fix: Use properly FPS-independent smoothing function

Author: twpol

Source/ORTS.Common/SmoothedData.cs

@@ -1,4 +1,4 @@
-// COPYRIGHT 2010, 2011 by the Open Rails project.
+// COPYRIGHT 2010, 2011 by the Open Rails project.
 // 
 // This file is part of Open Rails.
 // 
@@ -28,6 +28,7 @@ public class SmoothedData
 
         public readonly float SmoothPeriodS;
 
+        protected float rate = 0;
         protected float value = float.NaN;
         protected float smoothedValue = float.NaN;
 
@@ -39,6 +40,8 @@ public SmoothedData()
         public SmoothedData(float smoothPeriodS)
         {
             SmoothPeriodS = smoothPeriodS;
+            // Convert the input assuming 60 FPS (arbitary)
+            rate = (float)(-60 * Math.Log(1 - 1 / (60 * SmoothPeriodS)));
         }
 
         public void Update(float periodS, float newValue)
@@ -56,13 +59,11 @@ public void Update(float periodS, float newValue)
 
         protected void SmoothValue(ref float smoothedValue, float periodS, float newValue)
         {
-            var rate = SmoothPeriodS / periodS;
-            if (float.IsNaN(smoothedValue) || float.IsInfinity(smoothedValue))
-                smoothedValue = newValue;
-            else if (rate < 1)
+            var ratio = (float)Math.Exp(-rate * periodS);
+            if (float.IsNaN(smoothedValue) || float.IsInfinity(smoothedValue) || ratio < 0.5)
                 smoothedValue = newValue;
             else
-                smoothedValue = (smoothedValue * (rate - 1) + newValue) / rate;
+                smoothedValue = smoothedValue * ratio + newValue * (1 - ratio);
         }
 
         public void ForceSmoothValue(float forcedValue)

Source/Tests/Orts.Common/SmoothedData.cs

@@ -24,11 +24,11 @@ public static class SmoothedData
     {
         [Theory]
         // FPS-like tests
-        [InlineData(5, 3, 0.318)]
-        [InlineData(10, 3, 0.337)]
-        [InlineData(30, 3, 0.350)]
+        [InlineData(5, 3, 0.353)]
+        [InlineData(10, 3, 0.353)]
+        [InlineData(30, 3, 0.353)]
         [InlineData(60, 3, 0.353)]
-        [InlineData(120, 3, 0.355)]
+        [InlineData(120, 3, 0.353)]
         // Physics-like tests
         [InlineData(60, 1, 0.000)] // Exhaust particles
         [InlineData(60, 2, 0.066)] // Smoke colour