Add stretch tuning API and implement

src/sfizz.h

@@ -137,6 +137,15 @@ SFIZZ_EXPORTED_API void sfizz_set_tuning_frequency(sfizz_synth_t* synth, float f
  */
 SFIZZ_EXPORTED_API float sfizz_get_tuning_frequency(sfizz_synth_t* synth);
 
+/**
+ * @brief      Configure stretch tuning using a predefined parametric Railsback curve.
+ *             A ratio 1/2 is supposed to match the average piano; 0 disables (the default).
+ *
+ * @param      synth          The sfizz synth.
+ * @param      ratio          The parameter in domain 0-1.
+ */
+SFIZZ_EXPORTED_API void sfizz_load_stretch_tuning_by_ratio(sfizz_synth_t* synth, float ratio);
+
 /**
  * @brief      Return the number of regions in the currently loaded SFZ file.
  *

src/sfizz.hpp

@@ -119,6 +119,14 @@ public:
      */
     float getTuningFrequency() const;
 
+    /**
+     * @brief Configure stretch tuning using a predefined parametric Railsback curve.
+     * A ratio 1/2 is supposed to match the average piano; 0 disables (the default).
+     *
+     * @param ratio The parameter in domain 0-1.
+     */
+    void loadStretchTuningByRatio(float ratio);
+
     /**
      * @brief Return the current number of regions loaded.
      */

src/sfizz/Resources.h

@@ -13,6 +13,7 @@
 #include "Wavetables.h"
 #include "Curve.h"
 #include "Tuning.h"
+#include "absl/types/optional.h"
 
 namespace sfz
 {
@@ -29,6 +30,7 @@ struct Resources
     EQPool eqPool { midiState };
     WavetablePool wavePool;
     Tuning tuning;
+    absl::optional<StretchTuning> stretch;
 
     void setSampleRate(float samplerate)
     {

src/sfizz/Synth.cpp

@@ -516,6 +516,17 @@ float sfz::Synth::getTuningFrequency() const
     return resources.tuning.getTuningFrequency();
 }
 
+void sfz::Synth::loadStretchTuningByRatio(float ratio)
+{
+    CHECK(ratio >= 0.0f && ratio <= 1.0f);
+    ratio = clamp(ratio, 0.0f, 1.0f);
+
+    if (ratio > 0.0f)
+        resources.stretch = StretchTuning::createRailsbackFromRatio(ratio);
+    else
+        resources.stretch.reset();
+}
+
 sfz::Voice* sfz::Synth::findFreeVoice() noexcept
 {
     auto freeVoice = absl::c_find_if(voices, [](const std::unique_ptr<Voice>& voice) {

src/sfizz/Synth.h

@@ -150,6 +150,12 @@ public:
      * @return The frequency which indicates where standard tuning A4 is (default 440 Hz).
      */
     float getTuningFrequency() const;
+    /**
+     * @brief Configure stretch tuning using a predefined parametric Railsback curve.
+     *
+     * @param ratio The parameter in domain 0-1.
+     */
+    void loadStretchTuningByRatio(float ratio);
     /**
      * @brief Get the current number of regions loaded
      *

src/sfizz/Voice.cpp

@@ -87,6 +87,11 @@ void sfz::Voice::startVoice(Region* region, int delay, int number, float value,
     const float numberRetuned = resources.tuning.getKeyFractional12TET(number);
 
     pitchRatio = region->getBasePitchVariation(numberRetuned, value);
+
+    // apply stretch tuning if set
+    if (resources.stretch)
+        pitchRatio *= resources.stretch->getRatioForFractionalKey(numberRetuned);
+
     baseVolumedB = region->getBaseVolumedB(number);
     baseGain = region->getBaseGain();
     if (triggerType != TriggerType::CC)

src/sfizz/sfizz.cpp

@@ -58,6 +58,11 @@ float sfz::Sfizz::getTuningFrequency() const
     return synth->getTuningFrequency();
 }
 
+void sfz::Sfizz::loadStretchTuningByRatio(float ratio)
+{
+    return synth->loadStretchTuningByRatio(ratio);
+}
+
 int sfz::Sfizz::getNumRegions() const noexcept
 {
     return synth->getNumRegions();

src/sfizz/sfizz_wrapper.cpp

@@ -67,6 +67,12 @@ float sfizz_get_tuning_frequency(sfizz_synth_t* synth)
     return self->getTuningFrequency();
 }
 
+void sfizz_load_stretch_tuning_by_ratio(sfizz_synth_t* synth, float ratio)
+{
+    auto self = reinterpret_cast<sfz::Synth*>(synth);
+    self->loadStretchTuningByRatio(ratio);
+}
+
 void sfizz_free(sfizz_synth_t* synth)
 {
     delete reinterpret_cast<sfz::Synth*>(synth);