Add Fluidsynth-backed MIDI controls

Robust.Client/Audio/Midi/IMidiRenderer.cs

@@ -216,4 +216,30 @@ public interface IMidiRenderer : IDisposable
     internal void InternalDispose();
 
     byte MinVolume { get; set; }
+
+    /// <summary>
+    ///     Directly sets the SF2 GEN_FILTERFC generator (low-pass cutoff) for a channel
+    ///     via fluid_synth_set_gen. A lower value muffles the channel.
+    ///     Valid range: ~1500 (fully closed) to 13500 (fully open / no filter).
+    /// </summary>
+    void SetChannelFilterCutoff(int channel, float cutoffCents);
+
+    /// <summary>
+    ///     Scales MIDI file playback speed via fluid_player_set_tempo
+    ///     1.0 = normal, 2.0 = double speed, 0.5 = half speed.
+    ///     Stored and re-applied when a new MIDI file is opened.
+    ///     Has no effect during MIDI input mode (no player).
+    /// </summary>
+    void SetTempoScale(double scale);
+
+    /// <summary>
+    ///     Per-channel pitch offset in semitones via SF2 GEN_COARSETUNE.
+    ///     Range typically -12 to +12 (one octave). 0 = no shift.
+    /// </summary>
+    void SetChannelPitch(int channel, int semitones);
+
+    /// <summary>
+    ///     Enables or disables per-channel reverb send (CC91).
+    /// </summary>
+    void SetChannelReverb(int channel, bool enabled);
 }

Robust.Client/Audio/Midi/MidiRenderer.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections;
+using System.Runtime.InteropServices;
 using JetBrains.Annotations;
 using NFluidsynth;
 using Robust.Client.Graphics;
@@ -48,6 +49,25 @@ internal sealed partial class MidiRenderer : IMidiRenderer
     private const int Buffers = SampleRate / 2205;
     private readonly object _playerStateLock = new();
     private readonly SequencerClientId _synthRegister;
+
+    // P/Invoke for fluid_synth_set_gen — lets us set SF2 generator values per channel
+    // directly, bypassing MIDI CC limitations.
+    [DllImport("libfluidsynth-3", CallingConvention = CallingConvention.Cdecl)]
+    private static extern int fluid_synth_set_gen(IntPtr synth, int chan, int param, float value);
+
+    private const int GenFilterCutoff = 8; // GEN_FILTERFC in the SF2 spec
+    // P/Invoke for fluid_player_set_tempo — scales MIDI playback BPM by a multiplier.
+    // FLUID_PLAYER_TEMPO_INTERNAL (0) applies the value as a speed multiplier while
+    // still honouring in-file SetTempo events: 1.0 = normal, 2.0 = double, 0.5 = half.
+    [DllImport("libfluidsynth-3", CallingConvention = CallingConvention.Cdecl)]
+    private static extern int fluid_player_set_tempo(IntPtr player, int tempoType, double tempo);
+
+    private const int FluidPlayerTempoInternal = 0; // FLUID_PLAYER_TEMPO_INTERNAL
+    private const int GenCoarseTune = 51;           // GEN_COARSETUNE — semitone pitch offset
+    private const int ReverbCc = 91;                // CC91 (Effects 1 Depth / Reverb Send)
+
+    private double _tempoScale = 1.0;
+
     private readonly SequencerClientId _robustRegister;
     private readonly SequencerClientId _debugRegister;
 
@@ -362,6 +382,8 @@ public bool OpenMidi(ReadOnlySpan<byte> buffer)
             _player.Seek(0);
             _player.Play();
             _player.SetLoop(LoopMidi ? -1 : 1);
+            if (_tempoScale != 1.0)
+                fluid_player_set_tempo(_player.Handle, FluidPlayerTempoInternal, _tempoScale);
         }
 
         return true;
@@ -558,6 +580,53 @@ private void SendMidiEvent(RobustMidiEvent midiEvent)
         SendMidiEvent(midiEvent, true);
     }
 
+    public void SetChannelFilterCutoff(int channel, float cutoffCents)
+    {
+        if (Disposed)
+            return;
+
+        lock (_playerStateLock)
+        {
+            fluid_synth_set_gen(_synth.Handle, channel, GenFilterCutoff, cutoffCents);
+        }
+    }
+
+    public void SetTempoScale(double scale)
+    {
+        if (Disposed)
+            return;
+
+        _tempoScale = scale;
+        lock (_playerStateLock)
+        {
+            if (_player != null)
+                fluid_player_set_tempo(_player.Handle, FluidPlayerTempoInternal, scale);
+        }
+    }
+
+    public void SetChannelPitch(int channel, int semitones)
+    {
+        if (Disposed)
+            return;
+
+        lock (_playerStateLock)
+        {
+            fluid_synth_set_gen(_synth.Handle, channel, GenCoarseTune, (float)semitones);
+        }
+    }
+
+    public void SetChannelReverb(int channel, bool enabled)
+    {
+        if (Disposed)
+            return;
+
+        lock (_playerStateLock)
+        {
+            _rendererState.Controllers.AsSpan[channel].AsSpan[ReverbCc] = (byte) (enabled ? 127 : 0);
+            _synth.CC(channel, ReverbCc, enabled ? 127 : 0);
+        }
+    }
+
     public void SendMidiEvent(RobustMidiEvent midiEvent, bool raiseEvent)
     {
         if (Disposed)