Rendering a qwt plot spectrogram

[moijhd]

Hi,

I am trying to render a QwtPlotSpectrogram into a PDF file using a QwtPlotRenderer.

Here some code that does not get the result I expected :

int main()
{
   QwtPlot *plot;
    plot = new QwtPlot();
    plot->setTitle("Plot");
    plot->setAxisScale(QwtPlot::xBottom, 0, 10);
    plot->setAxisScale(QwtPlot::yLeft, 0, 10);
    plot->enableAxis(QwtPlot::yRight);
    plot->setAxisScale(QwtPlot::yRight, 0, 100*100);
    plot->show();
 
    QVector<double> xData;
    QVector<double> zData;
    for(int i = 0; i < 100; i++)
    {
        zData.push_back(i*i);
        xData.push_back(i);
    }
 
    QwtMatrixRasterData *matrix;
    matrix = new QwtMatrixRasterData();
    matrix->setValueMatrix(zData, 10);
    matrix->setInterval(
                Qt::XAxis,
                QwtInterval(0, 10)
                );
    matrix->setInterval(
                Qt::YAxis,
                QwtInterval(0, 10)
                );
 
    matrix->setInterval(
                Qt::ZAxis,
                QwtInterval(0, 100*100)
                );
 
    QwtPlotSpectrogram *spectrogram;
    spectrogram = new QwtPlotSpectrogram();
    spectrogram->setData(
                matrix
                );
    spectrogram->attach(plot);
 
    QwtPlotCurve *curve;
    curve = new QwtPlotCurve();
    curve->setSamples(xData, zData);
    curve->attach(plot);
 
    QwtPlotRenderer *plotRenderer;
    plotRenderer = new QwtPlotRenderer();
    plotRenderer->setDiscardFlag(QwtPlotRenderer::DiscardBackground);
    plotRenderer->setDiscardFlag(QwtPlotRenderer::DiscardCanvasBackground);
    plotRenderer->setDiscardFlag(QwtPlotRenderer::DiscardCanvasFrame);
 
    plot->replot();
 
    QPrinter printer;
    printer.setOutputFormat(QPrinter::PdfFormat);
    printer.setOutputFileName("test01.pdf");
    printer.setResolution(QPrinter::HighResolution);
    printer.setColorMode(QPrinter::Color);
    printer.setDuplex(QPrinter::DuplexAuto);
    printer.setFullPage(true);
    printer.setOrientation(QPrinter::Landscape);
    printer.setPageMargins(0, 0, 0, 0, QPrinter::Millimeter);
    printer.setPageOrder(QPrinter::FirstPageFirst);
 
    QPainter *painter;
    painter = new QPainter();
    painter->begin(&printer);
 
    plotRenderer->render(plot, painter, painter->viewport());
 
    painter->end();
 
    return 0;
}

int main()
{
   QwtPlot *plot;
    plot = new QwtPlot();
    plot->setTitle("Plot");
    plot->setAxisScale(QwtPlot::xBottom, 0, 10);
    plot->setAxisScale(QwtPlot::yLeft, 0, 10);
    plot->enableAxis(QwtPlot::yRight);
    plot->setAxisScale(QwtPlot::yRight, 0, 100*100);
    plot->show();

    QVector<double> xData;
    QVector<double> zData;
    for(int i = 0; i < 100; i++)
    {
        zData.push_back(i*i);
        xData.push_back(i);
    }

    QwtMatrixRasterData *matrix;
    matrix = new QwtMatrixRasterData();
    matrix->setValueMatrix(zData, 10);
    matrix->setInterval(
                Qt::XAxis,
                QwtInterval(0, 10)
                );
    matrix->setInterval(
                Qt::YAxis,
                QwtInterval(0, 10)
                );

    matrix->setInterval(
                Qt::ZAxis,
                QwtInterval(0, 100*100)
                );

    QwtPlotSpectrogram *spectrogram;
    spectrogram = new QwtPlotSpectrogram();
    spectrogram->setData(
                matrix
                );
    spectrogram->attach(plot);

    QwtPlotCurve *curve;
    curve = new QwtPlotCurve();
    curve->setSamples(xData, zData);
    curve->attach(plot);

    QwtPlotRenderer *plotRenderer;
    plotRenderer = new QwtPlotRenderer();
    plotRenderer->setDiscardFlag(QwtPlotRenderer::DiscardBackground);
    plotRenderer->setDiscardFlag(QwtPlotRenderer::DiscardCanvasBackground);
    plotRenderer->setDiscardFlag(QwtPlotRenderer::DiscardCanvasFrame);

    plot->replot();

    QPrinter printer;
    printer.setOutputFormat(QPrinter::PdfFormat);
    printer.setOutputFileName("test01.pdf");
    printer.setResolution(QPrinter::HighResolution);
    printer.setColorMode(QPrinter::Color);
    printer.setDuplex(QPrinter::DuplexAuto);
    printer.setFullPage(true);
    printer.setOrientation(QPrinter::Landscape);
    printer.setPageMargins(0, 0, 0, 0, QPrinter::Millimeter);
    printer.setPageOrder(QPrinter::FirstPageFirst);

    QPainter *painter;
    painter = new QPainter();
    painter->begin(&printer);

    plotRenderer->render(plot, painter, painter->viewport());

    painter->end();

    return 0;
}

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Please see attachments to understand.

Thanks !

---

Added after 16 minutes:

printer.setResolution(QPrinter::HighResolution);

printer.setResolution(QPrinter::HighResolution);

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was the problem.

I removed it. And I guess I should prefer QPrinter::setResolution instead, if I want to increase the quality of the document.

[moijhd]

This is quite not the end : the result is a JPG picture : how to get a vectoriel picture ?

[Uwe]

The renderer should always create a correct image - even with a very low resolution. So the seconds screenshot looks like a bug to me - in Qwt or in your code. Please upload a small compilable demo, that shows the issue, that I can have a look at it.

Qt offers 2 types of scalable vector graphic formats: SVG and PDF. I recommend PDF as the SVG renderer ignores painter clipping and because of this not all plots are rendered properly. But even when using SVG or PDF - drawing a raster graphic ( a spectrogram is one ) to a format that is able to handle vector graphics doesn't make it a vector graphic.

Also note, that the resolution of the resulting image also depends on the resolution of your data !

Have a look at QwtPlotRenderer::renderDocument() or QwtPlotRenderer::exportTo(). These high level APIs should be more convenient for your use case.

Uwe

[moijhd]

The second screenshot bug was due to "printer.setResolution(QPrinter::HighResolution);" . I don't know why. I removed it and it rendered fine. The code I provided is compilable.

Regarding the raster graphic, there is no way to make it a vector graphic ?

[Uwe]

Regarding the raster graphic, there is no way to make it a vector graphic ?

The reason why it is a raster graphic is because you are displaying raster data. "Vectorizing" data pixels to rectangles won't change anything.

What you could try to do is to enable QwtMatrixRasterData::BilinearInterpolation. It doesn't make your output scalable, but the image will always be rendered in device resolution ( not limited by the resolution of your data ). But note, that rendering the image will be slower then.

Uwe

PS: try to use "spectrogram->setRenderThreadCount( 0 );" to enable multithreaded rendering