Qwt 6.0.1 three times slower than 5.2.0

[Spitfire]

I'm using Qt 4.6.3,
Painter and pen settings are default (QPainter::TextAntialiasing, pen solid line, 1px black).

When I've launched the app on Ubuntu with -graphicssystem raster both versions are slower, but not even close to what's happening on windows.
5.2.0 plots 2m points in 340ms (as oposite to 250ms), 6.0.1 plots 2m points in 770ms (as opposite to 400ms).

I'll take your suggestion and tomorrow I'll write simple pure Qt app to verify if the issue lies with Qt or not and post it here.

Anyway, it may be worth considering adding a way of selecting which rendering system to use (int or double) as not everybody (for example me ) needs double precision.

I'll keep you updated.

[Uwe]

Painter and pen settings are default (QPainter::TextAntialiasing, pen solid line, 1px black).

The default is a pen width of 0 - a pen width of 1 is running into a different render path and might be significantly slower,

Uwe

[Spitfire]

I've done simple Qt example of painting using float and integers (attached) and on windows - the results is the same.
Floats are dead slow.

I did some search through Qt bug database and I found an issue complaining about drawpolyline being slow with antialiasing, but it's not the case here.

Any chances of getting a way of using ints instead of doubles for drawing in Qwt 6.x.x?
I can't see any other way of making it any faster

[Uwe]

Do you have a similar effects on 32bit systems - and of course with Qt 4.7.x ?

Uwe

[Spitfire]

I don't have access to any 32bit system at the moment, but I'll check it over the weekend and let you know.

[Uwe]

I've done simple Qt example of painting using float and integers (attached) and on windows - the results is the same.

I checked your code on my box ( Linux/X11, Qt 4.7.3 ):

• With basic X11 using floats is significantly faster ( about 10-30 % )
• Enabling the raster paint engine ( -graphicssystem raster ) I can confirm that doubles are much slower ( about 200-250 % )

So the "right" solution means obviously more than simply using integers instead of floats.

I will check the code of the raster paint engine why it performs so bad for floats.

Uwe

[Spitfire]

I've tested 32bit Windows XP box.

The problem is the same, with 5.2.0 plot is rendered in 1700ms, with 6.0.1 it takes 5600ms.

Let me know if you find anything interesting.
Cheers!

[Uwe]

In SVN trunk ( Qwt 6.1 ) QwtPlotCurve tries to use QPolygon instead of QPolygonF ( depending on various parameters ).

If you don't want to have this optimization you can enable the new flag QwtPlotItem::RenderFloats. Like expected setting this flag slows down the frame rate of the refreshtest example, when using the raster graphics system.

Uwe

[Uwe]

Just one more hint: in Qwt 5.2 was a flag QwtPlotCurve::PaintFiltered that removes points that are mapped to the same position as their predecessor. As long as your samples are not only noise it should have a much bigger impact on the performance than the QPolygon/QPolygonF flag.

I have reintroduced this optimization to Qwt 6.1 ( QwtPlotCurve::FilterPoints ). In situations, where the curve is rendered to an integer based paint engine ( all beside Svg and Pdf ) I would expect, that it will reduce the polygon heavily for a 2 million point series.

You can check the refreshtest example to see its effect.

Uwe