Source/platform/audio/ffmpeg/FFTFrameFFMPEG.cpp

/*
 * Copyright (C) 2011 Google Inc. All rights reserved.
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// FFTFrame implementation using FFmpeg's RDFT algorithm,
// suitable for use on Windows and Linux.

#include "config.h"

#if ENABLE(WEB_AUDIO)

#if USE(WEBAUDIO_FFMPEG)

#include "platform/audio/FFTFrame.h"

#include "platform/audio/VectorMath.h"

extern "C" {
    #include <libavcodec/avfft.h>
}

#include "wtf/MathExtras.h"

namespace blink {

#if ENABLE(ASSERT)
const int kMaxFFTPow2Size = 24;
#endif

// Normal constructor: allocates for a given fftSize.
FFTFrame::FFTFrame(unsigned fftSize)
    : m_FFTSize(fftSize)
    , m_log2FFTSize(static_cast<unsigned>(log2(fftSize)))
    , m_realData(fftSize / 2)
    , m_imagData(fftSize / 2)
    , m_forwardContext(nullptr)
    , m_inverseContext(nullptr)
    , m_complexData(fftSize)
{
    // We only allow power of two.
    ASSERT(1UL << m_log2FFTSize == m_FFTSize);

    m_forwardContext = contextForSize(fftSize, DFT_R2C);
    m_inverseContext = contextForSize(fftSize, IDFT_C2R);
}

// Creates a blank/empty frame (interpolate() must later be called).
FFTFrame::FFTFrame()
    : m_FFTSize(0)
    , m_log2FFTSize(0)
    , m_forwardContext(nullptr)
    , m_inverseContext(nullptr)
{
}

// Copy constructor.
FFTFrame::FFTFrame(const FFTFrame& frame)
    : m_FFTSize(frame.m_FFTSize)
    , m_log2FFTSize(frame.m_log2FFTSize)
    , m_realData(frame.m_FFTSize / 2)
    , m_imagData(frame.m_FFTSize / 2)
    , m_forwardContext(nullptr)
    , m_inverseContext(nullptr)
    , m_complexData(frame.m_FFTSize)
{
    m_forwardContext = contextForSize(m_FFTSize, DFT_R2C);
    m_inverseContext = contextForSize(m_FFTSize, IDFT_C2R);

    // Copy/setup frame data.
    unsigned nbytes = sizeof(float) * (m_FFTSize / 2);
    memcpy(realData(), frame.realData(), nbytes);
    memcpy(imagData(), frame.imagData(), nbytes);
}

void FFTFrame::initialize()
{
}

void FFTFrame::cleanup()
{
}

FFTFrame::~FFTFrame()
{
    av_rdft_end(m_forwardContext);
    av_rdft_end(m_inverseContext);
}

void FFTFrame::doFFT(const float* data)
{
    // Copy since processing is in-place.
    float* p = m_complexData.data();
    memcpy(p, data, sizeof(float) * m_FFTSize);

    // Compute Forward transform.
    av_rdft_calc(m_forwardContext, p);

    // De-interleave to separate real and complex arrays.
    int len = m_FFTSize / 2;

    float* real = m_realData.data();
    float* imag = m_imagData.data();
    for (int i = 0; i < len; ++i) {
        int baseComplexIndex = 2 * i;
        // m_realData[0] is the DC component and m_imagData[0] is the nyquist component
        // since the interleaved complex data is packed.
        real[i] = p[baseComplexIndex];
        imag[i] = p[baseComplexIndex + 1];
    }
}

void FFTFrame::doInverseFFT(float* data)
{
    // Prepare interleaved data.
    float* interleavedData = getUpToDateComplexData();

    // Compute inverse transform.
    av_rdft_calc(m_inverseContext, interleavedData);

    // Scale so that a forward then inverse FFT yields exactly the original data. For some reason
    // av_rdft_calc above returns values that are half of what I expect. Hence make the scale factor
    // twice as large to compensate for that.
    const float scale = 2.0 / m_FFTSize;
    VectorMath::vsmul(interleavedData, 1, &scale, data, 1, m_FFTSize);
}

float* FFTFrame::getUpToDateComplexData()
{
    // FIXME: if we can't completely get rid of this method, SSE
    // optimization could be considered if it shows up hot on profiles.
    int len = m_FFTSize / 2;
    const float* real = m_realData.data();
    const float* imag = m_imagData.data();
    float* c = m_complexData.data();
    for (int i = 0; i < len; ++i) {
        int baseComplexIndex = 2 * i;
        c[baseComplexIndex] = real[i];
        c[baseComplexIndex + 1] = imag[i];
    }
    return const_cast<float*>(m_complexData.data());
}

RDFTContext* FFTFrame::contextForSize(unsigned fftSize, int trans)
{
    // FIXME: This is non-optimal. Ideally, we'd like to share the contexts for FFTFrames of the same size.
    // But FFmpeg's RDFT uses a scratch buffer inside the context and so they are not thread-safe.
    // We could improve this by sharing the FFTFrames on a per-thread basis.
    ASSERT(fftSize);
    int pow2size = static_cast<int>(log2(fftSize));
    ASSERT(pow2size < kMaxFFTPow2Size);

    RDFTContext* context = av_rdft_init(pow2size, (RDFTransformType)trans);
    return context;
}

} // namespace blink

#endif // USE(WEBAUDIO_FFMPEG)

#endif // ENABLE(WEB_AUDIO)