annotate PeakInterpolator.cpp @ 43:53260757fc9f

Switch from cubic to much simpler & faster parabolic interpolator

/* -*- c-basic-offset: 4 indent-tabs-mode: nil -*-  vi:set ts=8 sts=4 sw=4: */
/*
    This file is Copyright (c) 2012 Chris Cannam
  
    Permission is hereby granted, free of charge, to any person
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    The above copyright notice and this permission notice shall be
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    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
    EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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    NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR
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*/

#include "PeakInterpolator.h"

#include <iostream>

static double cubicInterpolate(const double y[4], double x)
{
    double a0 = y[3] - y[2] - y[0] + y[1];
    double a1 = y[0] - y[1] - a0;
    double a2 = y[2] - y[0];
    double a3 = y[1];
    return
        a0 * x * x * x +
        a1 * x * x +
        a2 * x +
        a3;
}

double
PeakInterpolator::findPeakLocation(const double *data, int size)
{
    double maxval;
    int maxidx = 0;
    int i;
    for (i = 0; i < size; ++i) {
        if (i == 0 || data[i] > maxval) {
            maxval = data[i];
            maxidx = i;
        }
    }
    return findPeakLocation(data, size, maxidx);
}

double
PeakInterpolator::findPeakLocation(const double *data, int size, int peakIndex)
{
    // after jos, 
    // https://ccrma.stanford.edu/~jos/sasp/Quadratic_Interpolation_Spectral_Peaks.html

    if (peakIndex < 1 || peakIndex > size - 2) {
        return peakIndex;
    }

    double alpha = data[peakIndex-1];
    double beta  = data[peakIndex];
    double gamma = data[peakIndex+1];

    double denom = (alpha - 2*beta + gamma);

    if (denom == 0) {
        // flat
        return peakIndex;
    }

    double p = ((alpha - gamma) / denom) / 2.0;

    return double(peakIndex) + p;
}