uni-42394: handle non-identity transforms when transfering animation

Author: Benoit Hudson

Assets/FbxExporters/Editor/FbxExporter.cs

@@ -1889,8 +1889,8 @@ protected void ExportAnimationClip (AnimationClip uniAnimClip, GameObject uniRoo
             /// Transfers transform animation from source to dest. Replaces dest's Unity Animation Curves with updated animations.
             /// NOTE: Source must be the parent of dest.
             /// </summary>
-            /// <param name="source">Source.</param>
-            /// <param name="dest">Destination.</param>
+            /// <param name="source">Source animated object.</param>
+            /// <param name="dest">Destination, child of the source.</param>
             /// <param name="sampleRate">Sample rate.</param>
             /// <param name="unityCurves">Unity curves.</param>
             private void TransferMotion(Transform source, Transform dest, float sampleRate, ref Dictionary<GameObject, List<UnityCurve>> unityCurves){
@@ -1933,14 +1933,23 @@ private void TransferMotion(Transform source, Transform dest, float sampleRate,
                     scaleKeyFrames[k] = new Keyframe[sampleTimes.Count];
                 }
 
+                // If we have a point in local coords represented as a column-vector x, the equation of x in coordinates relative to source's parent is:
+                //   x_grandparent = source * dest * x
+                // Now we're going to change dest to dest' which has the animation from source. And we're going to change
+                // source to source' which has no animation. The equation of x will become:
+                //   x_grandparent = source' * dest' * x
+                // We're not changing x_grandparent and x, so we need that:
+                //   source * dest = source' * dest'
+                // We know dest and source (both animated) and source' (static). Solve for dest':
+                //   dest' = (source')^-1 * source * dest
                 int keyIndex = 0;
+                var sourceStaticMatrixInverse = Matrix4x4.TRS(source.localPosition, source.localRotation, source.localScale).inverse;
                 foreach (var currSampleTime in sampleTimes) 
                 {
                     var sourceLocalMatrix = GetTransformMatrix (currSampleTime, source, sourceUnityCurves);
                     var destLocalMatrix = GetTransformMatrix (currSampleTime, dest, destUnityCurves);
 
-                    // child * parent
-                    var newLocalMatrix = sourceLocalMatrix * destLocalMatrix;
+                    var newLocalMatrix = sourceStaticMatrixInverse * sourceLocalMatrix * destLocalMatrix;
 
                     FbxVector4 translation, rotation, scale;
                     GetTRSFromMatrix (newLocalMatrix, out translation, out rotation, out scale);