Add a low-pass filter effect to music when certain popup dialogs are shown

osu.Game/Audio/Effects/LowPassFilter.cs

@@ -0,0 +1,75 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using ManagedBass.Fx;
+using osu.Framework.Audio.Mixing;
+using osu.Framework.Bindables;
+using osu.Framework.Graphics;
+
+namespace osu.Game.Audio.Effects
+{
+    public class LowPassFilter : Component
+    {
+        private const float filter_cutoff_start = 2000;
+        private const float filter_cutoff_end = 150;
+        private const float filter_sweep_duration = 100;
+        private readonly Bindable<float> filterFreq = new Bindable<float>(filter_cutoff_start);
+        private readonly AudioMixer mixer;
+        private readonly BQFParameters filter;
+
+        /// <summary>
+        /// A toggle-able low-pass filter with a subtle filter-sweep effect when toggled that can be attached to an <see cref="AudioMixer"/>.
+        /// </summary>
+        public LowPassFilter(AudioMixer mixer)
+        {
+            this.mixer = mixer;
+            filter = new BQFParameters
+            {
+                lFilter = BQFType.LowPass,
+                fCenter = filterFreq.Value
+            };
+        }
+
+        public void Enable()
+        {
+            attachFilter();
+            this.TransformBindableTo(filterFreq, filter_cutoff_end, filter_sweep_duration);
+        }
+
+        public void Disable()
+        {
+            this.TransformBindableTo(filterFreq, filter_cutoff_start, filter_sweep_duration)
+                .OnComplete(_ => detatchFilter());
+        }
+
+        private void attachFilter()
+        {
+            mixer.Effects.Add(filter);
+            filterFreq.ValueChanged += updateFilter;
+        }
+
+        private void detatchFilter()
+        {
+            filterFreq.ValueChanged -= updateFilter;
+            mixer.Effects.Remove(filter);
+        }
+
+        private void updateFilter(ValueChangedEvent<float> cutoff)
+        {
+            var filterIndex = mixer.Effects.IndexOf(filter);
+            if (filterIndex < 0) return;
+
+            var existingFilter = mixer.Effects[filterIndex] as BQFParameters;
+            if (existingFilter == null) return;
+
+            existingFilter.fCenter = cutoff.NewValue;
+            mixer.Effects[filterIndex] = existingFilter;
+        }
+
+        protected override void Dispose(bool isDisposing)
+        {
+            base.Dispose(isDisposing);
+            detatchFilter();
+        }
+    }
+}