Extra samples in the wavetables, to help interpolations

src/sfizz/Wavetables.cpp

@@ -143,17 +143,18 @@ WavetableRange WavetableRange::getRangeForFrequency(float f)
 }
 
 //------------------------------------------------------------------------------
+constexpr unsigned WavetableMulti::_tableExtra;
+
 WavetableMulti WavetableMulti::createForHarmonicProfile(
     const HarmonicProfile& hp, unsigned tableSize, double refSampleRate)
 {
     WavetableMulti wm;
-    constexpr unsigned multiSize = wm.multiSize();
+    constexpr unsigned multiSize = WavetableMulti::multiSize();
 
     // amplitude to match ARIA's default generator output
     constexpr double amplitude = 0.25;
 
-    wm._tableSize = tableSize;
-    wm._multiData.reset(new float[tableSize * multiSize]);
+    wm.allocateStorage(tableSize);
 
     for (unsigned m = 0; m < multiSize; ++m) {
         WavetableRange range = WavetableRange::getRangeForOctave(m);
@@ -165,13 +166,34 @@ WavetableMulti WavetableMulti::createForHarmonicProfile(
         // Therefore it's desired to cut harmonics at C=0.5*Fs/Fs'=0.5*Fs/(F*N).
         double cutoff = (0.5 * refSampleRate / tableSize) / freq;
 
-        float* ptr = &wm._multiData[m * tableSize];
+        float* ptr = const_cast<float*>(wm.getTablePointer(m));
         absl::Span<float> table(ptr, tableSize);
 
         hp.generate(table, amplitude, cutoff);
     }
 
+    wm.fillExtra();
+
     return wm;
 }
 
+void WavetableMulti::allocateStorage(unsigned tableSize)
+{
+    _multiData.reset(new float[(tableSize + _tableExtra) * multiSize()]());
+    _tableSize = tableSize;
+}
+
+void WavetableMulti::fillExtra()
+{
+    unsigned tableSize = _tableSize;
+    constexpr unsigned tableExtra = _tableExtra;
+    constexpr unsigned multiSize = WavetableMulti::multiSize();
+
+    for (unsigned m = 0; m < multiSize; ++m) {
+        float* ptr = const_cast<float*>(getTablePointer(m));
+        for (unsigned i = 0; i < tableExtra; ++i)
+            ptr[tableSize + i] = ptr[i % tableSize];
+    }
+}
+
 } // namespace sfz

src/sfizz/Wavetables.h

@@ -86,9 +86,7 @@ public:
     // get the N-th table in the multisample
     absl::Span<const float> getTable(unsigned index) const
     {
-        unsigned size = _tableSize;
-        const float* ptr = &_multiData[index * _tableSize];
-        return { ptr, size };
+        return { getTablePointer(index), _tableSize };
     }
 
     // get the table which is adequate for a given playback frequency
@@ -104,9 +102,24 @@ public:
         const HarmonicProfile& hp, unsigned tableSize, double refSampleRate = 44100.0);
 
 private:
+    // get a pointer to the beginning of the N-th table
+    const float* getTablePointer(unsigned index) const
+    {
+        return &_multiData[index * (_tableSize + _tableExtra)];
+    }
+
+    // allocate the internal data for tables of the given size
+    void allocateStorage(unsigned tableSize);
+
+    // fill extra data at table ends with repetitions of the first samples
+    void fillExtra();
+
     // length of each individual table of the multisample
     unsigned _tableSize = 0;
 
+    // number X of extra elements, for safe interpolations up to X-th order.
+    static constexpr unsigned _tableExtra = 4;
+
     // internal storage, having `multiSize` rows and `tableSize` columns.
     std::unique_ptr<float[]> _multiData;
 };