app using "excessive" CPU

[mzimmers]

I suspect it has something to do with this line:

painter.drawPixmap(rect(), pixmap()->scaled(size(), Qt::KeepAspectRatio, Qt::SmoothTransformation));

I don't need to scale it; what do I replace the call to scaled() with?

[mrjj]

@mzimmers said in app using "excessive" CPU:

painter.drawPixmap(rect(), pixmap()->scaled(size(), Qt::KeepAspectRatio, Qt::SmoothTransformation));

painter.drawPixmap(rect(), pixmap());
actaully scale it to rect so @Bonnie version is correct.

to draw it at original size.

or you can simply scale it once outside paintEvent and reuse the scaled version.

[Bonnie]

To not scale at all, should not use a rect as parameter (unless the rect size is the same as the pixmap size).

painter.drawPixmap(0, 0, pixmap());

(0, 0) is the draw position.

[J.Hilk]

@mzimmers great you found it! But paintEvent itself should not be called that much regularly! I suspect your logo is being resized (wiggles) all the time!

[JonB]

@mzimmers
Ah ha! We have been looking for where you "wiggle", I suspect @J-Hilk has found it!?

[mzimmers]

Thank you all for the replies. I tried mrjj's/Bonnie's suggestions, and they didn't change the CPU usage.

But really, the question (IMO) is WHY is my logo being wiggled? I have matched the dimensions of the .png file to the QWidget (a QLabel) that displays it. I've set the QLabel's size to fixed. What could be causing this flood of paint events?

The technique I'm using is to promote the QLabel to a class I've created (LogoLabel). Here's the entirety of its paint event:

void LogoLabel::paintEvent(QPaintEvent *event)
{
    Q_UNUSED(event)
    QPainter painter(this);
    const QString filename(":/logos/CYBERDATA_IP_ENDPOINT_CO_small.png");

    bool rc;

    rc = m_pixmap.load(filename);
    if (rc)
    {
        setPixmap(m_pixmap);
        Qt::KeepAspectRatio, Qt::SmoothTransformation));
        painter.drawPixmap(0, 0, *pixmap());
    }
}

[ollarch]

Hi,

You could reimplement "reseizeEvent" and do the Pixmap scale there. On "paintEvent", just paint it or just set the pixmap to the Label and it will paint it.

[Bonnie]

@mzimmers Why are you doing the loading in the paintEvent???

void LogoLabel::paintEvent(QPaintEvent *event)
{
    Q_UNUSED(event)
     //m_pixmap should already be loaded
    if (!m_pixmap.isNull())
    {
        QPainter painter(this);
        painter.drawPixmap(0, 0, m_pixmap);
    }
}

I remembered you had a post said you write your label class because you want the QLabel to scale smoother.
Now, if you do not need to scale, you only need to load the pixmap once after it is created.

label.setPixmap(QPixmap(":/logos/CYBERDATA_IP_ENDPOINT_CO_small.png"));

If you still need to scale, I think you should scale it in the resizeEvent.

void LogoLabel::resizeEvent(QResize*event)
{
    QLabel::resizeEvent(event);
    if(QPixmap *pix = pixmap())
        m_pixmap = pix->scaled(event->size(), Qt::KeepAspectRatio, Qt::SmoothTransformation);
}

Actually, I don't think you need to subclass QLabel...Subclassing QWidget is enough if you reimplement all these events...

[mzimmers]

@Bonnie you nailed it. I moved the load to the c'tor, and everything works much better now.

A couple of notes:

1. I turned on my event filter, and it's no longer spewing zillions of events, so I guess my keyPress idea would have worked (if Windows wasn't so lame).
2. I tried making my painter a member variable and initializing it in the c'tor, but that didn't work. Any idea why?

Thanks to everyone who looked at this.

[Bonnie]

@mzimmers
You can only create and use a painter of a widget in its paintEvent.

[jsulm]

@mzimmers To add to @Bonnie it is actually explained in the documentation https://doc.qt.io/qt-5/qpainter.html :
"Warning: When the paintdevice is a widget, QPainter can only be used inside a paintEvent() function or in a function called by paintEvent()."