Qt Reference Documentation

# levelmeter.cpp Example File

demos/spectrum/app/levelmeter.cpp

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     #include "levelmeter.h"

     #include <math.h>

     #include <QPainter>
     #include <QTimer>
     #include <QDebug>

     // Constants
     const int RedrawInterval = 100; // ms
     const qreal PeakDecayRate = 0.001;
     const int PeakHoldLevelDuration = 2000; // ms

     LevelMeter::LevelMeter(QWidget *parent)
         :   QWidget(parent)
         ,   m_rmsLevel(0.0)
         ,   m_peakLevel(0.0)
         ,   m_decayedPeakLevel(0.0)
         ,   m_peakDecayRate(PeakDecayRate)
         ,   m_peakHoldLevel(0.0)
         ,   m_redrawTimer(new QTimer(this))
         ,   m_rmsColor(Qt::red)
         ,   m_peakColor(255, 200, 200, 255)
     {
         setSizePolicy(QSizePolicy::Fixed, QSizePolicy::Preferred);
         setMinimumWidth(30);

         connect(m_redrawTimer, SIGNAL(timeout()), this, SLOT(redrawTimerExpired()));
         m_redrawTimer->start(RedrawInterval);
     }

     LevelMeter::~LevelMeter()
     {

     }

     void LevelMeter::reset()
     {
         m_rmsLevel = 0.0;
         m_peakLevel = 0.0;
         update();
     }

     void LevelMeter::levelChanged(qreal rmsLevel, qreal peakLevel, int numSamples)
     {
         // Smooth the RMS signal
         const qreal smooth = pow(qreal(0.9), static_cast<qreal>(numSamples) / 256); // TODO: remove this magic number
         m_rmsLevel = (m_rmsLevel * smooth) + (rmsLevel * (1.0 - smooth));

         if (peakLevel > m_decayedPeakLevel) {
             m_peakLevel = peakLevel;
             m_decayedPeakLevel = peakLevel;
             m_peakLevelChanged.start();
         }

         if (peakLevel > m_peakHoldLevel) {
             m_peakHoldLevel = peakLevel;
             m_peakHoldLevelChanged.start();
         }

         update();
     }

     void LevelMeter::redrawTimerExpired()
     {
         // Decay the peak signal
         const int elapsedMs = m_peakLevelChanged.elapsed();
         const qreal decayAmount = m_peakDecayRate * elapsedMs;
         if (decayAmount < m_peakLevel)
             m_decayedPeakLevel = m_peakLevel - decayAmount;
         else
             m_decayedPeakLevel = 0.0;

         // Check whether to clear the peak hold level
         if (m_peakHoldLevelChanged.elapsed() > PeakHoldLevelDuration)
             m_peakHoldLevel = 0.0;

         update();
     }

     void LevelMeter::paintEvent(QPaintEvent *event)
     {
         Q_UNUSED(event)

         QPainter painter(this);
         painter.fillRect(rect(), Qt::black);

         QRect bar = rect();

         bar.setTop(rect().top() + (1.0 - m_peakHoldLevel) * rect().height());
         bar.setBottom(bar.top() + 5);
         painter.fillRect(bar, m_rmsColor);
         bar.setBottom(rect().bottom());

         bar.setTop(rect().top() + (1.0 - m_decayedPeakLevel) * rect().height());
         painter.fillRect(bar, m_peakColor);

         bar.setTop(rect().top() + (1.0 - m_rmsLevel) * rect().height());
         painter.fillRect(bar, m_rmsColor);
     }