annotate src/SubsequenceMatchVampPlugin.cpp @ 246:aac9ad4064ea subsequence tip

Fix incorrect handling of silent tail in the non-subsequence MATCH phase; some debug output changes

/* -*- c-basic-offset: 4 indent-tabs-mode: nil -*-  vi:set ts=8 sts=4 sw=4: */

/*
    Vamp feature extraction plugin using the MATCH audio alignment
    algorithm.

    Centre for Digital Music, Queen Mary, University of London.
    Copyright (c) 2007-2020 Simon Dixon, Chris Cannam, and Queen Mary
    University of London, Copyright (c) 2014-2015 Tido GmbH.
    
    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License as
    published by the Free Software Foundation; either version 2 of the
    License, or (at your option) any later version.  See the file
    COPYING included with this distribution for more information.
*/

#include "SubsequenceMatchVampPlugin.h"
#include "FullDTW.h"
#include "MatchPipeline.h"

#include <vamp/vamp.h>
#include <vamp-sdk/RealTime.h>

#include <vector>
#include <algorithm>

using std::string;
using std::vector;
using std::cerr;
using std::cout;
using std::endl;

//#define DEBUG_SUBSEQUENCE_MATCH 1

// We want to ensure our freq map / crossover bin are always valid
// with a fixed FFT length in seconds, so must reject low sample rates
static float sampleRateMin = 5000.f;

static float defaultStepTime = 0.020f;
static int defaultCoarseDownsample = 50;
static double defaultAnchoredDiagonalWeight = 2.0;
static double defaultSubsequenceDiagonalWeight = 0.75;

SubsequenceMatchVampPlugin::SubsequenceMatchVampPlugin(float inputSampleRate) :
    Plugin(inputSampleRate),
    m_stepSize(int(inputSampleRate * defaultStepTime + 0.001)),
    m_stepTime(defaultStepTime),
    m_blockSize(2048),
    m_coarseDownsample(defaultCoarseDownsample),
    m_downsamplePeaks(false),
    m_serialise(false),
    m_smooth(false),
    m_channelCount(0),
    m_params(defaultStepTime),
    m_defaultParams(defaultStepTime),
    m_feParams(inputSampleRate),
    m_defaultFeParams(44100), // parameter descriptors can't depend on samplerate
    m_secondReferenceFrequency(m_defaultFeParams.referenceFrequency), // must be declared/initialised after m_defaultFeParams
    m_fcParams(),
    m_defaultFcParams(),
    m_dParams(),
    m_defaultDParams(),
    m_fdParams(defaultStepTime),
    m_defaultFdParams(defaultStepTime)
{
    // for the coarse subsequence span aligner:
    m_fdParams.diagonalWeight = m_defaultFdParams.diagonalWeight =
        defaultSubsequenceDiagonalWeight;

    // for the MATCH phase following subsequence span identification:
    m_params.diagonalWeight = m_defaultParams.diagonalWeight =
        defaultAnchoredDiagonalWeight;

    // and of course
    m_fdParams.subsequence = m_defaultFdParams.subsequence = true;

    if (inputSampleRate < sampleRateMin) {
        cerr << "SubsequenceMatchVampPlugin::SubsequenceMatchVampPlugin: input sample rate "
             << inputSampleRate << " < min supported rate "
             << sampleRateMin << ", plugin will refuse to initialise" << endl;
    }
}

SubsequenceMatchVampPlugin::~SubsequenceMatchVampPlugin()
{
}

string
SubsequenceMatchVampPlugin::getIdentifier() const
{
    return "match-subsequence";
}

string
SubsequenceMatchVampPlugin::getName() const
{
    return "Match Subsequence Aligner";
}

string
SubsequenceMatchVampPlugin::getDescription() const
{
    return "Calculate alignment between a reference performance and a performance known to represent only part of the same material";
}

string
SubsequenceMatchVampPlugin::getMaker() const
{
    return "Simon Dixon (plugin by Chris Cannam)";
}

int
SubsequenceMatchVampPlugin::getPluginVersion() const
{
    return 3;
}

string
SubsequenceMatchVampPlugin::getCopyright() const
{
    return "GPL";
}

SubsequenceMatchVampPlugin::ParameterList
SubsequenceMatchVampPlugin::getParameterDescriptors() const
{
    ParameterList list;

    ParameterDescriptor desc;

    desc.identifier = "freq1";
    desc.name = "Tuning frequency of first input";
    desc.description = "Tuning frequency (concert A) for the reference audio";
    desc.minValue = 220.0;
    desc.maxValue = 880.0;
    desc.defaultValue = float(m_defaultFeParams.referenceFrequency);
    desc.isQuantized = false;
    desc.unit = "Hz";
    list.push_back(desc);

    desc.identifier = "freq2";
    desc.name = "Tuning frequency of second input";
    desc.description = "Tuning frequency (concert A) for the other audio.";
    desc.minValue = 220.0;
    desc.maxValue = 880.0;
    desc.defaultValue = float(m_defaultFeParams.referenceFrequency);
    desc.isQuantized = false;
    desc.unit = "Hz";
    list.push_back(desc);

    desc.identifier = "minfreq";
    desc.name = "Minimum frequency";
    desc.description = "Minimum frequency to include in features";
    desc.minValue = 0.0;
    desc.maxValue = float(m_inputSampleRate / 4.f);
    desc.defaultValue = float(m_defaultFeParams.minFrequency);
    desc.isQuantized = false;
    desc.unit = "Hz";
    list.push_back(desc);

    desc.identifier = "maxfreq";
    desc.name = "Maximum frequency";
    desc.description = "Maximum frequency to include in features";
    desc.minValue = 1000.0;
    desc.maxValue = float(m_inputSampleRate / 2.f);
    desc.defaultValue = float(m_defaultFeParams.maxFrequency);
    desc.isQuantized = false;
    desc.unit = "Hz";
    list.push_back(desc);
    
    desc.unit = "";

    desc.identifier = "coarsedownsample";
    desc.name = "Coarse alignment downsample factor";
    desc.description = "Downsample factor for features used in first coarse subsequence-alignment step";
    desc.minValue = 1;
    desc.maxValue = 200;
    desc.defaultValue = float(defaultCoarseDownsample);
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    list.push_back(desc);
    
    desc.identifier = "downsamplemethod";
    desc.name = "Coarse alignment downsample method";
    desc.description = "Downsample method for features used in first coarse subsequence-alignment step";
    desc.minValue = 0;
    desc.maxValue = 1;
    desc.defaultValue = 0;
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    desc.valueNames.clear();
    desc.valueNames.push_back("Average");
    desc.valueNames.push_back("Peak");
    list.push_back(desc);
    
    desc.identifier = "usechroma";
    desc.name = "Feature type";
    desc.description = "Whether to use warped spectrogram or chroma frequency map";
    desc.minValue = 0;
    desc.maxValue = 1;
    desc.defaultValue = m_defaultFeParams.useChromaFrequencyMap ? 1 : 0;
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    desc.valueNames.clear();
    desc.valueNames.push_back("Spectral");
    desc.valueNames.push_back("Chroma");
    list.push_back(desc);

    desc.valueNames.clear();

    desc.identifier = "usespecdiff";
    desc.name = "Use feature difference";
    desc.description = "Whether to use half-wave rectified feature-to-feature difference instead of straight spectral or chroma feature (does not apply to downsampled features)";
    desc.minValue = 0;
    desc.maxValue = 1;
    desc.defaultValue = float(m_defaultFcParams.order);
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    list.push_back(desc);

    desc.identifier = "framenorm";
    desc.name = "Frame normalisation";
    desc.description = "Type of normalisation to use for features";
    desc.minValue = 0;
    desc.maxValue = 2;
    desc.defaultValue = float(m_defaultFcParams.norm);
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    desc.valueNames.clear();
    desc.valueNames.push_back("None");
    desc.valueNames.push_back("Sum to 1");
    desc.valueNames.push_back("Long-term average");
    list.push_back(desc);
    desc.valueNames.clear();
    desc.defaultValue = float(m_defaultFcParams.silenceThreshold);

    desc.identifier = "metric";
    desc.name = "Distance metric";
    desc.description = "Metric for distance calculations";
    desc.minValue = 0;
    desc.maxValue = 2;
    desc.defaultValue = float(m_defaultDParams.metric);
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    desc.valueNames.clear();
    desc.valueNames.push_back("Manhattan");
    desc.valueNames.push_back("Euclidean");
    desc.valueNames.push_back("Cosine");
    list.push_back(desc);
    desc.valueNames.clear();

    desc.identifier = "distnorm";
    desc.name = "Distance normalisation";
    desc.description = "Type of normalisation to use for distance metric";
    desc.minValue = 0;
    desc.maxValue = 2;
    desc.defaultValue = float(m_defaultDParams.norm);
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    desc.valueNames.clear();
    desc.valueNames.push_back("None");
    desc.valueNames.push_back("Sum of frames");
    desc.valueNames.push_back("Log sum of frames");
    list.push_back(desc);
    desc.valueNames.clear();

#ifdef USE_COMPACT_TYPES
    desc.identifier = "scale";
    desc.name = "Distance scale";
    desc.description = "Scale factor to use when mapping distance metric into byte range for storage";
    desc.minValue = 1;
    desc.maxValue = 1000;
    desc.defaultValue = float(m_defaultDParams.scale);
    desc.isQuantized = false;
    list.push_back(desc);
#endif
    
    desc.identifier = "silencethreshold";
    desc.name = "Silence threshold";
    desc.description = "Total frame energy threshold below which a feature will be regarded as silent";
    desc.minValue = 0;
    desc.maxValue = 0.1f;
    desc.defaultValue = float(m_defaultFcParams.silenceThreshold);
    desc.isQuantized = false;
    list.push_back(desc);

    desc.identifier = "noise";
    desc.name = "Add noise";
    desc.description = "Whether to mix in a small constant white noise term when calculating feature distance. This can improve alignment against sources containing cleanly synthesised audio";
    desc.minValue = 0;
    desc.maxValue = 1;
    desc.defaultValue = float(m_defaultDParams.noise);
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    list.push_back(desc);
    
    desc.identifier = "gradientlimit";
    desc.name = "Gradient limit";
    desc.description = "Limit of number of frames that will be accepted from one source without a frame from the other source being accepted";
    desc.minValue = 1;
    desc.maxValue = 10;
    desc.defaultValue = float(m_defaultParams.maxRunCount);
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    list.push_back(desc);

    desc.identifier = "zonewidth";
    desc.name = "Search zone width";
    desc.description = "Width of the search zone (error margin) either side of the ongoing match position, in seconds";
    desc.minValue = 1;
    desc.maxValue = 60;
    desc.defaultValue = float(m_defaultParams.blockTime);
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    desc.unit = "s";
    list.push_back(desc);

    desc.identifier = "diagonalweight";
    desc.name = "Diagonal weight, anchored";
    desc.description = "Weight applied to cost of diagonal step relative to horizontal or vertical step, during the anchored (non-subsequence) alignment step";
    desc.minValue = 0.5;
    desc.maxValue = 2.0;
    desc.defaultValue = float(m_defaultParams.diagonalWeight);
    desc.isQuantized = false;
    desc.unit = "";
    list.push_back(desc);

    desc.identifier = "diagonalweightsubsequence";
    desc.name = "Diagonal weight, subsequence";
    desc.description = "Weight applied to cost of diagonal step relative to horizontal or vertical step, during the coarse subsequence alignment step";
    desc.minValue = 0.5;
    desc.maxValue = 2.0;
    desc.defaultValue = float(m_defaultFdParams.diagonalWeight);
    desc.isQuantized = false;
    desc.unit = "";
    list.push_back(desc);
    
    desc.identifier = "smooth";
    desc.name = "Use path smoothing";
    desc.description = "Smooth the path by replacing steps with diagonals. (This was enabled by default in earlier versions of the MATCH plugin, but the default now is to produce an un-smoothed path.)";
    desc.minValue = 0;
    desc.maxValue = 1;
    desc.defaultValue = 0;
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    desc.unit = "";
    list.push_back(desc);

    desc.identifier = "serialise";
    desc.name = "Serialise plugin invocations";
    desc.description = "Reduce potential memory load at the expense of multiprocessor performance by serialising multi-threaded plugin runs";
    desc.minValue = 0;
    desc.maxValue = 1;
    desc.defaultValue = 0;
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    list.push_back(desc);
    
    return list;
}

float
SubsequenceMatchVampPlugin::getParameter(std::string name) const
{
    if (name == "serialise") {
        return m_serialise ? 1.0 : 0.0; 
    } else if (name == "framenorm") {
        return float(m_fcParams.norm);
    } else if (name == "distnorm") {
        return float(m_dParams.norm);
    } else if (name == "usespecdiff") {
        return float(m_fcParams.order);
    } else if (name == "usechroma") {
        return m_feParams.useChromaFrequencyMap ? 1.0 : 0.0;
    } else if (name == "gradientlimit") {
        return float(m_params.maxRunCount);
    } else if (name == "diagonalweight") {
        return float(m_params.diagonalWeight);
    } else if (name == "diagonalweightsubsequence") {
        return float(m_fdParams.diagonalWeight);
    } else if (name == "zonewidth") {
        return float(m_params.blockTime);
    } else if (name == "smooth") {
        return m_smooth ? 1.0 : 0.0;
    } else if (name == "silencethreshold") {
        return float(m_fcParams.silenceThreshold);
    } else if (name == "metric") {
        return float(m_dParams.metric);
    } else if (name == "noise") {
        return m_dParams.noise;
    } else if (name == "scale") {
        return float(m_dParams.scale);
    } else if (name == "freq1") {
        return float(m_feParams.referenceFrequency);
    } else if (name == "freq2") {
        return float(m_secondReferenceFrequency);
    } else if (name == "minfreq") {
        return float(m_feParams.minFrequency);
    } else if (name == "maxfreq") {
        return float(m_feParams.maxFrequency);
    } else if (name == "coarsedownsample") {
        return float(m_coarseDownsample);
    } else if (name == "downsamplemethod") {
        return m_downsamplePeaks ? 1.0 : 0.0;
    }
    
    return 0.0;
}

void
SubsequenceMatchVampPlugin::setParameter(std::string name, float value)
{
    if (name == "serialise") {
        m_serialise = (value > 0.5);
    } else if (name == "framenorm") {
        m_fcParams.norm = FeatureConditioner::Normalisation(int(value + 0.1));
    } else if (name == "distnorm") {
        m_dParams.norm = DistanceMetric::DistanceNormalisation(int(value + 0.1));
    } else if (name == "usespecdiff") {
        m_fcParams.order = FeatureConditioner::OutputOrder(int(value + 0.1));
    } else if (name == "usechroma") {
        m_feParams.useChromaFrequencyMap = (value > 0.5);
    } else if (name == "gradientlimit") {
        m_params.maxRunCount = int(value + 0.1);
    } else if (name == "diagonalweight") {
        m_params.diagonalWeight = value;
    } else if (name == "diagonalweightsubsequence") {
        m_fdParams.diagonalWeight = value;
    } else if (name == "zonewidth") {
        m_params.blockTime = value;
    } else if (name == "smooth") {
        m_smooth = (value > 0.5);
    } else if (name == "silencethreshold") {
        m_fcParams.silenceThreshold = value;
    } else if (name == "metric") {
        m_dParams.metric = DistanceMetric::Metric(int(value + 0.1));
    } else if (name == "noise") {
        m_dParams.noise = DistanceMetric::NoiseAddition(int(value + 0.1));
    } else if (name == "scale") {
        m_dParams.scale = value;
    } else if (name == "freq1") {
        m_feParams.referenceFrequency = value;
    } else if (name == "freq2") {
        m_secondReferenceFrequency = value;
    } else if (name == "minfreq") {
        m_feParams.minFrequency = value;
    } else if (name == "maxfreq") {
        m_feParams.maxFrequency = value;
    } else if (name == "coarsedownsample") {
        m_coarseDownsample = int(value + 0.1);
    } else if (name == "downsamplemethod") {
        m_downsamplePeaks = (value > 0.5);
    }
}

SubsequenceMatchVampPlugin::InputDomain
SubsequenceMatchVampPlugin::getInputDomain() const
{
    return FrequencyDomain;
}

size_t
SubsequenceMatchVampPlugin::getPreferredStepSize() const
{
    return int(m_inputSampleRate * defaultStepTime + 0.001);
}

size_t
SubsequenceMatchVampPlugin::getPreferredBlockSize() const
{
    return m_defaultFeParams.fftSize;
}

size_t
SubsequenceMatchVampPlugin::getMinChannelCount() const
{
    return 2;
}

size_t
SubsequenceMatchVampPlugin::getMaxChannelCount() const
{
    return 2;
}

bool
SubsequenceMatchVampPlugin::initialise(size_t channels, size_t stepSize, size_t blockSize)
{
    if (m_inputSampleRate < sampleRateMin) {
        cerr << "SubsequenceMatchVampPlugin::SubsequenceMatchVampPlugin: input sample rate "
             << m_inputSampleRate << " < min supported rate "
             << sampleRateMin << endl;
        return false;
    }
    if (channels < getMinChannelCount() ||
	channels > getMaxChannelCount()) {
        return false;
    }
    if (stepSize > blockSize/2 ||
        blockSize != getPreferredBlockSize()) {
        return false;
    }

    m_stepSize = int(stepSize);
    m_stepTime = float(stepSize) / m_inputSampleRate;
    m_blockSize = int(blockSize);

    m_params.hopTime = m_stepTime;
    m_feParams.fftSize = m_blockSize;

    m_channelCount = channels;

    reset();
        
    return true;
}

void
SubsequenceMatchVampPlugin::reset()
{
    m_featureExtractors.clear();
    m_features.clear();
    m_startTime = Vamp::RealTime::zeroTime;

    FeatureExtractor::Parameters feParams(m_feParams);
    
    for (size_t c = 0; c < m_channelCount; ++c) {
        if (c > 0 && m_secondReferenceFrequency != 0.0) {
            feParams.referenceFrequency = m_secondReferenceFrequency;
        }
        m_featureExtractors.push_back(FeatureExtractor(feParams));
        m_features.push_back(featureseq_t());
    }
}

SubsequenceMatchVampPlugin::OutputList
SubsequenceMatchVampPlugin::getOutputDescriptors() const
{
    OutputList list;

    float outRate = 1.0f / m_stepTime;

    OutputDescriptor desc;
    desc.identifier = "path";
    desc.name = "Path";
    desc.description = "Alignment path";
    desc.unit = "";
    desc.hasFixedBinCount = true;
    desc.binCount = 1;
    desc.hasKnownExtents = false;
    desc.isQuantized = true;
    desc.quantizeStep = 1;
    desc.sampleType = OutputDescriptor::VariableSampleRate;
    desc.sampleRate = outRate;
    m_pathOutNo = int(list.size());
    list.push_back(desc);

    desc.identifier = "b_a";
    desc.name = "B-A Timeline";
    desc.description = "Timing in performance A corresponding to moments in performance B";
    desc.unit = "sec";
    desc.hasFixedBinCount = true;
    desc.binCount = 1;
    desc.hasKnownExtents = false;
    desc.isQuantized = false;
    desc.sampleType = OutputDescriptor::VariableSampleRate;
    desc.sampleRate = outRate;
    m_baOutNo = int(list.size());
    list.push_back(desc);

    desc.identifier = "span";
    desc.name = "Subsequence Span";
    desc.description = "Region in performance A corresponding to the whole of performance B";
    desc.unit = "";
    desc.hasFixedBinCount = true;
    desc.binCount = 0;
    desc.hasKnownExtents = false;
    desc.isQuantized = false;
    desc.sampleType = OutputDescriptor::VariableSampleRate;
    desc.sampleRate = outRate;
    desc.hasDuration = true;
    m_spanOutNo = int(list.size());
    list.push_back(desc);
    
    return list;
}

SubsequenceMatchVampPlugin::FeatureSet
SubsequenceMatchVampPlugin::process(const float *const *inputBuffers,
                                    Vamp::RealTime timestamp)
{
    if (m_featureExtractors.empty()) {
        cerr << "SubsequenceMatchVampPlugin::process: Plugin has not been (properly?) initialised" << endl;
        return {};
    }

    if (m_features[0].empty()) {
        m_startTime = timestamp;
    }
    
    for (size_t c = 0; c < m_featureExtractors.size(); ++c) {
        m_features[c].push_back(m_featureExtractors[c].process
                                (inputBuffers[c]));
    }        

    return {};
}

size_t
SubsequenceMatchVampPlugin::findNonEmptyLength(const featureseq_t &ff)
{
    bool haveNonEmpty = false;
    size_t lastNonEmpty = 0;
    for (size_t i = ff.size(); i > 0; ) {
        --i;
        if (MatchPipeline::isAboveEndingThreshold(ff[i])) {
            haveNonEmpty = true;
            lastNonEmpty = i;
            break;
        }
    }
    if (haveNonEmpty) {
        return lastNonEmpty + 1;
    } else {
        return 0;
    }
}

featureseq_t
SubsequenceMatchVampPlugin::downsample(const featureseq_t &ff,
                                       size_t inLength)
{
    if (ff.empty()) {
        return ff;
    }

    FeatureConditioner::Parameters fcParams(m_fcParams);
    fcParams.order = FeatureConditioner::OutputFeatures; // not the difference
    FeatureConditioner fc(fcParams);
    
    int featureSize = m_featureExtractors[0].getFeatureSize();
    
    featureseq_t d;

    size_t i = 0;
    while (i < inLength) {
        feature_t acc(featureSize, 0);
        int j = 0;
        while (j < m_coarseDownsample) {
            if (i >= ff.size()) break;
            feature_t feature = fc.process(ff[i]);
            if (m_downsamplePeaks) {
                for (int k = 0; k < featureSize; ++k) {
                    if (feature[k] > acc[k]) {
                        acc[k] = feature[k];
                    }
                }
            } else {
                for (int k = 0; k < featureSize; ++k) {
                    acc[k] += feature[k];
                }
            }
            ++i;
            ++j;
        }
        if (!m_downsamplePeaks && j > 0) {
            for (int k = 0; k < featureSize; ++k) {
                acc[k] /= float(j);
            }
        }
        d.push_back(acc);
    }

    return d;
}

SubsequenceMatchVampPlugin::FeatureSet
SubsequenceMatchVampPlugin::getRemainingFeatures()
{
    if (m_featureExtractors.empty()) {
        cerr << "SubsequenceMatchVampPlugin::getRemainingFeatures: Plugin has not been (properly?) initialised" << endl;
        return {};
    }
    
#ifdef _WIN32
    static HANDLE mutex;
#else
    static pthread_mutex_t mutex;
#endif
    static bool mutexInitialised = false;

    if (m_serialise) {
        if (!mutexInitialised) {
#ifdef _WIN32
            mutex = CreateMutex(NULL, FALSE, NULL);
#else
            pthread_mutex_init(&mutex, 0);
#endif
            mutexInitialised = true;
        }
#ifdef _WIN32
        WaitForSingleObject(mutex, INFINITE);
#else
        pthread_mutex_lock(&mutex);
#endif
    }

    FeatureSet returnFeatures = performAlignment();
    
    if (m_serialise) {
#ifdef _WIN32
        ReleaseMutex(mutex);
#else
        pthread_mutex_unlock(&mutex);
#endif
    }

    return returnFeatures;
}
    
SubsequenceMatchVampPlugin::FeatureSet
SubsequenceMatchVampPlugin::performAlignment()
{
    size_t refLength = findNonEmptyLength(m_features[0]);
    featureseq_t downsampledRef = downsample(m_features[0], refLength);

#ifdef DEBUG_SUBSEQUENCE_MATCH
    cerr << "SubsequenceMatchVampPlugin: reference downsampled sequence length = " << downsampledRef.size() << " (from " << refLength << " non-empty of " << m_features[0].size() << " total)" << endl;
#endif
    
    FullDTW dtw(m_fdParams, m_dParams);
    
    FeatureSet returnFeatures;
    int featureSize = m_featureExtractors[0].getFeatureSize();

    int rate = int(m_inputSampleRate + 0.5);
    
    for (size_t c = 1; c < m_channelCount; ++c) {

        size_t otherLength = findNonEmptyLength(m_features[c]);
        featureseq_t downsampledOther = downsample(m_features[c], otherLength);

#ifdef DEBUG_SUBSEQUENCE_MATCH
        cerr << "SubsequenceMatchVampPlugin: other downsampled sequence length = " << downsampledOther.size() << " (from " << otherLength << " non-empty of " << m_features[c].size() << " total)" << endl;
#endif

        vector<size_t> subsequenceAlignment = dtw.align(downsampledRef,
                                                        downsampledOther);

        if (subsequenceAlignment.empty()) {
            cerr << "No subsequenceAlignment??" << endl;
            continue;
        }
        
        int64_t first = subsequenceAlignment[0];
        int64_t last = subsequenceAlignment[subsequenceAlignment.size()-1];

#ifdef DEBUG_SUBSEQUENCE_MATCH
        cerr << "Subsequence alignment maps 0 -> " << subsequenceAlignment.size()-1 << " to " << first << " -> " << last << endl;
#endif

        if (last <= first) {
            cerr << "NOTE: Invalid span (" << first << " to " << last
                 << "), reverting to aligning against whole of reference"
                 << endl;
            first = 0;
            last = downsampledRef.size() - 1;
        } else if (first < 0 || last >= long(downsampledRef.size())) {
            cerr << "NOTE: Span end points (" << first << " to "
                 << last << ") out of range (0 to " << downsampledRef.size()-1
                 << "), reverting to aligning against whole of reference"
                 << endl;
            first = 0;
            last = downsampledRef.size() - 1;
        }
            
        Feature span;
        span.hasTimestamp = true;
        span.timestamp = Vamp::RealTime::frame2RealTime
            (first * m_coarseDownsample * m_stepSize, rate);
        span.hasDuration = true;
        span.duration = Vamp::RealTime::frame2RealTime
            ((last - first) * m_coarseDownsample * m_stepSize, rate);
        returnFeatures[m_spanOutNo].push_back(span);

        size_t firstAtOriginalRate = first * m_coarseDownsample;
        size_t lastAtOriginalRate = (last + 1) * m_coarseDownsample;

        if (lastAtOriginalRate >= m_features[0].size()) {
            lastAtOriginalRate = m_features[0].size() - 1;
        }
        
        featureseq_t referenceSubsequence
            (m_features[0].begin() + firstAtOriginalRate,
             m_features[0].begin() + lastAtOriginalRate);

#ifdef DEBUG_SUBSEQUENCE_MATCH
        cerr << "Reference subsequence length = " << referenceSubsequence.size()
             << endl;
        cerr << "Other sequence length = " << otherLength << endl;
#endif

        MatchPipeline pipeline(m_feParams,
                               m_fcParams,
                               m_dParams,
                               m_params,
                               m_secondReferenceFrequency);

        size_t sequenceLength = std::max(referenceSubsequence.size(),
                                         otherLength);
        
#ifdef DEBUG_SUBSEQUENCE_MATCH
        cerr << "MATCH input sequences have length " << sequenceLength << endl;
#endif
    
        for (size_t i = 0; i < sequenceLength; ++i) {
            feature_t f1(featureSize, 0);
            feature_t f2(featureSize, 0);
            if (i < referenceSubsequence.size()) {
                f1 = referenceSubsequence[i];
            }
            if (i < otherLength) {
                f2 = m_features[c][i];
            }
            pipeline.feedFeatures(f1, f2);
        }

        pipeline.finish();
            
        vector<int> pathx;
        vector<int> pathy;
        int len = pipeline.retrievePath(m_smooth, pathx, pathy);

        int prevy = 0;

#ifdef DEBUG_SUBSEQUENCE_MATCH
        cerr << "MATCH path has length " << len;
        if (len > 0) {
            cerr << " and goes from ("
                 << pathx[0] << ", " << pathy[0] << ") to ("
                 << pathx[len-1] << ", " << pathy[len-1] << ")";
            if (len > 2) {
                cerr << " with penultimate point at ("
                     << pathx[len-2] << ", " << pathy[len-2] << ")";
            }
            cerr << endl;
        } else {
            cerr << endl;
        }
#endif
    
        for (int i = 0; i < len; ++i) {

            int x = pathx[i];
            int y = pathy[i] + int(first * m_coarseDownsample);
            
            Vamp::RealTime xt = Vamp::RealTime::frame2RealTime
                (x * m_stepSize, rate) + m_startTime;
            Vamp::RealTime yt = Vamp::RealTime::frame2RealTime
                (y * m_stepSize, rate) + m_startTime;

            Feature feature;
            feature.hasTimestamp = true;
            feature.timestamp = xt;
            feature.values.clear();
            feature.values.push_back(float(yt.sec + double(yt.nsec)/1.0e9));
            returnFeatures[m_pathOutNo].push_back(feature);
        
            if (y != prevy) {
                feature.hasTimestamp = true;
                feature.timestamp = yt;
                feature.values.clear();
                feature.values.push_back(float(xt.sec + xt.msec()/1000.0));
                returnFeatures[m_baOutNo].push_back(feature);
            }

            prevy = y;
        }
    }

#ifdef DEBUG_SUBSEQUENCE_MATCH
    cerr << endl;
#endif
    
    return returnFeatures;
}