Flutter Soloud Filters

flutter_soloud exposes the SoLoud engine's filter graph as typed Dart objects. Filters live at three scopes — global output (SoLoud.instance.filters), a single loaded sound (audioSource.filters), and a mixing bus (bus.filters). Every filter is activated with activate() and tuned through FilterParam objects (param.value, fadeFilterParameter, oscillateFilterParameter). There is no "node graph" to wire like the Web Audio API and no per-player effect config like just_audio — you activate a filter, then mutate its parameters live.

Minimal example

import 'package:flutter_soloud/flutter_soloud.dart';

Future<void> main() async {
  await SoLoud.instance.init();

  // Global echo on the whole output.
  final echo = SoLoud.instance.filters.echoFilter;
  echo.activate();
  echo.delay.value = 0.2; // seconds
  echo.decay.value = 0.7;

  // Per-sound filter: MUST be activated before play().
  final sound = await SoLoud.instance.loadAsset('assets/audio/track.mp3');
  sound.filters.pitchShiftFilter.activate();
  final handle = SoLoud.instance.play(sound, looping: true);
  sound.filters.pitchShiftFilter.semitones(soundHandle: handle).value = 3;
}

The API shape

• SoLoud.instance.filters → FiltersGlobal. Getters: biquadResonantFilter, echoFilter, lofiFilter, flangerFilter, bassBoostFilter, waveShaperFilter, robotizeFilter, freeverbFilter, pitchShiftFilter, limiterFilter, compressorFilter, parametricEqFilter. Params are getters: filters.echoFilter.delay.value = 0.2.
• audioSource.filters → FiltersSingle. Same filters, except the biquad getter is named biquadFilter (global scope calls it biquadResonantFilter). Params are methods taking the playing voice's handle: sound.filters.echoFilter.delay(soundHandle: handle).value = 0.2.
• bus.filters → FiltersSingle(busId: ...) on a Bus from SoLoud.instance.createMixingBus(). Params apply bus-wide; no soundHandle needed.
• FilterBase: activate(), deactivate(), isActive (bool), index (filter slot, -1 when inactive), filterType.
• FilterParam: double get value / set value(double), fadeFilterParameter({required double to, required Duration time}), oscillateFilterParameter({required double from, required double to, required Duration time}).
• queryXxx getters on each filter (e.g. filters.echoFilter.queryDelay) expose the parameter's min, max, def (default) and a human-readable toString() — use these for slider ranges instead of hardcoding.

Which params matter for the common use (full table in references/filter-parameters.md):

Every filter also has wet (0–1, default 1): 1 = fully processed, 0 = dry passthrough.

Recipes:

// Anti-clip safety: global limiter with defaults is usually enough.
// Activate it AFTER all other global filters so it sits last in the chain.
final limiter = SoLoud.instance.filters.limiterFilter;
limiter.activate();

// Taming many simultaneous loud voices (from example/lib/filters/compressor.dart):
final comp = SoLoud.instance.filters.compressorFilter;
comp.activate();
comp.threshold.value = -6;   // dB
comp.ratio.value = 3;
comp.attackTime.value = 10;  // ms
comp.releaseTime.value = 100;

// Time stretch without pitch change (per-sound scope only):
sound.filters.pitchShiftFilter.timeStretch(handle, 1.25); // 25% faster, same pitch
// Global scope: do it manually — setRelativePlaySpeed(handle, speed) then
// pitchShiftFilter.shift.value = 1 / speed.

// Parametric EQ (bands are logarithmic, 30 Hz .. 16 kHz):
final eq = SoLoud.instance.filters.parametricEqFilter;
eq.activate();
eq.numBands.value = 10;
eq.stftWindowSize.value = 2048; // must be a power of two, 32..4096
eq.bandGain(0).value = 2;       // boost the lowest band
final hz = eq.bandFrequency(0); // center frequency of band 0 (30 Hz)

Traps

• Per-sound filters must be activated before play(). Only voices started after activate() carry the filter; already-playing handles are unaffected. To change the filter set, stop the handle and play again.
• Per-sound filters do not work on web. Any activate()/param access with a soundHash on web throws SoLoudFilterForSingleSoundOnWebDartException. Global filters work on web; gate per-sound usage with kIsWeb if your app targets it.
• Max 8 filters per stream/sound (FILTERS_PER_STREAM in SoLoud). The 12 types each occupy one slot per scope.
• Out-of-range param.value sets are silently ignored — the setter logs a warning and returns without throwing. Check bounds with the queryXxx.min/max getters.
• Naming inconsistencies vs. intuition: the single/bus-scope biquad getter is biquadFilter (global: biquadResonantFilter); freeverb's damping param is damp (not damping); lofi's params are samplerate/bitdepth (all lowercase). The online docs get some of these wrong — trust the code in lib/src/filters/.
• freeverb is 2-channel only. As a global filter, init the engine with the default 2 channels (don't pass channels: to SoLoud.instance.init); as a per-sound filter the sound itself must be stereo.
• parametricEq stftWindowSize must be a power of two in 32..4096, otherwise the set is ignored with a warning. Larger window = more frequency accuracy, more latency and CPU.
• fadeFilterParameter/oscillateFilterParameter throw SoLoudNotInitializedException if the engine isn't initialized; FilterParam.value can throw SoLoudCppException on native errors. Engine re-init (deinit() + init()) drops all active filters — reactivate afterwards.
• Filter order matters: activation order is the processing order. Put gain-reducing filters (compressor, limiter) last — activate them after everything else (tip from example/lib/filters/limiter.dart).
• On Android, the default low-latency output profile leaves little CPU headroom for heavy filters (FFT pitch shift, large-EQ windows); pass lowLatency: false to SoLoud.instance.init if you hear underruns.

More depth

• references/filter-parameters.md — every parameter of all 12 filters with min/max/default and the matching queryXxx getter.
• Interactive demos in the plugin repo: example/lib/filters/ (compressor.dart, limiter.dart, parametric_eq.dart, pitchshift.dart) and the automated examples under example/tests/tests/ (global_filters.dart, sound_filters.dart, equalizer_filter.dart, compressor_filter.dart, limiter_filter.dart, pitch_shifter_filter.dart).

Keeping this skill current

This skill ships inside the flutter_soloud package, so upgrading flutter_soloud can carry a newer revision of it than the copy installed in the project. To check, run:

dart run flutter_soloud:skills --check

It reports the installed and bundled skill versions and exits non-zero when an update is available. Offer to update with dart run flutter_soloud:skills (which touches only the skills, never pubspec or build files).