Fix fader display for Equalizer shelves and peaks

The peak values for the shelves and peaks of the Equalizer plugin are computed in `EqEffect::peakBand`. The previous implementation evaluated the bins of the corresponding frequency spectrum and determined the "loudest" one. The value of this bin was then converted to dbFS and mapped to the interval [0, inf[ where all values less or equal to -60 dbFS were mapped to 0 and a value of 40 dbFS was mapped to 1. So effectively everything was mapped somewhere into [0, 1] yet in a quite "distorted" way because a signal of 40 dbFS resulted in being displayed as unity in the fader.

This commit directly returns the value of the maximum bin, i.e. it does not map first to dbFS and then linearize the result anymore. This should work because the `Fader` class assumes a "linear" input signal and if the value of the bin was previously mapped to dbFS it should have some "linear" character. Please note that this is still somewhat of a "proxy" value because ideally the summed amplitude of all relevant bins in the frequency range would be shown and not just the "loudest" one.

## Other changes
Rename `peakBand` to `linearPeakBand` to make more clear that a linear value is returned.

Handle a potential division by zero by checking the value of `fft->getEnergy()` before using it.

Index into `fft->m_bands` so that no parallel incrementing of the pointer is needed. This also enables the removal of the local variable `b`.

plugins/Eq/EqControls.h

@@ -66,6 +66,8 @@ class EqControls : public EffectControls
 	float m_inPeakR;
 	float m_outPeakL;
 	float m_outPeakR;
+
+	// The following are linear peaks
 	float m_lowShelfPeakL, m_lowShelfPeakR;
 	float m_para1PeakL, m_para1PeakR;
 	float m_para2PeakL, m_para2PeakR;

plugins/Eq/EqEffect.cpp

@@ -287,21 +287,26 @@ bool EqEffect::processAudioBuffer( sampleFrame *buf, const fpp_t frames )
 
 
 
-float EqEffect::peakBand( float minF, float maxF, EqAnalyser *fft, int sr )
+float EqEffect::linearPeakBand(float minF, float maxF, EqAnalyser* fft, int sr)
 {
-	float peak = -60;
-	float *b = fft->m_bands;
-	float h = 0;
-	for( int x = 0; x < MAX_BANDS; x++, b++ )
+	auto const fftEnergy = fft->getEnergy();
+	if (fftEnergy == 0)
 	{
-		if( bandToFreq( x ,sr) >= minF && bandToFreq( x,sr ) <= maxF )
+		// No energy -> no signal
+		return 0;
+	}
+
+	float peakLinear = 0.;
+
+	for (int i = 0; i < MAX_BANDS; ++i)
+	{
+		if (bandToFreq(i, sr) >= minF && bandToFreq(i, sr) <= maxF)
 		{
-			h = 20 * ( log10( *b / fft->getEnergy() ) );
-			peak = h > peak ? h : peak;
+			peakLinear = std::max(peakLinear, fft->m_bands[i] / fftEnergy);
 		}
 	}
 
-	return ( peak + 60 ) / 100;
+	return peakLinear;
 }
 
 
@@ -310,41 +315,41 @@ float EqEffect::peakBand( float minF, float maxF, EqAnalyser *fft, int sr )
 void EqEffect::setBandPeaks( EqAnalyser *fft, int samplerate )
 {
 	m_eqControls.m_lowShelfPeakR = m_eqControls.m_lowShelfPeakL =
-			peakBand( m_eqControls.m_lowShelfFreqModel.value()
+			linearPeakBand( m_eqControls.m_lowShelfFreqModel.value()
 					  * ( 1 - m_eqControls.m_lowShelfResModel.value() * 0.5 ),
 					  m_eqControls.m_lowShelfFreqModel.value(),
 					  fft , samplerate );
 
 	m_eqControls.m_para1PeakL = m_eqControls.m_para1PeakR =
-			peakBand( m_eqControls.m_para1FreqModel.value()
+			linearPeakBand( m_eqControls.m_para1FreqModel.value()
 					  * ( 1 - m_eqControls.m_para1BwModel.value() * 0.5 ),
 					  m_eqControls.m_para1FreqModel.value()
 					  * ( 1 + m_eqControls.m_para1BwModel.value() * 0.5 ),
 					  fft , samplerate );
 
 	m_eqControls.m_para2PeakL = m_eqControls.m_para2PeakR =
-			peakBand( m_eqControls.m_para2FreqModel.value()
+			linearPeakBand( m_eqControls.m_para2FreqModel.value()
 					  * ( 1 - m_eqControls.m_para2BwModel.value() * 0.5 ),
 					  m_eqControls.m_para2FreqModel.value()
 					  * ( 1 + m_eqControls.m_para2BwModel.value() * 0.5 ),
 					  fft , samplerate );
 
 	m_eqControls.m_para3PeakL = m_eqControls.m_para3PeakR =
-			peakBand( m_eqControls.m_para3FreqModel.value()
+			linearPeakBand( m_eqControls.m_para3FreqModel.value()
 					  * ( 1 - m_eqControls.m_para3BwModel.value() * 0.5 ),
 					  m_eqControls.m_para3FreqModel.value()
 					  * ( 1 + m_eqControls.m_para3BwModel.value() * 0.5 ),
 					  fft , samplerate );
 
 	m_eqControls.m_para4PeakL = m_eqControls.m_para4PeakR =
-			peakBand( m_eqControls.m_para4FreqModel.value()
+			linearPeakBand( m_eqControls.m_para4FreqModel.value()
 					  * ( 1 - m_eqControls.m_para4BwModel.value() * 0.5 ),
 					  m_eqControls.m_para4FreqModel.value()
 					  * ( 1 + m_eqControls.m_para4BwModel.value() * 0.5 ),
 					  fft , samplerate );
 
 	m_eqControls.m_highShelfPeakL = m_eqControls.m_highShelfPeakR =
-			peakBand( m_eqControls.m_highShelfFreqModel.value(),
+			linearPeakBand( m_eqControls.m_highShelfFreqModel.value(),
 					  m_eqControls.m_highShelfFreqModel.value()
 					  * ( 1 + m_eqControls.m_highShelfResModel.value() * 0.5 ),
 					  fft, samplerate );

plugins/Eq/EqEffect.h

@@ -87,7 +87,7 @@ class EqEffect : public Effect
 	float m_inGain;
 	float m_outGain;
 
-	float peakBand( float minF, float maxF, EqAnalyser *, int );
+	float linearPeakBand(float minF, float maxF, EqAnalyser*, int);
 
 	inline float bandToFreq ( int index , int sampleRate )
 	{