Add a glow fade based on density of hits.

osu.Game/Rulesets/Scoring/ScoreProcessor.cs

@@ -19,6 +19,11 @@ namespace osu.Game.Rulesets.Scoring
         /// </summary>
         public event Action Failed;
 
+        /// <summary>
+        /// Invoked when a new judgement has occurred. This occurs after the judgement has been processed by the <see cref="ScoreProcessor"/>.
+        /// </summary>
+        public event Action<Judgement> NewJudgement;
+
         /// <summary>
         /// The current total score.
         /// </summary>
@@ -105,6 +110,15 @@ namespace osu.Game.Rulesets.Scoring
             Failed?.Invoke();
         }
 
+        /// <summary>
+        /// Notifies subscribers of <see cref="NewJudgement"/> that a new judgement has occurred.
+        /// </summary>
+        /// <param name="judgement">The judgement to notify subscribers of.</param>
+        protected void NotifyNewJudgement(Judgement judgement)
+        {
+            NewJudgement?.Invoke(judgement);
+        }
+
         /// <summary>
         /// Retrieve a score populated with data for the current play this processor is responsible for.
         /// </summary>
@@ -177,6 +191,8 @@ namespace osu.Game.Rulesets.Scoring
 
                 Judgements.Add(judgement);
                 OnNewJudgement(judgement);
+
+                NotifyNewJudgement(judgement);
             }
             else
                 OnJudgementChanged(judgement);

osu.Game/Rulesets/UI/HudOverlay.cs

@@ -94,7 +94,7 @@ namespace osu.Game.Rulesets.UI
             }
         }
 
-        public void BindProcessor(ScoreProcessor processor)
+        public virtual void BindProcessor(ScoreProcessor processor)
         {
             ScoreCounter?.Current.BindTo(processor.TotalScore);
             AccuracyCounter?.Current.BindTo(processor.Accuracy);
@@ -102,7 +102,7 @@ namespace osu.Game.Rulesets.UI
             HealthDisplay?.Current.BindTo(processor.Health);
         }
 
-        public void BindHitRenderer(HitRenderer hitRenderer)
+        public virtual void BindHitRenderer(HitRenderer hitRenderer)
         {
             hitRenderer.InputManager.Add(KeyCounter.GetReceptor());
         }

osu.Game/Rulesets/UI/StandardHealthDisplay.cs

@@ -3,15 +3,43 @@
 
 using OpenTK;
 using OpenTK.Graphics;
+using osu.Framework.Extensions.Color4Extensions;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Framework.Graphics.Sprites;
 using osu.Game.Graphics;
+using osu.Game.Rulesets.Judgements;
+using osu.Game.Rulesets.Objects.Drawables;
+using osu.Game.Rulesets.Scoring;
+using System;
 
 namespace osu.Game.Rulesets.UI
 {
     public class StandardHealthDisplay : HealthDisplay, IHasAccentColour
     {
+        /// <summary>
+        /// The base opacity of the glow.
+        /// </summary>
+        private const float base_glow_opacity = 0.6f;
+
+        /// <summary>
+        /// The number of sequential hits required within <see cref="glow_fade_delay"/> to reach the maximum glow opacity.
+        /// </summary>
+        private const int glow_max_hits = 8;
+
+        /// <summary>
+        /// The amount of time to delay before fading the glow opacity back to <see cref="base_glow_opacity"/>.
+        /// <para>
+        /// This is calculated to require a stream snapped to 1/4 at 150bpm to reach the maximum glow opacity with <see cref="glow_max_hits"/> hits.
+        /// </para>
+        /// </summary>
+        private const float glow_fade_delay = 100;
+
+        /// <summary>
+        /// The amount of time to fade the glow to <see cref="base_glow_opacity"/> after <see cref="glow_fade_delay"/>.
+        /// </summary>
+        private const double glow_fade_time = 500;
+
         private readonly Container fill;
 
         public Color4 AccentColour
@@ -32,7 +60,7 @@ namespace osu.Game.Rulesets.UI
 
                 fill.EdgeEffect = new EdgeEffect
                 {
-                    Colour = glowColour,
+                    Colour = glowColour.Opacity(base_glow_opacity),
                     Radius = 8,
                     Roundness = 4,
                     Type = EdgeEffectType.Glow,
@@ -65,6 +93,21 @@ namespace osu.Game.Rulesets.UI
             };
         }
 
+        public void BindProcessor(ScoreProcessor processor)
+        {
+            processor.NewJudgement += onNewJudgement;
+        }
+
+        private void onNewJudgement(Judgement judgement)
+        {
+            if (judgement.Result == HitResult.Miss)
+                return;
+
+            fill.FadeEdgeEffectTo(Math.Min(1, fill.EdgeEffect.Colour.Linear.A + (1f - base_glow_opacity) / glow_max_hits), 50, EasingTypes.OutQuint);
+            fill.Delay(glow_fade_delay);
+            fill.FadeEdgeEffectTo(base_glow_opacity, glow_fade_time, EasingTypes.OutQuint);
+        }
+
         protected override void SetHealth(float value) => fill.ResizeTo(new Vector2(value, 1), 200, EasingTypes.OutQuint);
     }
 }

osu.Game/Rulesets/UI/StandardHudOverlay.cs

@@ -8,6 +8,7 @@ using osu.Framework.Graphics;
 using osu.Framework.Graphics.Primitives;
 using osu.Game.Graphics;
 using osu.Game.Graphics.UserInterface;
+using osu.Game.Rulesets.Scoring;
 using osu.Game.Screens.Play;
 
 namespace osu.Game.Rulesets.UI
@@ -75,8 +76,16 @@ namespace osu.Game.Rulesets.UI
             if (shd != null)
             {
                 shd.AccentColour = colours.BlueLighter;
-                shd.GlowColour = colours.BlueDarker.Opacity(0.6f);
+                shd.GlowColour = colours.BlueDarker;
-            }
+            }
+        }
+
+        public override void BindProcessor(ScoreProcessor processor)
+        {
+            base.BindProcessor(processor);
+
+            var shd = HealthDisplay as StandardHealthDisplay;
+            shd?.BindProcessor(processor);
         }
     }
 }