UT-3746 add unreleased changes to changelog (#545)

* add unreleased changes to changelog

* code review fix - add more details + update doc

Author: vkovec

CHANGELOG.md

@@ -9,14 +9,23 @@
 - Rename "Show Convert UI" in project settings to "Display Options Window" and use it for both the Convert and Export windows instead of just the Convert to Prefab window.
 - Made FBX SDK bindings Editor only so they are not copied into builds by default. In order to use at runtime, 
   add the FBXSDK_RUNTIME define to Edit > Project Settings... > Player > Other Settings > Scripting Define Symbols.
+- Use DeepConvertScene from Autodesk® FBX SDK to convert from Unity (left handed system with odd parity) to Autodesk® Maya® (right handed system with odd parity) axis system.
+    - Previously the conversion was performed by the FBX exporter.
+    - For the majority of cases there will be no noticeable difference in the final export result. If any custom export code that is affected by the axis system has been added or modified, it will
+      need to use Unity's axis system (left handed with odd parity), instead of converting directly to Autodesk® Maya®'s.
+    - There may be discrepancies in the Aim constraint and Parent constraint export result compared to before, however these should still import correctly into Autodesk® Maya® and other DCCs.
+- Export animation curve tangents instead of baking animation.
+    - For more details see https://docs.unity3d.com/Packages/[email protected]/manual/exporting.html#animation
 
 ### Fixed
 - Fix Export Model and Convert to Prefab Variant setting presets not serializing settings properly.
 - Fix NullReferenceException when modifying a preset for the FBX export settings in Edit > Project Settings > Fbx Export.
 - Fix error in an export when the project settings are not writeable (e.g. if you're using Perforce).
 - FBX SDK bindings no longer included in builds, fixing an issue with shipping on the Mac App Store.
 
-
+### Known issues
+- Negative scale will not export properly.
+    
 ## [3.2.1-preview.2] - 2020-08-05
 ### Added
 - Add an export option to preserve model import settings when overwriting an fbx file.

com.unity.formats.fbx/Documentation~/exporting.md

@@ -195,7 +195,14 @@ In addition, it also exports the following animated attributes:
   - Up Vector (Aim Constraint)
   - Aim Vector (Aim Constraint)
 
-
+Animation curve tangents are exported, with the exception of rotation curves for objects with a prerotation (i.e. bones of skinned meshes).
+
+The reason for this is that prerotation and rotation are combined in Unity, but exported and stored separately in FBX. 
+The rotation curves for prerotation and rotation are also stored together as a single curve in Unity.
+The rotations are separated at export into two separate fields (prerotation and local rotation), however splitting the rotation curves in the same
+way is not as straightforward. In order for the rotation curve to export properly (with a static prerotation and animated local rotation), the prerotation would need to be removed at each key.
+This would affect not only the values at each key, but also the key tangents, which would need to be recalculated. 
+For this reason, the rotation curves of skinned mesh bones are baked at each frame (with the prerotation factored out), to ensure that the result matches the original animation.
 
 ## Export Options window