Make scroll wheel zoom work with decimation

src/plotview.cpp

@@ -186,7 +186,7 @@ bool PlotView::viewportEvent(QEvent *event) {
         QWheelEvent *wheelEvent = (QWheelEvent*)event;
         if (QApplication::keyboardModifiers() & Qt::ControlModifier) {
             bool canZoomIn = zoomLevel < fftSize;
-            bool canZoomOut = zoomLevel > 1;
+            bool canZoomOut = zoomLevel > -6;
             int delta = wheelEvent->angleDelta().y();
             if ((delta > 0 && canZoomIn) || (delta < 0 && canZoomOut)) {
                 scrollZoomStepsAccumulated += delta;
@@ -395,9 +395,12 @@ void PlotView::setFFTAndZoom(int size, int zoom)
         spectrogramPlot->setFFTSize(size);
 
     // Set new zoom level
-    zoomLevel = zoom;
-    if (spectrogramPlot != nullptr)
-        spectrogramPlot->setZoomLevel(zoom);
+    zoomLevel = std::max(1,zoom);
+    nfftSkip = std::max(1,-1*zoom);
+    if (spectrogramPlot != nullptr) {
+        spectrogramPlot->setZoomLevel(zoomLevel);
+        spectrogramPlot->setSkip(nfftSkip);
+    }
 
     // Update horizontal (time) scrollbar
     horizontalScrollBar()->setSingleStep(10);
@@ -524,7 +527,7 @@ void PlotView::resizeEvent(QResizeEvent * event)
 
 size_t PlotView::samplesPerColumn()
 {
-    return fftSize / zoomLevel;
+    return fftSize * nfftSkip / zoomLevel;
 }
 
 void PlotView::scrollContentsBy(int dx, int dy)

src/plotview.h

@@ -73,6 +73,7 @@ public slots:
 
     int fftSize = 1024;
     int zoomLevel = 1;
+    int nfftSkip = 1;
     int powerMin;
     int powerMax;
     bool cursorsEnabled;

src/spectrogramcontrols.cpp

@@ -52,7 +52,7 @@ SpectrogramControls::SpectrogramControls(const QString & title, QWidget * parent
     layout->addRow(new QLabel(tr("FFT size:")), fftSizeSlider);
 
     zoomLevelSlider = new QSlider(Qt::Horizontal, widget);
-    zoomLevelSlider->setRange(0, 10);
+    zoomLevelSlider->setRange(-6, 10);
     zoomLevelSlider->setPageStep(1);
 
     layout->addRow(new QLabel(tr("Zoom:")), zoomLevelSlider);
@@ -139,7 +139,13 @@ void SpectrogramControls::setDefaults()
 void SpectrogramControls::fftOrZoomChanged(void)
 {
     int fftSize = pow(2, fftSizeSlider->value());
-    int zoomLevel = std::min(fftSize, (int)pow(2, zoomLevelSlider->value()));
+    int zoomLevel = zoomLevelSlider->value();
+    if (zoomLevel >= 0)
+        // zooming in by power-of-two steps
+        zoomLevel = std::min(fftSize, (int)pow(2, zoomLevel));
+    else
+        // zooming out (skipping FFTs) by power-of-two steps
+        zoomLevel = -1*std::min(fftSize, (int)pow(2, -1*zoomLevel));
     emit fftOrZoomChanged(fftSize, zoomLevel);
 }
 

src/spectrogramplot.cpp

@@ -35,6 +35,7 @@ SpectrogramPlot::SpectrogramPlot(std::shared_ptr<SampleSource<std::complex<float
 {
     setFFTSize(fftSize);
     zoomLevel = 1;
+    nfftSkip = 1;
     powerMax = 0.0f;
     powerMin = -50.0f;
     sampleRate = 0;
@@ -222,7 +223,7 @@ void SpectrogramPlot::paintMid(QPainter &painter, QRect &rect, range_t<size_t> s
 
 QPixmap* SpectrogramPlot::getPixmapTile(size_t tile)
 {
-    QPixmap *obj = pixmapCache.object(TileCacheKey(fftSize, zoomLevel, tile));
+    QPixmap *obj = pixmapCache.object(TileCacheKey(fftSize, zoomLevel, nfftSkip, tile));
     if (obj != 0)
         return obj;
 
@@ -241,13 +242,13 @@ QPixmap* SpectrogramPlot::getPixmapTile(size_t tile)
         }
     }
     obj->convertFromImage(image);
-    pixmapCache.insert(TileCacheKey(fftSize, zoomLevel, tile), obj);
+    pixmapCache.insert(TileCacheKey(fftSize, zoomLevel, nfftSkip, tile), obj);
     return obj;
 }
 
 float* SpectrogramPlot::getFFTTile(size_t tile)
 {
-    std::array<float, tileSize>* obj = fftCache.object(TileCacheKey(fftSize, zoomLevel, tile));
+    std::array<float, tileSize>* obj = fftCache.object(TileCacheKey(fftSize, zoomLevel, nfftSkip, tile));
     if (obj != nullptr)
         return obj->data();
 
@@ -259,7 +260,7 @@ float* SpectrogramPlot::getFFTTile(size_t tile)
         sample += getStride();
         ptr += fftSize;
     }
-    fftCache.insert(TileCacheKey(fftSize, zoomLevel, tile), destStorage);
+    fftCache.insert(TileCacheKey(fftSize, zoomLevel, nfftSkip, tile), destStorage);
     return destStorage->data();
 }
 
@@ -296,7 +297,7 @@ void SpectrogramPlot::getLine(float *dest, size_t sample)
 
 int SpectrogramPlot::getStride()
 {
-    return fftSize / zoomLevel;
+    return fftSize * nfftSkip / zoomLevel;
 }
 
 float SpectrogramPlot::getTunerPhaseInc()
@@ -377,6 +378,11 @@ void SpectrogramPlot::setZoomLevel(int zoom)
     zoomLevel = zoom;
 }
 
+void SpectrogramPlot::setSkip(int skip)
+{
+    nfftSkip = skip;
+}
+
 void SpectrogramPlot::setSampleRate(double rate)
 {
     sampleRate = rate;

src/spectrogramplot.h

@@ -54,6 +54,7 @@ public slots:
     void setPowerMax(int power);
     void setPowerMin(int power);
     void setZoomLevel(int zoom);
+    void setSkip(int skip);
     void tunerMoved();
 
 private:
@@ -69,6 +70,7 @@ public slots:
 
     int fftSize;
     int zoomLevel;
+    int nfftSkip;
     float powerMax;
     float powerMin;
     double sampleRate;
@@ -92,19 +94,22 @@ class TileCacheKey
 {
 
 public:
-    TileCacheKey(int fftSize, int zoomLevel, size_t sample) {
+    TileCacheKey(int fftSize, int zoomLevel, int nfftSkip, size_t sample) {
         this->fftSize = fftSize;
         this->zoomLevel = zoomLevel;
+        this->nfftSkip = nfftSkip;
         this->sample = sample;
     }
 
     bool operator==(const TileCacheKey &k2) const {
         return (this->fftSize == k2.fftSize) &&
                (this->zoomLevel == k2.zoomLevel) &&
+               (this->nfftSkip == k2.nfftSkip) &&
                (this->sample == k2.sample);
     }
 
     int fftSize;
     int zoomLevel;
+    int nfftSkip;
     size_t sample;
 };