Add follow points.

osu.Game.Modes.Osu/Objects/Drawables/FollowPoint.cs

@@ -0,0 +1,64 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using OpenTK;
+using osu.Framework.Allocation;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Graphics.Sprites;
+using osu.Framework.Graphics.Textures;
+using osu.Framework.Graphics.Transformations;
+
+namespace osu.Game.Modes.Osu.Objects.Drawables
+{
+    public class FollowPoint : Container
+    {
+        private Sprite followPoint;
+
+        public double StartTime;
+        public double EndTime;
+        public Vector2 EndPosition;
+
+        public FollowPoint()
+        {
+            Origin = Anchor.Centre;
+            Alpha = 0;
+
+            Children = new Drawable[]
+            {
+                followPoint = new Sprite
+                {
+                    Size = new Vector2(12f),
+                    Origin = Anchor.Centre,
+                    BlendingMode = BlendingMode.Additive,
+                    Alpha = 0.5f
+                },
+            };
+        }
+
+        protected override void LoadComplete()
+        {
+            base.LoadComplete();
+
+            DelayReset();
+
+            Delay(StartTime);
+            FadeIn(DrawableOsuHitObject.TIME_FADEIN);
+            ScaleTo(1.5f);
+            ScaleTo(1, DrawableOsuHitObject.TIME_FADEIN, EasingTypes.Out);
+            MoveTo(EndPosition, DrawableOsuHitObject.TIME_FADEIN, EasingTypes.Out);
+
+            Delay(EndTime - StartTime);
+            FadeOut(DrawableOsuHitObject.TIME_FADEIN);
+
+            Delay(DrawableOsuHitObject.TIME_FADEIN);
+            Expire(true);
+        }
+
+        [BackgroundDependencyLoader]
+        private void load(TextureStore textures)
+        {
+            followPoint.Texture = textures.Get(@"Play/osu/ring-glow");
+        }
+    }
+}

osu.Game.Modes.Osu/UI/OsuPlayfield.cs

@@ -1,13 +1,16 @@
 // Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
 // Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
 
+using OpenTK;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Game.Modes.Objects.Drawables;
 using osu.Game.Modes.Osu.Objects;
 using osu.Game.Modes.Osu.Objects.Drawables;
 using osu.Game.Modes.UI;
-using OpenTK;
+using System;
+using System.Collections.Generic;
+using System.Linq;
 
 namespace osu.Game.Modes.Osu.UI
 {
@@ -15,6 +18,7 @@ namespace osu.Game.Modes.Osu.UI
     {
         private Container approachCircles;
         private Container judgementLayer;
+        private Container followPointsLayer;
 
         public override Vector2 Size
         {
@@ -36,11 +40,16 @@ namespace osu.Game.Modes.Osu.UI
 
             Add(new Drawable[]
             {
-                judgementLayer = new Container
+                followPointsLayer = new Container
                 {
                     RelativeSizeAxes = Axes.Both,
                     Depth = 1,
                 },
+                judgementLayer = new Container
+                {
+                    RelativeSizeAxes = Axes.Both,
+                    Depth = 0,
+                },
                 approachCircles = new Container
                 {
                     RelativeSizeAxes = Axes.Both,
@@ -63,11 +72,69 @@ namespace osu.Game.Modes.Osu.UI
             base.Add(h);
         }
 
+        public override void PostProcess()
+        {
+            AddFollowPoints();
+        }
+
         private void judgement(DrawableHitObject h, JudgementInfo j)
         {
             HitExplosion explosion = new HitExplosion((OsuJudgementInfo)j, (OsuHitObject)h.HitObject);
 
             judgementLayer.Add(explosion);
         }
+
+        public void AddFollowPoints(int startIndex = 0, int endIndex = -1)
+        {
+            var followLineDistance = 32;
+            var followLinePreEmpt = 800;
+
+            var hitObjects = new List<OsuHitObject>(HitObjects.Children
+                .Select(d => (OsuHitObject)d.HitObject)
+                .OrderBy(h => h.StartTime));
+
+            if (endIndex < 0)
+                endIndex = hitObjects.Count - 1;
+
+            for (int i = startIndex + 1; i <= endIndex; i++)
+            {
+                var prevHitObject = hitObjects[i - 1];
+                var currHitObject = hitObjects[i];
+
+                if (prevHitObject.StartTime > currHitObject.StartTime)
+                    throw new Exception();
+
+                if (!currHitObject.NewCombo && !(prevHitObject is Spinner) && !(currHitObject is Spinner))
+                {
+                    Vector2 startPosition = prevHitObject.EndPosition;
+                    Vector2 endPosition = currHitObject.Position;
+                    double startTime = prevHitObject.EndTime;
+                    double endTime = currHitObject.StartTime;
+
+                    Vector2 distanceVector = endPosition - startPosition;
+                    int distance = (int)distanceVector.Length;
+                    float rotation = (float)Math.Atan2(distanceVector.Y, distanceVector.X);
+                    double duration = endTime - startTime;
+
+                    for (int d = (int)(followLineDistance * 1.5); d < distance - followLineDistance; d += followLineDistance)
+                    {
+                        float fraction = ((float)d / distance);
+                        Vector2 pointStartPosition = startPosition + (fraction - 0.1f) * distanceVector;
+                        Vector2 pointEndPosition = startPosition + fraction * distanceVector;
+                        double fadeOutTime = startTime + fraction * duration;
+                        double fadeInTime = fadeOutTime - followLinePreEmpt;
+
+                        followPointsLayer.Add(new FollowPoint()
+                        {
+                            StartTime = fadeInTime,
+                            EndTime = fadeOutTime,
+                            Position = pointStartPosition,
+                            EndPosition = pointEndPosition,
+                            Rotation = rotation,
+                        });
+                    }
+                }
+            }
+        }
     }
 }

osu.Game.Modes.Osu/osu.Game.Modes.Osu.csproj

@@ -47,6 +47,7 @@
     <Compile Include="Objects\Drawables\Pieces\ApproachCircle.cs" />
     <Compile Include="Objects\Drawables\Pieces\CirclePiece.cs" />
     <Compile Include="Objects\Drawables\DrawableSlider.cs" />
+    <Compile Include="Objects\Drawables\FollowPoint.cs" />
     <Compile Include="Objects\Drawables\Pieces\ExplodePiece.cs" />
     <Compile Include="Objects\Drawables\Pieces\FlashPiece.cs" />
     <Compile Include="Objects\Drawables\Pieces\GlowPiece.cs" />

osu.Game/Modes/UI/HitRenderer.cs

@@ -83,6 +83,7 @@ namespace osu.Game.Modes.UI
 
                 Playfield.Add(drawableObject);
             }
+            Playfield.PostProcess();
         }
 
         private void onJudgement(DrawableHitObject o, JudgementInfo j) => TriggerOnJudgement(j);

osu.Game/Modes/UI/Playfield.cs

@@ -1,10 +1,10 @@
 // Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
 // Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
 
+using OpenTK;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
 using osu.Game.Modes.Objects.Drawables;
-using OpenTK;
 
 namespace osu.Game.Modes.UI
 {
@@ -32,6 +32,10 @@ namespace osu.Game.Modes.UI
             });
         }
 
+        public virtual void PostProcess()
+        {
+        }
+
         public class ScaledContainer : Container
         {
             protected override Vector2 DrawScale => new Vector2(DrawSize.X / 512);