fix: half float delta position dither

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -13,6 +13,8 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Changed
 
+- Changed `NetworkTransform.UseHalfFloatPrecision` to synchronize position with a resolution of approximately 1mm regardless of how far an object has travelled. Previously the resolution could degrade to approximately 3cm. This does not increase bandwidth, but projects using `NetworkTransform.UseUnreliableDeltas` will send full precision position updates more often. (#4128)
+
 
 ### Deprecated
 
@@ -22,6 +24,8 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Issue where objects using `NetworkTransform.UseHalfFloatPrecision` appeared to jitter on non-authority instances while they were stationary or coming to rest, even though the authority was not moving them. (#4128)
+
 
 ### Security
 

com.unity.netcode.gameobjects/Runtime/Components/NetworkDeltaPosition.cs

@@ -11,7 +11,14 @@
     [Serializable]
     public struct NetworkDeltaPosition : INetworkSerializable
     {
-        internal const float MaxDeltaBeforeAdjustment = 64f;
+        /// <summary>
+        /// How far the delta may grow before it is folded into the base position.
+        /// </summary>
+        /// <remarks>
+        /// This determines the transmitted position resolution, since a half float's step size grows with its
+        /// magnitude. Keeping the delta small keeps that step small: at 2 the coarsest step is roughly 1mm.
+        /// </remarks>
+        internal const float MaxDeltaBeforeAdjustment = 2f;
 
         /// <summary>
         /// The HalfVector3 used to synchronize the delta in position
@@ -138,14 +145,29 @@
         {
             CollapsedDeltaIntoBase = false;
             NetworkTick = networkTick;
-            DeltaPosition = (vector3 + PrecisionLossDelta) - CurrentBasePosition;
             for (int i = 0; i < HalfVector3.Length; i++)
             {
                 if (HalfVector3.AxisToSynchronize[i])
                 {
+                    var rawDelta = vector3[i] - CurrentBasePosition[i];
+
+                    // Adding the previous rounding loss back in keeps the average position accurate while the
+                    // value is moving, but it also changes the value being sent. Once the value stops moving
+                    // that is all it does, which makes a stationary object appear to oscillate.
+                    var movedSinceLastSend = Mathf.Abs(vector3[i] - PreviousPosition[i]);
+                    var applyPrecisionLoss = movedSinceLastSend >= HalfPrecisionQuantum(rawDelta);
+
+                    DeltaPosition[i] = applyPrecisionLoss ? rawDelta + PrecisionLossDelta[i] : rawDelta;
+
                     HalfVector3.Axis[i] = math.half(DeltaPosition[i]);
                     HalfDeltaConvertedBack[i] = Mathf.HalfToFloat(HalfVector3.Axis[i].value);
-                    PrecisionLossDelta[i] = DeltaPosition[i] - HalfDeltaConvertedBack[i];
+
+                    // Left unchanged when skipped so it is still applied once movement resumes.
+                    if (applyPrecisionLoss)
+                    {
+                        PrecisionLossDelta[i] = DeltaPosition[i] - HalfDeltaConvertedBack[i];
+                    }
+
                     if (Mathf.Abs(HalfDeltaConvertedBack[i]) >= MaxDeltaBeforeAdjustment)
                     {
                         CurrentBasePosition[i] += HalfDeltaConvertedBack[i];
@@ -165,6 +187,26 @@
             }
         }
 
+        /// <summary>
+        /// The smallest change a half float can represent at the magnitude of the value passed in.
+        /// </summary>
+        /// <param name="value">The value to get the step size for.</param>
+        /// <returns>The distance to the next representable half float value.</returns>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static float HalfPrecisionQuantum(float value)
+        {
+            // The step size is symmetric about zero, so the sign is dropped.
+            var magnitude = (ushort)(math.half(value).value & 0x7FFF);
+
+            // Guard only: stepping past the largest finite half float would give infinity.
+            if (magnitude >= 0x7BFF)
+            {
+                return MaxDeltaBeforeAdjustment;
+            }
+
+            return Mathf.HalfToFloat((ushort)(magnitude + 1)) - Mathf.HalfToFloat(magnitude);
+        }
+
         /// <summary>
         /// Constructor
         /// </summary>