelected - widthDiff/pos_diff;
 
    if (startIdx < 0)
        startIdx = 0;
 
    for (qint16 i = 0, idx = startIdx; idx < m_iSelected; ++i, ++idx) {
        const QImage &img = m_ImagesList[idx];
 
        drawItemAt(p, QPoint(selPt.x()-(m_iSelected-i)*pos_diff, selPt.y()),
                   img, img.width() -size_diff, img.height()-size_diff, -55);
    }
    qDebug() << "Exiting CMerchandizeBrowser::drawItemsBeforeSelected ...";    // debug
}

void CMerchandizeBrowser::drawItemsBeforeSelected(QPainter *p)
{
    qDebug() << "Entered CMerchandizeBrowser::drawItemsBeforeSelected ...";    // debug
    Q_ASSERT(p!=0);
    QPoint selPt(width()/2-m_ImagesList[m_iSelected].width()/2,
                 height()/2-m_ImagesList[m_iSelected].height()/4);
    qint16 widthDiff = selPt.x();
    qint16 startIdx = m_iSelected - widthDiff/pos_diff;

    if (startIdx < 0)
        startIdx = 0;

    for (qint16 i = 0, idx = startIdx; idx < m_iSelected; ++i, ++idx) {
        const QImage &img = m_ImagesList[idx];

        drawItemAt(p, QPoint(selPt.x()-(m_iSelected-i)*pos_diff, selPt.y()),
                   img, img.width() -size_diff, img.height()-size_diff, -55);
    }
    qDebug() << "Exiting CMerchandizeBrowser::drawItemsBeforeSelected ...";    // debug
}

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[jpn]

Where is the QPainter object originally created? Where does CMerchandizeBrowser::drawItemsBeforeSelected() get called from?

[MarkoSan]

I think painter is created here, in line 4:

void CMerchandizeBrowser::paintEvent(QPaintEvent *e)
{
    qDebug() << "Entered CMerchandizeBrowser::paintEvent ...";    // debug
    QPainter p(this);
    p.setClipRect(e->rect());
    p.fillRect(rect(), Qt::black);
    p.setRenderHint(QPainter::Antialiasing);
    p.setRenderHint(QPainter::SmoothPixmapTransform);
#ifdef SCALE_VIEW
    static qint16 diff = 0;
    qint16 w =  width() - diff;
    qint16 h = height() - diff;
    QTransform trans;
    trans.translate(width()/2, height()/2);
    trans.scale(qreal(w)/width(), qreal(h)/height());
    trans.translate(-width()/2, -height()/2);
    p.setTransform(trans);
#endif
    drawItemsBeforeSelected(&p);
    drawItemsAfterSelected(&p);
    drawSelectedItem(&p);
#ifdef SCALE_VIEW
    static bool goingUp = true;
    if (goingUp)
        diff += 3;
    else
        diff -= 3;
    if (diff >= 200 || diff <= 0)
        goingUp = !goingUp;
#endif
    if (m_iSelected>=m_ImagesList.count())
        m_iSelected=0;
    //QTimer::singleShot(1000/60, this, SLOT(update()));
    QTimer::singleShot(100/60, this, SLOT(update()));
    qDebug() << "Exiting CMerchandizeBrowser::paintEvent ...";    // debug
}

void CMerchandizeBrowser::paintEvent(QPaintEvent *e)
{
    qDebug() << "Entered CMerchandizeBrowser::paintEvent ...";    // debug
    QPainter p(this);
    p.setClipRect(e->rect());
    p.fillRect(rect(), Qt::black);
    p.setRenderHint(QPainter::Antialiasing);
    p.setRenderHint(QPainter::SmoothPixmapTransform);
#ifdef SCALE_VIEW
    static qint16 diff = 0;
    qint16 w =  width() - diff;
    qint16 h = height() - diff;
    QTransform trans;
    trans.translate(width()/2, height()/2);
    trans.scale(qreal(w)/width(), qreal(h)/height());
    trans.translate(-width()/2, -height()/2);
    p.setTransform(trans);
#endif
    drawItemsBeforeSelected(&p);
    drawItemsAfterSelected(&p);
    drawSelectedItem(&p);
#ifdef SCALE_VIEW
    static bool goingUp = true;
    if (goingUp)
        diff += 3;
    else
        diff -= 3;
    if (diff >= 200 || diff <= 0)
        goingUp = !goingUp;
#endif
    if (m_iSelected>=m_ImagesList.count())
        m_iSelected=0;
    //QTimer::singleShot(1000/60, this, SLOT(update()));
    QTimer::singleShot(100/60, this, SLOT(update()));
    qDebug() << "Exiting CMerchandizeBrowser::paintEvent ...";    // debug
}

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[high_flyer]

You say the app is crashing, do you know where?

[jpn]

void CMerchandizeBrowser::paintEvent(QPaintEvent *e)
{
    QPainter p(this);
    [...]
    //QTimer::singleShot(1000/60, this, SLOT(update()));
    QTimer::singleShot(100/60, this, SLOT(update()));
}

void CMerchandizeBrowser::paintEvent(QPaintEvent *e)
{
    QPainter p(this);
    [...]
    //QTimer::singleShot(1000/60, this, SLOT(update()));
    QTimer::singleShot(100/60, this, SLOT(update()));
}

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Creation of QPainter looks look to me. But I hope you're aware that

• 100/60 = 1
• 1000/60 = 16

so you're attempting to schedule a paint event every one or every 16th millisecond. Of course, the platform won't succeed in delivering timer events at so tight interval and Qt will also optimize it a bit behind the curtains by merging pending paint events but it might still not be a wise thing to do.

[MarkoSan]

But this code, even with short timer, worked fine in 4.3.2. I simply do not get it. Let me try to change this timer.

Same warning!

[wysota]

That doesn't mean anything. I suggest you use a debugger to find out where the crash occurs instead of shooting blind.