Unity-Technologies
@@ -22,6 +22,8 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Issue where lerp smoothing was applied per frame instead of over time, which caused the `Lerp` and `SmoothDampening` interpolation types to smooth by different amounts at different frame rates. Results at 60fps are unchanged. (#4130)
+- Issue where setting a maximum interpolation time of 1.0 would stop a `NetworkTransform` from interpolating at all when using the `Lerp` or `SmoothDampening` interpolation types. (#4130)
 
 ### Security
 

@@ -209,12 +209,23 @@ public void Reset(T currentValue)
         internal bool LerpSmoothEnabled;
 
         /// <summary>
-        /// Determines how much smoothing will be applied to the 2nd lerp when using the <see cref="Update(float, double, double)"/> (i.e. lerping and not smooth dampening).
+        /// The frame rate that <see cref="MaximumInterpolationTime"/> is relative to when lerp smoothing.
+        /// </summary>
+        private const float k_LerpSmoothReferenceFrameRate = 60.0f;
+
+        /// <summary>
+        /// Keeps a <see cref="MaximumInterpolationTime"/> of 1.0f from retaining the entire delta each frame,
+        /// which would stop the value from ever advancing towards the target.
+        /// </summary>
+        private const float k_MaximumLerpSmoothRetention = 0.99f;
+
+        /// <summary>
+        /// Determines how much smoothing will be applied to the 2nd lerp.
         /// </summary>
         /// <remarks>
-        /// There's two factors affecting interpolation: <br />
-        /// - Buffering: Which can be adjusted in set in the <see cref="NetworkManager.NetworkTimeSystem"/>.<br />
-        /// - Interpolation time: The divisor applied to delta time where the quotient is used as the lerp time.
+        /// Higher values are smoother, lower values are more precise. The amount of smoothing applied is
+        /// frame rate independent.<br />
+        /// Buffering also affects interpolation and can be adjusted via <see cref="NetworkManager.NetworkTimeSystem"/>.
         /// </remarks>
         [Range(0.016f, 1.0f)]
         public float MaximumInterpolationTime = 0.1f;
@@ -420,6 +431,22 @@ internal void ResetCurrentState()
             }
         }
 
+        /// <summary>
+        /// Calculates the frame rate independent lerp smoothing "t" for the current frame.
+        /// </summary>
+        /// <remarks>
+        /// Raising the retained portion to the number of reference frames elapsed makes the smoothing rate
+        /// a function of elapsed time rather than of how often this is called.
+        /// </remarks>
+        /// <param name="deltaTime">The last frame time.</param>
+        /// <returns>The lerp smoothing time to apply for this frame.</returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private float GetLerpSmoothTime(float deltaTime)
+        {
+            var retained = Mathf.Clamp(MaximumInterpolationTime, 0.0f, k_MaximumLerpSmoothRetention);
+            return 1.0f - Mathf.Pow(retained, deltaTime * k_LerpSmoothReferenceFrameRate);
+        }
+
         /// <summary>
         /// Interpolation Update to use when smooth dampening is enabled on a <see cref="Components.NetworkTransform"/>.
         /// </summary>
@@ -459,7 +486,7 @@ internal T Update(float deltaTime, double tickLatencyAsTime, double minDeltaTime
                     if (LerpSmoothEnabled)
                     {
                         // Apply the smooth lerp to the target to help smooth the final value.
-                        InterpolateState.CurrentValue = Interpolate(InterpolateState.CurrentValue, InterpolateState.NextValue, Mathf.Clamp(1.0f - MaximumInterpolationTime, 0.0f, 1.0f));
+                        InterpolateState.CurrentValue = Interpolate(InterpolateState.CurrentValue, InterpolateState.NextValue, GetLerpSmoothTime(deltaTime));
                     }
                     else
                     {

@@ -1136,7 +1136,7 @@ public enum InterpolationTypes
             /// Uses a 1 to 2 phase interpolation approach where:<br />
             /// <list type="bullet">
             /// <item><description>The first phase lerps from the previous state update value to the next state update value.</description></item>
-            /// <item><description>The second phase (optional) performs lerp smoothing where the current respective transform value is lerped towards the result of the first phase at a rate of 1.0 minus the respective maximum interpolation time.</description></item>
+            /// <item><description>The second phase (optional) performs lerp smoothing where the current respective transform value is lerped towards the result of the first phase at a frame rate independent rate determined by the respective maximum interpolation time.</description></item>
             /// </list>
             /// </summary>
             /// <remarks>
@@ -1156,7 +1156,7 @@ public enum InterpolationTypes
             /// Uses a 1 to 2 phase smooth dampening approach where:<br />
             /// <list type="bullet">
             /// <item><description>The first phase smooth dampens towards the current tick state update being processed by the accumulated delta time relative to the time to target.</description></item>
-            /// <item><description>The second phase (optional) performs lerp smoothing where the current respective transform value is lerped towards the result of the first phase at a rate of delta time divided by the respective max interpolation time.</description></item>
+            /// <item><description>The second phase (optional) performs lerp smoothing where the current respective transform value is lerped towards the result of the first phase at a frame rate independent rate determined by the respective maximum interpolation time.</description></item>
             /// </list>
             /// </summary>
             /// <remarks>
@@ -1236,7 +1236,10 @@ public enum InterpolationTypes
         /// Controls position interpolation smoothing.
         /// </summary>
         /// <remarks>
-        /// When enabled, the <see cref="BufferedLinearInterpolator{T}"/> will apply a final lerping pass where the "t" parameter is calculated by dividing the frame time divided by the <see cref="PositionMaxInterpolationTime"/>.
+        /// When enabled, the <see cref="BufferedLinearInterpolator{T}"/> will apply a final lerping pass towards
+        /// the interpolated result at a rate determined by <see cref="PositionMaxInterpolationTime"/>.<br />
+        /// This is frame rate independent for all <see cref="InterpolationTypes"/>, but the same value will not
+        /// produce the same result under <see cref="InterpolationTypes.LegacyLerp"/> as it does under the others.
         /// </remarks>
         public bool PositionLerpSmoothing = true;
         private bool m_PreviousPositionLerpSmoothing;
@@ -1257,7 +1260,10 @@ public enum InterpolationTypes
         /// Controls rotation interpolation smoothing.
         /// </summary>
         /// <remarks>
-        /// When enabled, the <see cref="BufferedLinearInterpolator{T}"/> will apply a final lerping pass where the "t" parameter is calculated by dividing the frame time divided by the <see cref="RotationMaxInterpolationTime"/>.
+        /// When enabled, the <see cref="BufferedLinearInterpolator{T}"/> will apply a final lerping pass towards
+        /// the interpolated result at a rate determined by <see cref="RotationMaxInterpolationTime"/>.<br />
+        /// This is frame rate independent for all <see cref="InterpolationTypes"/>, but the same value will not
+        /// produce the same result under <see cref="InterpolationTypes.LegacyLerp"/> as it does under the others.
         /// </remarks>
         public bool RotationLerpSmoothing = true;
         private bool m_PreviousRotationLerpSmoothing;
@@ -1278,7 +1284,10 @@ public enum InterpolationTypes
         /// Controls scale interpolation smoothing.
         /// </summary>
         /// <remarks>
-        /// When enabled, the <see cref="BufferedLinearInterpolator{T}"/> will apply a final lerping pass where the "t" parameter is calculated by dividing the frame time divided by the <see cref="ScaleMaxInterpolationTime"/>.
+        /// When enabled, the <see cref="BufferedLinearInterpolator{T}"/> will apply a final lerping pass towards
+        /// the interpolated result at a rate determined by <see cref="ScaleMaxInterpolationTime"/>.<br />
+        /// This is frame rate independent for all <see cref="InterpolationTypes"/>, but the same value will not
+        /// produce the same result under <see cref="InterpolationTypes.LegacyLerp"/> as it does under the others.
         /// </remarks>
         public bool ScaleLerpSmoothing = true;
         private bool m_PreviousScaleLerpSmoothing;

@@ -301,5 +301,87 @@ public void TestDuplicatedValues()
             Assert.That(interp, Is.EqualTo(2f));
             // Since there is no extrapolation, the rest of this test was removed.
         }
+
+        #region Lerp Smoothing
+
+        // Deliberately not round numbers, so exactly representable values cannot mask a defect.
+        private const double k_SmoothTickInterval = 1.0d / 30.0d;
+        private const int k_SmoothTickLatency = 2;
+        private const float k_SmoothStartValue = 3.17f;
+        private const float k_SmoothVelocity = 2.3f;
+        private const double k_SmoothMoveDuration = 1.53d;
+        private const double k_SmoothTotalDuration = 2.11d;
+
+        /// <summary>
+        /// Drives the lerp and smooth dampening interpolation path with lerp smoothing enabled, where an
+        /// authority moves at a constant velocity and then holds still while the non-authority renders at
+        /// <paramref name="frameDeltaTime"/>.
+        /// </summary>
+        /// <returns>The interpolated value once <see cref="k_SmoothTotalDuration"/> has elapsed.</returns>
+        private float RunLerpSmoothing(float maximumInterpolationTime, float frameDeltaTime, bool lerp)
+        {
+            var interpolator = new BufferedLinearInterpolatorFloat
+            {
+                MaximumInterpolationTime = maximumInterpolationTime,
+                LerpSmoothEnabled = true,
+            };
+            interpolator.ResetTo(k_SmoothStartValue, 0.0d);
+
+            var restValue = k_SmoothStartValue + (float)(k_SmoothVelocity * k_SmoothMoveDuration);
+            var maxDeltaTime = k_SmoothTickLatency * k_SmoothTickInterval;
+            var nextTick = 1;
+            var currentValue = k_SmoothStartValue;
+
+            for (var time = 0.0d; time < k_SmoothTotalDuration; time += frameDeltaTime)
+            {
+                // Deliver every state update whose send time has already passed.
+                while (nextTick * k_SmoothTickInterval <= time)
+                {
+                    var sentTime = nextTick * k_SmoothTickInterval;
+                    var sentValue = sentTime <= k_SmoothMoveDuration
+                        ? k_SmoothStartValue + (float)(k_SmoothVelocity * sentTime)
+                        : restValue;
+                    interpolator.AddMeasurement(sentValue, sentTime);
+                    nextTick++;
+                }
+
+                currentValue = interpolator.Update(frameDeltaTime, time - maxDeltaTime, k_SmoothTickInterval, maxDeltaTime, lerp);
+            }
+
+            return currentValue;
+        }
+
+        /// <summary>
+        /// Lerp smoothing must still advance the value at 1.0f, the maximum legal value of the
+        /// <see cref="Components.NetworkTransform.PositionMaxInterpolationTime"/> family of fields.
+        /// </summary>
+        [Test]
+        public void LerpSmoothingDoesNotFreezeAtMaximumInterpolationTime([Values] bool lerp)
+        {
+            var result = RunLerpSmoothing(1.0f, 1.0f / 60.0f, lerp);
+
+            Assert.That(result, Is.GreaterThan(k_SmoothStartValue + 1.0f),
+                $"Interpolated value only advanced {result - k_SmoothStartValue} from {k_SmoothStartValue} over " +
+                $"{k_SmoothTotalDuration}s of authority motion. The maximum interpolation time froze the transform.");
+        }
+
+        /// <summary>
+        /// The rate at which lerp smoothing converges must not depend on the frame rate.
+        /// </summary>
+        [Test]
+        public void LerpSmoothingIsFrameRateIndependent()
+        {
+            // Heavier than the default, where the frame rate dependency is measurable.
+            const float maximumInterpolationTime = 0.87f;
+
+            var atThirtyFps = RunLerpSmoothing(maximumInterpolationTime, 1.0f / 30.0f, true);
+            var atTwoFortyFps = RunLerpSmoothing(maximumInterpolationTime, 1.0f / 240.0f, true);
+
+            Assert.That(atThirtyFps, Is.EqualTo(atTwoFortyFps).Within(0.01f),
+                $"The same elapsed time and interpolation settings produced {atThirtyFps} at 30fps but " +
+                $"{atTwoFortyFps} at 240fps. The smoothing rate is scaling with the frame rate.");
+        }
+
+        #endregion
     }
 }