Method extractChroma

src/fl/audio/detector/key.cpp.hpp:209–245

//------------------------------------------------------------------------------

void KeyDetector::extractChroma(const fft::Bins& fft, float* chroma) {
    // Initialize chroma to zero
    for (int i = 0; i < 12; i++) {
        chroma[i] = 0.0f;
    }

    // Map fft::FFT bins to pitch classes using linearly-rebinned magnitudes.
    // Linear bins give evenly-spaced frequency mapping: freq = fmin + bin * binWidth.
    fl::span<const float> linearBins = fft.linear();
    const int numBins = static_cast<int>(linearBins.size());
    const float fmin = fft.linearFmin();
    const float fmax = fft.linearFmax();
    const float linearBinWidth = (numBins > 0) ? (fmax - fmin) / static_cast<float>(numBins) : 1.0f;

    // Process each linear bin
    for (int bin = 0; bin < numBins; bin++) {
        float magnitude = linearBins[bin];
        if (magnitude < 1e-6f) continue;

        // Calculate frequency for this linearly-spaced bin
        float freq = fmin + (static_cast<float>(bin) + 0.5f) * linearBinWidth;

        // Skip frequencies below 60 Hz (below C2)
        if (freq < 60.0f) continue;

        // Convert frequency to MIDI note number
        // MIDI note = 69 + 12 * log2(freq / 440)
        float midiNote = 69.0f + 12.0f * (fl::logf(freq / 440.0f) / fl::logf(2.0f));

        // Get pitch class (0-11)
        int pitchClass = static_cast<int>(midiNote + 0.5f) % 12;
        if (pitchClass < 0) pitchClass += 12;

        // Accumulate magnitude into pitch class
        chroma[pitchClass] += magnitude;
    }
}

void KeyDetector::normalizeChroma(float* chroma) {
    // Find maximum value