feat: added ball trail

Shared/Behaviours/BallTrail.cs

@@ -0,0 +1,55 @@
+using System.Collections.Generic;
+using System.Linq;
+
+using Engine.Core;
+using Engine.Physics2D;
+using Engine.Systems.Graphics;
+using Engine.Systems.Tween;
+
+namespace Pong.Behaviours;
+
+public class BallTrail : Behaviour2D, IPostPhysicsUpdate, IDrawableTriangle
+{
+    private float width = 10f;
+    private ColorRGBA colorBase = new ColorRGBA(255, 255, 255);
+    private int trailTickPeriod = 20;
+    private IEasing widthCurve = EaseInCirc.Instance;
+
+    private readonly LinkedList<Vector2D> trailPoints = new();
+
+    public void PostPhysicsUpdate(float delta)
+    {
+        trailPoints.AddFirst(Transform.Position);
+        if (trailPoints.Count > trailTickPeriod)
+            trailPoints.RemoveLast();
+    }
+
+    public void Draw(ITriangleBatch triangleBatch)
+    {
+        for (int i = 0; i < trailPoints.Count - 2; i++)
+        {
+            Line2D line = new(trailPoints.ElementAt(i), trailPoints.ElementAt(i + 1));
+            Vector2D perpendicularDirection = line.Direction.Perpendicular();
+
+            float t = widthCurve.Evaluate(1f - ((float)i / trailPoints.Count));
+            float tPlus = widthCurve.Evaluate(1f - ((float)(i + 1) / trailPoints.Count));
+
+            float firstWidth = t * width;
+            float endWidth = tPlus * width;
+
+            triangleBatch.Draw(
+                new(line.From + perpendicularDirection * firstWidth,
+                    line.From - perpendicularDirection * firstWidth,
+                    line.To + perpendicularDirection * endWidth),
+                colorBase
+            );
+
+            triangleBatch.Draw(
+                new(line.From - perpendicularDirection * firstWidth,
+                    line.To - perpendicularDirection * endWidth,
+                    line.To + perpendicularDirection * endWidth),
+                colorBase
+            );
+        }
+    }
+}

Shared/PongUniverse.cs

@@ -81,6 +81,7 @@ public static class PongUniverse
             .BehaviourController.AddBehaviour<Transform2D>().SetTransform(position: new Vector2D(0, 0f), scale: new Vector2D(10f, 10f))
             .BehaviourController.AddBehaviour<DrawableColliderCircle>(new Circle(Vector2D.Zero, 1f))
             .BehaviourController.AddBehaviour<Ball>()
+            .BehaviourController.AddBehaviour<BallTrail>()
             .BehaviourController.AddBehaviour<RigidBody2D>();
 
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