Add tutorials for blend animation and retargeting

Author: TokageItLab

tutorials/animation/animation_tree.rst

@@ -188,6 +188,47 @@ Transitions also have a few properties. Click any transition and it will be disp
 * *Priority* is used together with the ``travel()`` function from code (more on this later). Lower priority transitions are preferred when travelling through the tree.
 * *Disabled* toggles disabling this transition (when disabled, it will not be used during travel or auto advance).
 
+For better blending
+-------------------
+
+In Godot 4.0+, for blending results consistently, the property values to be blended always have a specific initial value.
+For example, in the case of two animations to be blended, if one animation has a property track and the other does not,
+the blended animation is calculated as if the latter animation had a property track with the initial value.
+
+When using Position/Rotation/Scale 3D tracks for Skeleton3D bones, the initial value is Bone Rest.
+For other properties, the initial value is ``0`` (exactly the value of ``zero()`` as Variant)
+and if the track is present in the ``RESET`` animation, the value of its first keyframe is used instead.
+
+For example, the following AnimationPlayer has two animations, but one of them lacks a Property track for Position.
+
+.. image:: img/blending1.png
+
+This means that the animation lacking that will treat those Positions as ``Vector2(0, 0)``.
+
+.. image:: img/blending2.gif
+
+This problem can be solved by adding a Property track for Position as an initial value to the RESET animation.
+
+.. image:: img/blending3.png
+
+.. image:: img/blending4.gif
+
+.. note:: Be aware that the RESET animation exists to define the default pose when loading an object originally.
+          It is assumed to have only one frame and is not expected to be played back using the timeline.
+
+Also keep in mind that the Rotation 3D tracks and the Property tracks for 2D rotation
+with Interpolation Type set to Linear Angle or Cubic Angle will prevent rotation more than 180 degrees
+from the initial value as blended animation.
+
+This behavior is useful for Skeleton3D to prevent the bones penetrating the body when blend animations.
+Therefore, Skeleton3D's Bone Rest values should be as close to the midpoint of the movable range as possible.
+**This means that for humanoid models, it is preferable to import them with T-pose**.
+
+.. image:: img/blending5.gif
+
+You can see that the shortest rotation path from Bone Rests is prioritized rather than the shortest rotation path between animations.
+
+If you need to rotate Skeleton3D itself more than 180 degrees by blend animations for movement, you can use Root Motion.
 
 Root motion
 -----------