Qt + WINAPI + direct painting

[JovianGhost]

I'm experimenting with direct painting to device contexts using WINAPI. Here's what my widget's paintEvent function looks like:

void ShotView::paintEvent(QPaintEvent *event)
{
    qDebug() << "Start paint function";
 
    HDC hdc = this->getDC();
 
    if (hdc == NULL)
    {
        qDebug() << "HDC error";
        return;
    }
 
    qDebug() << "Start painting";
 
    HPEN hPen = CreatePen(PS_SOLID, 4, RGB(0, 255, 0));
    if (hPen == NULL)
        qDebug() << "CreatePen error";
    if (SelectObject(hdc, hPen) == HGDI_ERROR)
        qDebug() << "SelectObject error";
    if (MoveToEx(hdc, 0, 0, NULL) == 0)
        qDebug() << "MoveToEx error";
    if (LineTo(hdc, 200, 200) == 0)
        qDebug() << "LineTo error";
 
    qDebug() << "End painting";
 
    this->releaseDC(hdc);
 
    qDebug() << "End paint function";
}

void ShotView::paintEvent(QPaintEvent *event)
{
    qDebug() << "Start paint function";

    HDC hdc = this->getDC();

    if (hdc == NULL)
    {
        qDebug() << "HDC error";
        return;
    }

    qDebug() << "Start painting";

    HPEN hPen = CreatePen(PS_SOLID, 4, RGB(0, 255, 0));
    if (hPen == NULL)
        qDebug() << "CreatePen error";
    if (SelectObject(hdc, hPen) == HGDI_ERROR)
        qDebug() << "SelectObject error";
    if (MoveToEx(hdc, 0, 0, NULL) == 0)
        qDebug() << "MoveToEx error";
    if (LineTo(hdc, 200, 200) == 0)
        qDebug() << "LineTo error";

    qDebug() << "End painting";

    this->releaseDC(hdc);

    qDebug() << "End paint function";
}

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When I run the program, I don't see a solid red line appearing as I should. The console output seems correct:

Start paint function
Start painting
End painting
End paint function

so it looks like everything in the function is getting called properly, but I don't see an output. It's been a while since I've done WINAPI stuff, but I think I have it right. Can anyone offer any insight?

[minimoog]

What about setting flag Qt::WA_PaintOnScreen?

[JovianGhost]

What about setting flag Qt::WA_PaintOnScreen?

I read in the docs this attribute is only supported under X11, and I'm running Windows.
Still, I tried it, no effect.

[edit]
I made a discovery. If I inherit from QWidget instead of QGraphicsView, it still doesn't work, but if I rapidly resize the widget I can see the line flickering, until it (presumably) gets painted over.

So what in QWidget is being called after paintEvent that's painting over my line?

[minimoog]

Set Qt::WA_PaintOnScreen in the QWidget constructor. Reimplement QPaintDevice:aintEngine() to return a null pointer. Just like is written in documentation on Qt::WA_PaintOnScreen.

It should be working.

[pherthyl]

Why are you using WinAPI to paint? Is there a problem with QPainter?

[JovianGhost]

I got it working, finally!

Set Qt::WA_PaintOnScreen in the QWidget constructor. Reimplement QPaintDevice:aintEngine() to return a null pointer. Just like is written in documentation on Qt::WA_PaintOnScreen.

It should be working.

That's what I did at first, but it didn't work. After you said this I went back to check it out and noticed something: when overriding paintEngine() I forgot to put the trailing "const" in the declaration and definition, so instead of

QPaintEngine* paintEngine() const { return 0; }

QPaintEngine* paintEngine() const { return 0; }

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I had

QPaintEngine* paintEngine() { return 0; }

QPaintEngine* paintEngine() { return 0; }

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which of course did nothing. So thanks!

Why are you using WinAPI to paint? Is there a problem with QPainter?

To be honest, I never really considered QPainter. I was trying to do this as low-level as possible to reduce the overhead introduced by Qt's painting framework. But it looks like QPainter will do exactly what I need, thanks!

[aamer4yu]

But it looks like QPainter will do exactly what I need, thanks!

Thats how it is done in Qt.

To be honest, I never really considered QPainter. I was trying to do this as low-level as possible to reduce the overhead introduced by Qt's painting framework.

Then why use Qt... there will be some overhead still :P
juzz kidding.. theres always tradeoff between complexity of code and runtime. And Qt's paint framework is not that slow..also if you use QPainter, your code will be cross platform !

[JovianGhost]

Well I've been experimenting with QPainter and so far it's working very well!

[aamer4yu]

Well I've been experimenting with QPainter and so far it's working very well!

Good to know ... I guess you had underestimated Qt's performance :P