Redrawing a single row of a QwtPlotSpectrogram

[MSUdom5]

Interesting, never used mutable before but it seems to work fine. However, I bet if my C++ professor were around, I'd probably get a slap on the hand...

[MSUdom5]

Got it working!! Very smooth scrolling animation now. Uwe, thanks so much for your help!

[MSUdom5]

Although I've achieved my original goal, I'm now looking to generate large plots quicker and more efficiently. How much of a change would be involved to use OpenGL to paint plots instead of QPainter? I'm assuming I'll need to subclass QwtPlot? Uwe, any advice/direction?

[Uwe]

First of all I would optimize the existing solution and turn all paint caches off, because you have your own image cache:

a) Spectrogram item

spectrogram->setCachePolicy(QwtPlotRasterItem::NoCache);

spectrogram->setCachePolicy(QwtPlotRasterItem::NoCache);

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b) Plot canvas

plot->canvas()->setPaintAttribute(QwtPlotCanvas::PaintCached, false);
plot->canvas()->setPaintAttribute(QwtPlotCanvas::PaintPacked, false);

plot->canvas()->setPaintAttribute(QwtPlotCanvas::PaintCached, false);
plot->canvas()->setPaintAttribute(QwtPlotCanvas::PaintPacked, false);

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c) Qt ( only supported on X11 )

plot->canvas()->setAttribute(Qt::WA_PaintOnScreen, true);

plot->canvas()->setAttribute(Qt::WA_PaintOnScreen, true);

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Another optimization on multicore systems is to render parts of the image parallel in several threads.

Uwe

[MSUdom5]

Thanks! I'd already applied a) (the default is NoCache I think). I just applied b), and it sped things up a little. I'm using Windows (unfortunately), so c) doesn't apply.

For the most part, I'm getting the performance I need, so moving rendering to the GPU isn't absolutely necessary for me. My profiler says that most of the work is being done in qdrawhelper_p in some low-level math functions, so I don't think there's a higher level bottleneck. However, for a 1024x768 canvas, I'd like to be able to do 30 draws a second. Right now I can only get to ~20. (For smaller canvas, I have no problem with 30).