qwt的examples下的spectrogram例子 [复制链接]

qwt的examples下的spectrogram例子内的plot.cpp文件的SpectrogramData类下的value函数在这例子中是什么时候调用的？
class SpectrogramData: public QwtRasterData
{
public:
    SpectrogramData():
        QwtRasterData(QwtDoubleRect(-1.5, -1.5, 3.0, 3.0))
    {
    }

    virtual QwtRasterData *copy() const
    {
        return new SpectrogramData();
    }

    virtual QwtDoubleInterval range() const
    {
        return QwtDoubleInterval(0.0, 10.0);
    }

    virtual double value(double x, double y) const
    {
        const double c = 0.842;

        const double v1 = x * x + (y-c) * (y+c);
        const double v2 = x * (y+c) + x * (y+c);

        return 1.0 / (v1 * v1 + v2 * v2);
    }};

还有图中画的轮廓线是根据以下一段程序形成的吗？它是怎么画出那个轮廓线的啊？
  QwtValueList contourLevels;
    for ( double level = 0.5; level < 10.0; level += 1.0 )
        contourLevels += level;
    d_spectrogram->setContourLevels(contourLevels);