fix: half float position encoding manufactures motion on resting objects

NetworkDeltaPosition carries the half float rounding loss of each update
into the next one, which keeps the average transmitted position accurate
while a value is moving. The loss alone is enough to change the encoded
delta, so once the value stops moving that mechanism keeps changing what is
sent even though the position has not moved. The encoded value alternates
between neighbouring representable values and a stationary object is
transmitted as one that oscillates. The rounding loss is now only carried
forward while the value moves by at least one representable step.

MaxDeltaBeforeAdjustment also determined the transmitted resolution, since
a half float's step size grows with its magnitude. At 64 the coarsest step
was 31.25mm, so objects away from their base position were reproduced in
~3cm increments. At 2 it is 0.977mm. Folding the delta into the base more
often costs no bandwidth with reliable deltas because both sides apply the
same rule to the same value, and the reconstructed position is unchanged by
the fold. UseUnreliableDeltas forces a full precision base synchronization
per fold, so those projects will send those more often.

Measured on 10 settling physics objects with half float enabled: 28-42mm of
oscillation before, none after, matching the same scene with half float
disabled. Objects in motion improve as well, peak error dropping from
12.5mm to 0.587mm.

Sender and receiver must agree on MaxDeltaBeforeAdjustment, so this is not
compatible across builds. NetworkConstants.PROTOCOL_VERSION already
participates in the connection config hash, so mismatched versions cannot
connect.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: NoelStephensUnity

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -13,22 +13,21 @@ 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.
+
 - All editor assembly definitions are renamed with `Unity.Netcode.GameObjects.x` variants
   - `Unity.Netcode.Editor` → `Unity.Netcode.GameObjects.Editor`
   - `Unity.Netcode.Editor.CodeGen` → `Unity.Netcode.GameObjects.Editor.CodeGen`
   - `Unity.Netcode.Editor.PackageChecker` → `Unity.Netcode.GameObjects.Editor.PackageChecker`
   - `Unity.Netcode.Editor.Tests` → `Unity.Netcode.GameObjects.Editor.Tests`
 
-
-
 ### Deprecated
 
-
 ### Removed
 
-
 ### 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.
 - Issue with not being able to spawn initially disabled in-scene placed objects. (#4093)
 - Issue with pre-instantiated network prefab instances being marked as in-scene placed. Now pre-instantiated network prefabs are dynamically spawned. (#4093)
 - Issue where a user could spawn runtime created `NetworkObject` that has a GlobalObjectIdHash of zero. These are not valid instances and will no longer be allowed to spawn. (#4093)

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

@@ -11,7 +11,14 @@ namespace Unity.Netcode.Components
     [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 @@ public void UpdateFrom(ref Vector3 vector3, int networkTick)
         {
             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 @@ public void UpdateFrom(ref Vector3 vector3, int networkTick)
             }
         }
 
+        /// <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>