Add method to set up initial filter coeffs

src/sfizz/SfzFilter.cpp

@@ -105,6 +105,29 @@ void Filter::clear()
         dsp->instanceClear();
 }
 
+void Filter::prepare(float cutoff, float q, float pksh)
+{
+    sfzFilterDsp *dsp = P->getDsp(P->fChannels, P->fType);
+
+    if (!dsp)
+        return;
+
+    // compute a dummy 1-frame cycle with smoothing off
+
+    float buffer[Impl::maxChannels] = {0};
+    float *inout[Impl::maxChannels];
+    bool en = dsp->isSmoothingEnabled();
+
+    for (unsigned i = 0; i < Impl::maxChannels; ++i)
+        inout[i] = &buffer[i];
+
+    dsp->instanceClear();
+    dsp->configureStandard(cutoff, q, pksh);
+    dsp->setSmoothingEnabled(false);
+    dsp->compute(1, inout, inout);
+    dsp->setSmoothingEnabled(en);
+}
+
 void Filter::process(const float *const in[], float *const out[], float cutoff, float q, float pksh, unsigned nframes)
 {
     unsigned channels = P->fChannels;
@@ -274,6 +297,29 @@ void FilterEq::clear()
         dsp->instanceClear();
 }
 
+void FilterEq::prepare(float cutoff, float bw, float pksh)
+{
+    sfzFilterDsp *dsp = P->getDsp(P->fChannels);
+
+    if (!dsp)
+        return;
+
+    // compute a dummy 1-frame cycle with smoothing off
+
+    float buffer[Impl::maxChannels] = {0};
+    float *inout[Impl::maxChannels];
+    bool en = dsp->isSmoothingEnabled();
+
+    for (unsigned i = 0; i < Impl::maxChannels; ++i)
+        inout[i] = &buffer[i];
+
+    dsp->instanceClear();
+    dsp->configureEq(cutoff, bw, pksh);
+    dsp->setSmoothingEnabled(false);
+    dsp->compute(1, inout, inout);
+    dsp->setSmoothingEnabled(en);
+}
+
 void FilterEq::process(const float *const in[], float *const out[], float cutoff, float bw, float pksh, unsigned nframes)
 {
     unsigned channels = P->fChannels;

src/sfizz/SfzFilter.h

@@ -36,6 +36,14 @@ public:
      */
     void clear();
 
+    /**
+       Clear the filter memory, and compute the initial coefficients unaffected
+       by any smoothing.
+
+       Make sure to set the filter type and channel count first.
+     */
+    void prepare(float cutoff, float q, float pksh);
+
     /**
        Process one cycle of the filter without modulating cutoff or Q.
        `cutoff` is a frequency expressed in Hz.
@@ -131,6 +139,14 @@ public:
      */
     void clear();
 
+    /**
+       Clear the filter memory, and compute the initial coefficients unaffected
+       by any smoothing.
+
+       Make sure to set the channel count first.
+     */
+    void prepare(float cutoff, float bw, float pksh);
+
     /**
        Process one cycle of the filter without modulating cutoff or bandwidth.
        `cutoff` is a frequency expressed in Hz.

src/sfizz/SfzFilterImpls.cxx

@@ -20,6 +20,19 @@ public:
 
     virtual void configureStandard(float, float, float) {}
     virtual void configureEq(float, float, float) {}
+
+    bool isSmoothingEnabled() const
+    {
+        return fSmoothEnable;
+    }
+
+    void setSmoothingEnabled(bool smooth)
+    {
+        fSmoothEnable = smooth;
+    }
+
+protected:
+    bool fSmoothEnable = true; // external variable used from faust code
 };
 
 #if defined(__GNUC__) || defined(__clang__)

src/sfizz/dsp/filters/sfz_filters.dsp

@@ -146,4 +146,8 @@ bandwidth = vslider("[04] Bandwidth [unit:octave]", 1.0, 0.1, 10.0, 0.01);
 // smoothCoefs = _ ; // No smoothing applied at all
 
 // This applies a quicker smoothing but which may render the filter unstable
-smoothCoefs = si.smooth(ba.tau2pole(0.001)) ; // time constant = 1ms
+smoothCoefs = si.smooth(ba.if(smoothEnable, pole, 0.0)) with {
+  timeConstant = 1e-3; // time constant = 1ms
+  pole = ba.tau2pole(timeConstant);
+  smoothEnable = fvariable(int fSmoothEnable, <math.h>);
+};