Reduce number of plotted points in spectrogram plot

[hberg539]

Hello,

we are using the spectogram plot to display a 2D Matrix with about 13000x64 values and i would like to optimize the plot speed (currently about 10 fps). Because there are only 1920px pixel in width (display size), much time is wasted to try to plot pixels which physically can't be displayed.

We are also using the magnifier, zoom and pan widget to zoom into an area of interest, therefore i can't modifiy the matrix size.

I looked into the source code but i can't really find the place to change the behaivor of qwt_plot_spectrogram. My goal is to reduce the number of plotted points to the size of the displayed canvas.

Does anybody how i can make this possible?

Many thanks in advance.

Best regards,
Kevin

[Uwe]

we are using the spectogram plot to display a 2D Matrix with about 13000x64 values and i would like to optimize the plot speed (currently about 10 fps). Because there are only 1920px pixel in width (display size), much time is wasted to try to plot pixels which physically can't be displayed.

No QwtPlotSpectrogram runs over the pixels ( in paint device resolution ! ) requesting a value for each pixel from the raster data object.

The resolution of the data only matters if it is lower than the paint device resolution. To have this optimization you need to tell the spectrogram about the resolution by implementing QwtRasterData::pixelHint(). When using QwtMatrixRasterData this implementation is available out of the box. See http://qwt.sourceforge.net/class_qwt...a1e7efe97d39d6

Having a valid QwtRasterData::pixelHint makes a lot of sense in your case as it limits the number of requests ( and color interpolations ) from 1920 x 1920 to 1920 x 64 !
But keep in mind, that 1920 x 64 still means 122880 pixels that need to be calculated and then scaled to 1920 x 1920.

I recommend to have a look at Qwt from SVN trunk, where the spectrogram has some performance improvements. In qwt_plot_spectrogram.cpp you can enable DEBUG_RENDER to get some performance debug printouts you can check with the spectrogram example. The performance of your plot should be better compared with the example as your data resolution is horizontally always less than the screen resolution. If it is worse you should have a closer look at your color map and/or raster data object.

Uwe