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Thread: Slow replot of fillCurve in QWT

  1. #1
    Join Date
    Jun 2016
    Qt products
    Unix/X11 Windows Android

    Default Slow replot of fillCurve in QWT

    Hello, everybody,
    I have a problem drawing curves with a filled area (QwtPlotCurve::fillCurve). The time needed to draw the drawing method (QwtPainter::drawPolygon) increases rapidly with the data points. Here is a measurement (time to draw; number of data points).

    _timer: 7 ms; size: 300
    _timer: 22 ms; size: 550
    _timer: 36 ms; size: 800
    _timer: 65 ms; size: 1050
    _timer: 91 ms; size: 1300
    _timer: 105 ms; size: 1450

    My plot is redrawn every 250 ms and should contain up to 60000 data points if possible.

    When I switch to the area plot, I set the following parameters (ADataPlot is derived from QwtPlotCurve, PC_DATA is the index of a curve).

    Qt Code:
    1. ADataCurve * curve = Curves.value(PC_DATA);
    2. if (curve)
    3. {
    4. QBrush brush;
    5. brush.setStyle(Qt::SolidPattern);
    6. curve->setBrush(brush);
    7. curve->setStyle(QwtPlotCurve::Sticks);
    8. curve->setOrientation(Qt::Vertical);
    9. }
    To copy to clipboard, switch view to plain text mode 

    Does anyone know how I can reduce the time I spend drawing? I am grateful for any help!

  2. #2
    Join Date
    Feb 2006
    Munich, Germany
    Thanked 847 Times in 796 Posts
    Qt products
    Qt3 Qt4 Qt/Embedded
    MacOS X Unix/X11 Windows

    Default Re: Slow replot of fillCurve in QWT

    When having 60000 points the QwtPlotCurve::FilterPointsAggressive ( Qwt >= SVN branch 6.2 ) flag will be quite effective. This will limit the number of points to a maximum of 4 * canvasWidth.
    There is also the option of using QwtWeedingCurveFitter, but this algo is pretty expensive and might not result in something better. But you can try with a small chunkSize ( ~10 ) and see what happens.

    Furthermore you could try to find a hardware accelerated backend. When being on X11 you only need to enable the X11 paintengine ( Qt4 or Qt5 >= 5.10 by "export QT_XCB_NATIVE_PAINTING=1" ).

    On other platforms you could try to get OpenGL running. For your situation ( expensive rendering + moderate update rates ) QwtPlotCanvas::OpenGLBuffer ( SVN trunk ). You could also try one of the OpenGL canvases - rendering will be the same as with QwtPlotCanvas::OpenGLBuffer, but the extra roundtrip for copying between graphics and main memory can be avoided.

    If you have the choice: going with X11 is the best option.


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