feat: added trail color easing

Shared/Behaviours/BallTrail.cs

@@ -11,10 +11,14 @@ 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 ColorRGBA colorBase = new ColorRGBA(255, 255, 255, 255);
+    private ColorRGBA colorEnd = new ColorRGBA(255, 255, 255, 0);
+    private IEasing colorEasing = EaseOutSine.Instance;
+
+    private int trailTickPeriod = 20;
+
     private readonly LinkedList<Vector2D> trailPoints = new();
 
     public void PostPhysicsUpdate(float delta)
@@ -31,24 +35,26 @@ public class BallTrail : Behaviour2D, IPostPhysicsUpdate, IDrawableTriangle
             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 t = (float)i / trailPoints.Count;
+            float tPlus = (float)(i + 1) / trailPoints.Count;
 
-            float firstWidth = t * width;
-            float endWidth = tPlus * width;
+            float firstWidth = width * widthCurve.Evaluate(1f - t);
+            float endWidth = width * widthCurve.Evaluate(1f - tPlus);
+
+            ColorRGBA color = colorBase.Lerp(colorEnd, colorEasing.Evaluate(t));
 
             triangleBatch.Draw(
                 new(line.From + perpendicularDirection * firstWidth,
                     line.From - perpendicularDirection * firstWidth,
                     line.To + perpendicularDirection * endWidth),
-                colorBase
+                color
             );
 
             triangleBatch.Draw(
                 new(line.From - perpendicularDirection * firstWidth,
                     line.To - perpendicularDirection * endWidth,
                     line.To + perpendicularDirection * endWidth),
-                colorBase
+                color
             );
         }
     }