Simple FFT

A spectrogram built from the live input using juce::dsp::FFT.
dsp::FFT with a windowing function
Filling a FIFO on the audio thread
Drawing into an Image from a Timer
Source: examples/Audio/SimpleFFTDemo.h on GitHub.
SimpleFFTDemo.h
cpp
/*******************************************************************************
 The block below describes the properties of this PIP. A PIP is a short snippet
 of code that can be read by the Projucer and used to generate a JUCE project.

 BEGIN_JUCE_PIP_METADATA

 name:             SimpleFFTDemo
 version:          1.0.0
 vendor:           JUCE
 website:          http://juce.com
 description:      Simple FFT application.

 dependencies:     juce_audio_basics, juce_audio_devices, juce_audio_formats,
                   juce_audio_processors, juce_audio_utils, juce_core,
                   juce_data_structures, juce_dsp, juce_events, juce_graphics,
                   juce_gui_basics, juce_gui_extra, juce_audio_processors_headless
 exporters:        xcode_mac, vs2022, vs2026, linux_make, androidstudio,
                   xcode_iphone

 moduleFlags:      JUCE_STRICT_REFCOUNTEDPOINTER=1

 type:             Component
 mainClass:        SimpleFFTDemo

 useLocalCopy:     1

 END_JUCE_PIP_METADATA

*******************************************************************************/

#pragma once


//==============================================================================
class SimpleFFTDemo final : public AudioAppComponent,
                            private Timer
{
public:
    SimpleFFTDemo() :
         #ifdef JUCE_DEMO_RUNNER
          AudioAppComponent (getSharedAudioDeviceManager (1, 0)),
         #endif
          forwardFFT (fftOrder),
          spectrogramImage (Image::RGB, 512, 512, true)
    {
        setOpaque (true);

       #ifndef JUCE_DEMO_RUNNER
        RuntimePermissions::request (RuntimePermissions::recordAudio,
                                     [this] (bool granted)
                                     {
                                         int numInputChannels = granted ? 2 : 0;
                                         setAudioChannels (numInputChannels, 2);
                                     });
       #else
        setAudioChannels (2, 2);
       #endif

        startTimerHz (60);
        setSize (700, 500);
    }

    ~SimpleFFTDemo() override
    {
        shutdownAudio();
    }

    //==============================================================================
    void prepareToPlay (int /*samplesPerBlockExpected*/, double /*newSampleRate*/) override
    {
        // (nothing to do here)
    }

    void releaseResources() override
    {
        // (nothing to do here)
    }

    void getNextAudioBlock (const AudioSourceChannelInfo& bufferToFill) override
    {
        if (bufferToFill.buffer->getNumChannels() > 0)
        {
            const auto* channelData = bufferToFill.buffer->getReadPointer (0, bufferToFill.startSample);

            for (auto i = 0; i < bufferToFill.numSamples; ++i)
                pushNextSampleIntoFifo (channelData[i]);

            bufferToFill.clearActiveBufferRegion();
        }
    }

    //==============================================================================
    void paint (Graphics& g) override
    {
        g.fillAll (Colours::black);

        g.setOpacity (1.0f);
        g.drawImage (spectrogramImage, getLocalBounds().toFloat());
    }

    void timerCallback() override
    {
        if (nextFFTBlockReady)
        {
            drawNextLineOfSpectrogram();
            nextFFTBlockReady = false;
            repaint();
        }
    }

    void pushNextSampleIntoFifo (float sample) noexcept
    {
        // if the fifo contains enough data, set a flag to say
        // that the next line should now be rendered..
        if (fifoIndex == fftSize)
        {
            if (! nextFFTBlockReady)
            {
                zeromem (fftData, sizeof (fftData));
                memcpy (fftData, fifo, sizeof (fifo));
                nextFFTBlockReady = true;
            }

            fifoIndex = 0;
        }

        fifo[fifoIndex++] = sample;
    }

    void drawNextLineOfSpectrogram()
    {
        auto rightHandEdge = spectrogramImage.getWidth() - 1;
        auto imageHeight   = spectrogramImage.getHeight();

        // first, shuffle our image leftwards by 1 pixel..
        spectrogramImage.moveImageSection (0, 0, 1, 0, rightHandEdge, imageHeight);

        // then render our FFT data..
        forwardFFT.performFrequencyOnlyForwardTransform (fftData);

        // find the range of values produced, so we can scale our rendering to
        // show up the detail clearly
        auto maxLevel = FloatVectorOperations::findMinAndMax (fftData, fftSize / 2);

        Image::BitmapData bitmap { spectrogramImage, rightHandEdge, 0, 1, imageHeight, Image::BitmapData::writeOnly };

        for (auto y = 1; y < imageHeight; ++y)
        {
            auto skewedProportionY = 1.0f - std::exp (std::log ((float) y / (float) imageHeight) * 0.2f);
            auto fftDataIndex = jlimit (0, fftSize / 2, (int) (skewedProportionY * (int) fftSize / 2));
            auto level = jmap (fftData[fftDataIndex], 0.0f, jmax (maxLevel.getEnd(), 1e-5f), 0.0f, 1.0f);

            bitmap.setPixelColour (0, y, Colour::fromHSV (level, 1.0f, level, 1.0f));
        }
    }

    enum
    {
        fftOrder = 10,
        fftSize  = 1 << fftOrder
    };

private:
    dsp::FFT forwardFFT;
    Image spectrogramImage;

    float fifo [fftSize];
    float fftData [2 * fftSize];
    int fifoIndex = 0;
    bool nextFFTBlockReady = false;

    JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (SimpleFFTDemo)
};
Simple FFT ​

Simple FFT
