Initial implementation

Note this won't work with the osu!bgm because it is not a beatmap

osu.Game/Screens/Menu/MainMenu.cs

@@ -54,7 +54,8 @@ namespace osu.Game.Screens.Menu
                             OnSolo = delegate { Push(consumeSongSelect()); },
                             OnMulti = delegate { Push(new Lobby()); },
                             OnExit = delegate { Exit(); },
-                        }
+                        },
+                        new MenuSideFlashes(),
                     }
                 }
             };

osu.Game/Screens/Menu/MenuSideFlashes.cs

@@ -0,0 +1,95 @@
+using OpenTK.Graphics;
+using osu.Framework.Allocation;
+using osu.Framework.Configuration;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Colour;
+using osu.Game.Graphics.Containers;
+using osu.Framework.Graphics.Sprites;
+using osu.Game.Beatmaps;
+using osu.Game.Beatmaps.Timing;
+using System;
+
+namespace osu.Game.Screens.Menu
+{
+    public class MenuSideFlashes : BeatSyncedContainer
+    {
+        public override bool HandleInput => false;
+
+        private Bindable<WorkingBeatmap> beatmap = new Bindable<WorkingBeatmap>();
+
+        private Box leftBox;
+        private Box rightBox;
+
+        private const int amplitude_dead_zone = 9000;
+        private const float alpha_multiplier = (short.MaxValue - amplitude_dead_zone) / 0.55f;
+        private const int box_max_alpha = 200;
+        private const double box_fade_in_time = 65;
+
+        public MenuSideFlashes()
+        {
+            RelativeSizeAxes = Axes.Both;
+            Anchor = Anchor.Centre;
+            Origin = Anchor.Centre;
+            BlendingMode = BlendingMode.Additive;
+            Children = new Drawable[]
+            {
+                leftBox = new Box
+                {
+                    Anchor = Anchor.CentreLeft,
+                    Origin = Anchor.CentreLeft,
+                    RelativeSizeAxes = Axes.Y,
+                    Width = 300,
+                    Alpha = 0,
+                    BlendingMode = BlendingMode.Additive,
+                    ColourInfo = ColourInfo.GradientHorizontal(new Color4(255, 255, 255, box_max_alpha), Color4.Transparent),
+                },
+                rightBox = new Box
+                {
+                    Anchor = Anchor.CentreRight,
+                    Origin = Anchor.CentreRight,
+                    RelativeSizeAxes = Axes.Y,
+                    Width = 300,
+                    Alpha = 0,
+                    BlendingMode = BlendingMode.Additive,
+                    ColourInfo = ColourInfo.GradientHorizontal(Color4.Transparent, new Color4(255, 255, 255, box_max_alpha)),
+                }
+            };
+        }
+
+        protected override void OnNewBeat(int newBeat, double beatLength, TimeSignatures timeSignature, bool kiai)
+        {
+            if (!beatmap?.Value?.Track?.IsRunning ?? false)
+            {
+                leftBox.FadeOut(50);
+                rightBox.FadeOut(50);
+            }
+            else if (newBeat >= 0)
+            {
+                short[] lev = beatmap.Value.Track.ChannelPeakAmplitudes;
+                bool nextIsLeft = newBeat % 2 == 0;
+                if (kiai ? nextIsLeft : newBeat % (int)timeSignature == 0)
+                {
+                    leftBox.ClearTransforms();
+                    leftBox.FadeTo(Math.Max(0, (lev[0] - amplitude_dead_zone) / alpha_multiplier), 65);
+                    using (leftBox.BeginDelayedSequence(box_fade_in_time))
+                        leftBox.FadeOut(beatLength, EasingTypes.In);
+                    leftBox.DelayReset();
+                }
+                if (kiai ? !nextIsLeft : newBeat % (int)timeSignature == 0)
+                {
+                    rightBox.ClearTransforms();
+                    rightBox.FadeTo(Math.Max(0, (lev[1] - amplitude_dead_zone) / alpha_multiplier), 65);
+                    using (rightBox.BeginDelayedSequence(box_fade_in_time))
+                        rightBox.FadeOut(beatLength, EasingTypes.In);
+                    rightBox.DelayReset();
+                }
+            }
+        }
+
+        [BackgroundDependencyLoader]
+        private void load(OsuGameBase game)
+        {
+            beatmap = game.Beatmap;
+        }
+    }
+}

osu.Game/osu.Game.csproj

@@ -183,6 +183,7 @@
     <Compile Include="Database\RulesetDatabase.cs" />
     <Compile Include="Rulesets\Scoring\Score.cs" />
     <Compile Include="Rulesets\Scoring\ScoreProcessor.cs" />
+    <Compile Include="Screens\Menu\MenuSideFlashes.cs" />
     <Compile Include="Screens\Play\HUD\HealthDisplay.cs" />
     <Compile Include="Screens\Play\HUDOverlay.cs" />
     <Compile Include="Screens\Play\HUD\StandardHealthDisplay.cs" />