[WIP] Add new audio-reactive palettes for smoothed FFT data

[Copilot]

• Update MAX_PALETTES from 3 to 5
• Add paletteBandAvg[NUM_GEQ_CHANNELS] static array alongside existing FFT arrays
• Add EMA update for paletteBandAvg in fillAudioPalettes() before the palette loop
• Add case 3 ("Track Character") — spectral centroid from smoothed bands drives hue (warm=bass, cool=treble)
• Add case 4 ("Spectral Balance") — bass/mid/high energy ratios drive hue with smooth transitions
• Reset paletteBandAvg in onUpdateBegin() alongside other FFT buffer resets
• Run npm test to validate
• Run pio run -e esp32dev hardware build

Original prompt

Problem

The AudioReactive usermod currently contains three colour palettes (defined in getCRGBForBand() at line 2211 of usermods/audioreactive/audio_reactive.cpp) that are updated every frame using raw fftResult[] values. While this makes them very responsive, it also makes them "twitchy" — they react to instantaneous FFT values rather than reflecting the overall character or balance of the music.

Goal

Add new audio-reactive palette(s) that use smoothed/averaged FFT data to reflect the overall tonal balance of the music. The key user experience should be:

• Warm red/orange hues when bass frequencies dominate (e.g., a heavy drop)
• Green/cyan hues during vocal/melodic passages (mid-frequency dominant)
• Blue/purple hues when high-frequency content dominates (cymbals, hi-hats, bright synths)
• Smooth, flowing transitions between colours rather than frame-by-frame twitching

Implementation Details

All changes should be in usermods/audioreactive/audio_reactive.cpp.

1. Add a smoothed band average buffer for palette use

Add a static float paletteBandAvg[NUM_GEQ_CHANNELS] array (alongside the existing fftCalc, fftAvg, fftResult arrays near line 227). This buffer should be updated with an exponential moving average (EMA) in fillAudioPalettes() before the palette colours are computed:

static float paletteBandAvg[NUM_GEQ_CHANNELS] = {0.0f};
static constexpr float PALETTE_SMOOTHING = 0.05f; // ~400ms time constant at 20ms cycle

// In fillAudioPalettes(), before the palette loop:
for (int i = 0; i < NUM_GEQ_CHANNELS; i++) {
  paletteBandAvg[i] += PALETTE_SMOOTHING * ((float)fftResult[i] - paletteBandAvg[i]);
}

2. Add a "Spectral Centroid / Track Character" palette

Add a new case 3: to the switch (pal) in getCRGBForBand(). This palette computes the spectral centroid (centre-of-mass of the frequency spectrum) from the smoothed band averages and maps it to hue:

case 3: {
  // "Track Character" palette - smoothed spectral centroid drives hue
  static const float bandFreq[NUM_GEQ_CHANNELS] = {
    65, 107, 172, 258, 365, 495, 689, 969,
    1270, 1658, 2153, 2713, 3359, 4091, 5792, 8182
  };
  float wSum = 0, tEnergy = 0;
  for (int i = 0; i < NUM_GEQ_CHANNELS; i++) {
    wSum += paletteBandAvg[i] * bandFreq[i];
    tEnergy += paletteBandAvg[i];
  }
  float centroid = (tEnergy > 1.0f) ? (wSum / tEnergy) : 500.0f;
  float logC = log2f(constrain(centroid, 60.0f, 8000.0f));
  // log2(60)≈5.9, log2(8000)≈13.0 → map to hue 0..200
  uint8_t baseHue = (uint8_t)mapf(logC, 5.9f, 13.0f, 0.0f, 200.0f);

  // Spread palette positions around centroid hue
  int8_t hueSpread = map(x, 0, 255, -30, 30);
  uint8_t saturation = constrain((int)(tEnergy / 6.0f) + 180, 180, 255);
  hsv = CHSV(baseHue + hueSpread, saturation, constrain(x, 30, 255));
  value = hsv;
  break;
}

3. Add a "Spectral Balance" palette

Add a new case 4: to the switch (pal) in getCRGBForBand(). This palette computes energy ratios across bass/mid/high regions and uses the weighted balance to derive hue:

case 4: {
  // "Spectral Balance" palette - bass vs mid vs high energy balance
  float bassEnergy = 0, midEnergy = 0, highEnergy = 0;
  for (int i = 0;  i < 4;  i++) bassEnergy += paletteBandAvg[i];
  for (int i = 4;  i < 10; i++) midEnergy  += paletteBandAvg[i];
  for (int i = 10; i < 16; i++) highEnergy += paletteBandAvg[i];

  float total = bassEnergy + midEnergy + highEnergy;
  if (total < 1.0f) total = 1.0f;

  float bassRatio = bassEnergy / total;
  float midRatio  = midEnergy  / total;
  float highRatio = highEnergy / total;

  // Weighted hue: bass→warm(20), mid→green(110), high→cool(190)
  uint8_t hue = (uint8_t)(bassRatio * 20.0f + midRatio * 110.0f + highRatio * 190.0f);

  // More concentrated spectrum = more saturated
  float maxRatio = fmaxf(bassRatio, fmaxf(midRatio, highRatio));
  uint8_t sat = constrain((int)(maxRatio * 255.0f * 1.5f), 180, 255);

  // Spread across palette position
  int8_t hueOffset = map(x, 0, 255, -25, 25);
  uint8_t val = constrain((int)(total / 8.0f) + (int)map(x, 0, 255, 30, 255), 30, 255);
  hsv = CHSV(hue + hueOffset, sat, val);
  value = hsv;
  break;
}

4. Update MAX_PALETTES

Change line 74 from:

#define MAX_PALETTES 3

to:

#define MAX_PALETTES 5

5. Reset smoothing buffer on audio reset

In onUpdateBegin() (around line 1683), add a reset of paletteBandAvg alongside the existing memset(fftCalc, ...) calls:

memset(paletteBandAvg, 0, sizeof(paletteBandAvg));

Key constraints

• All changes are in the single file usermods/audioreactive/audio_reactive.cpp
• Memory impact is minimal: one additional float[16] array = 64 bytes
• CPU impact is negligible: a few multiply-adds per frame
• The existing 3 palettes (cases 0, 1, 2) must remain unchanged for backward compatibility
• The paletteBandAvg array must be declared static at file scope alongside the existing FFT...

This pull request was created from Copilot chat.