Re: A performance question... - summary

[Giacomo <gia...@el...>]

Summing up, my application takes up to 20-22%CPU spread on two screens
without labels.

Drawing the labels as pixmaps, with the methods cited below, the CPU
usage grows 10%, thus exceeding 30%

Pixmaps are cached in a QMap<double, QPixmap> as follows:

void PlotGrid::createHash(const QStringList &labels, const
QList<double>& zpos)
{
int rWidth, rHeight;
QFont f("", 9);
QFontMetrics fm(f);
QColor bgColor = Qt::white;
bgColor.setAlpha(155);
m_maxLabelHeight = 0;
for(int i = 0; i < zpos.size(); i++)
{
	QString label = labels.at(i);
	rWidth = fm.width(label);
	rHeight = fm.height();
	if(rWidth > m_maxLabelHeight)
		m_maxLabelHeight = rWidth;
	QImage img(rWidth, rHeight, QImage::Format_ARGB6666_Premultiplied);
	QImage imgRot;
	QPainter p(&img);
	p.setPen(Qt::black);
	QMatrix matrix;
	matrix.rotate(90);
	img.fill(bgColor.rgb());
	QRect r(QPoint(0,0), QSize(rWidth, rHeight));

	p.drawText(QRect(0, 0, rWidth, rHeight), Qt::AlignHCenter|Qt::AlignVCenter, label);
	/* rotate image */
	imgRot = img.transformed(matrix, Qt::SmoothTransformation);
	/* populate map */
	m_map.insert(zpos.at(i), QPixmap::fromImage(imgRot));
}
}

Here's the draw method:

void PlotGrid::draw(QPainter *p, const QwtScaleMap &xMap, const QwtScaleMap &yMap, const QRect &rect) const
{
	double dx, previousDx = 0.0, dy = m_maxLabelHeight;
	p->save();
	QwtScaleDiv sd = xScaleDiv();
	QwtValueList values = sd.ticks(QwtScaleDiv::MajorTick);

	p->translate(0, p->viewport().height() - dy);
	p->setPen(Qt::black);

	for(int i = 0; i < values.size(); i++)
	{
  		QPixmap pix = m_map.value(values.at(i));
        	p->drawPixmap(xMap.transform(values.at(i)), 0, pix);
	}
	p->restore();
	QwtPlotGrid::draw(p, xMap, yMap, rect);
}

The other draw operation, for the horizontal labels in the attached
screenshot is equivalent, except that the QImage isn't rotated, so the 
code is even simpler.

By the way, I wasn't able to set an alpha channel on the
QImage/QPixmap :(

The green strip between the green lines raises the cpu of another 15-20%
(due to the alpha channel)... but I think this is inevitable...


Thanks for all your advice.


Il giorno ven, 01/07/2011 alle 17.09 +0200, Giacomo ha scritto:
> Il giorno ven, 01/07/2011 alle 16.55 +0200, Uwe Rathmann ha scritto:
> > > I will try replacing QImage with QPixmap then...
> 
> 
> Replacing QImage with QPixmap reduced the load of another 10%.
> The application now takes about 30% of CPU (Xorg)... still taking up
> twice the cpu of other plots with the same refresh rate, but without
> text/ text pixmaps.
> 
> On monday I will try  to polish the code even more.
> 
> > 
> > I would implement a small dummy application, where you paint a text string 
> > 1000 times to a widget and another one, where you draw a pixmap 1000 times ( 
> > it doesn't matter what is in it ).
> > 
> > Simply overload paintEvent, set up a QPainter and run the loops.
> > 
> > > I'll give a look at backingstore, don't know what it is.. has it to do
> > > with the PaintCached and PaintPacked attributes of QwtPlotCanvas?
> > 
> > Yes, QwtPlotCanvas::PaintCached is the backing store ( in Qwt 6 this attribute 
> > is renamed to BackingStore ). Note, that Qt has another backing store.
> 
> In my plot constructor I invoke these two
> 
>     setCanvasPaintCached(true);
>     setCanvasPaintPacked(true);
> 
> hope this is the right choice and combination of the two.
> 
> I'll let you know where I get on monday ...
> 
> thanks a lot and have a nice weekend.
> 
> Giacomo.
> 
> > 
> > Uwe
> 
> 
> 
> ------------------------------------------------------------------------------
> All of the data generated in your IT infrastructure is seriously valuable.
> Why? It contains a definitive record of application performance, security 
> threats, fraudulent activity, and more. Splunk takes this data and makes 
> sense of it. IT sense. And common sense.
> http://p.sf.net/sfu/splunk-d2d-c2
> _______________________________________________
> qwt-interest mailing list
> qwt...@li...
> https://lists.sourceforge.net/lists/listinfo/qwt-interest