Add tutorials for blend animation and retargeting

tutorials/animation/animation_tree.rst

@@ -188,6 +188,48 @@ 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+, in order for the blending results to be deterministic (reproducible and always consistent),
+the blended property values must 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`` 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.webp
+
+This means that the animation lacking that will treat those Positions as ``Vector2(0, 0)``.
+
+.. image:: img/blending2.webp
+
+This problem can be solved by adding a Property track for Position as an initial value to the ``RESET`` animation.
+
+.. image:: img/blending3.webp
+
+.. image:: img/blending4.webp
+
+.. 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 of more than 180 degrees
+from the initial value as blended animation.
+
+This can be useful for Skeleton3Ds to prevent the bones penetrating the body when blending 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 in a T-pose**.
+
+.. image:: img/blending5.webp
+
+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
 -----------

tutorials/assets_pipeline/index.rst

@@ -10,5 +10,6 @@ Assets pipeline
    importing_audio_samples
    importing_translations
    importing_scenes
+   retargeting_3d_skeletons
    exporting_3d_scenes
    escn_exporter/index

tutorials/assets_pipeline/retargeting_3d_skeletons.rst

@@ -0,0 +1,170 @@
+.. _doc_retargeting_3d_skeletons:
+
+Retargeting 3D Skeletons
+========================
+
+To share animations among multiple Skeletons
+--------------------------------------------
+
+Godot has Position/Rotation/Scale 3D tracks (hereafter called Transform tracks)
+with Nodepaths to bones for Skeleton bone animation. If you think that you can
+share animations between multiple Skeletons as long as the bone names are the
+same, that is not correct.
+
+Godot allows each bone to have a parent-child relationship and can have rotation
+and scale as well as position, which means that bones can have different Transform values.
+
+The Skeleton stores the Transform values necessary for the default pose as Bone Rest.
+If Bone Pose is equal to Bone Rest, it means that the Skeleton is in the default pose.
+
+.. note:: Godot 3.x and Godot 4.0+ have different Bone Pose behaviors.
+          In Godot 3.x, Bone Pose is relative to Bone Rest, but in Godot 4.0+,
+          it includes Bone Rest. See this `article <https://godotengine.org/article/animation-data-redesign-40>`__.
+
+Skeletal models have different Bone Rests depending on the environment from
+which they were exported. For example, the bones of a glTF model output from Blender
+have Edit Bone Orientation as the Bone Rest rotation. However, there are skeletal
+models without any Bone Rest rotations, such as the glTF model output from Maya.
+
+To share animations in Godot, it is necessary to match Bone Rests as well as Bone Names,
+moreover to remove unwanted tracks in some cases. In Godot 4.0+, you can do that using
+the scene importer.
+
+Options for Retargeting
+-----------------------
+
+Bone Map
+~~~~~~~~
+
+When you select the Skeleton3D node on the scene importer, you can find the Retarget
+section on the inspector.
+
+.. image:: img/retargeting1.webp
+
+With the Skeleton node selected in the importer, first set up a new BoneMap and SkeletonProfile.
+Godot has a preset called SkeletonProfileHumanoid for humanoid models.
+This tutorial will proceed with the assumption that you will use SkeletonProfileHumanoid.
+
+.. note:: If you need a profile for a model such as a beast or creature, you can export
+          a SkeletonProfile from SkeletonEditor with some information converted from Skeleton.
+
+When you use SkeletonProfileHumanoid, the auto-mapping will be performed when the
+SkeletonProfile is set. If the auto-mapping does not work well, you can map bones manually.
+
+.. image:: img/retargeting2.webp
+
+Any missing, duplicate or incorrect parent-child relationship mappings will be indicated
+by a magenta / red button (depending on the editor setting). It does not block the import process,
+but it warns that animations may not be shared correctly.
+
+.. note:: The auto-mapping uses pattern matching for the bone names. So we recommend
+          to use common English names for bones.
+
+After you set up the BoneMap, several options are available in the sections below.
+
+.. image:: img/retargeting3.webp
+
+Remove Tracks
+~~~~~~~~~~~~~
+
+If you import resources as an AnimationLibrary that will be shared, we recommend to enable these options.
+However, if you import resources as scenes, these should be disabled in some cases.
+For example, if you import a character with animated accessories,
+these options may cause the accessories to not animate.
+
+Except Bone Transform
+^^^^^^^^^^^^^^^^^^^^^
+
+Removes any tracks except the bone Transform track from the animations.
+
+Unimportant Positions
+^^^^^^^^^^^^^^^^^^^^^
+
+Removes Position tracks other than ``root_bone`` and ``scale_base_bone``
+defined in SkeletonProfile from the animations. In SkeletonProfileHumanoid,
+this means that to remove Position tracks other than Root and Hips.
+Since Godot 4.0+, animations include Bone Rest in the transform value. If you disable this option,
+the animation may change the body shape unpredictably.
+
+Unmapped Bones
+^^^^^^^^^^^^^^
+
+Removes unmapped bone transform tracks from the animations.
+
+Bone Renamer
+~~~~~~~~~~~~
+
+Rename Bones
+^^^^^^^^^^^^
+
+Rename the mapped bones.
+
+Unique Node
+^^^^^^^^^^^
+
+Makes Skeleton a unique node with the name specified in the Skeleton Name.
+This allows the animation track paths to be unified independent of the scene hierarchy.
+
+Rest Fixer
+~~~~~~~~~~
+
+Reference Poses defined in SkeletonProfileHumanoid have the following rules:
+
+* The humanoid is T-pose
+* The humanoid is facing +Z in the Right-Handed Y-UP Coordinate System
+* The humanoid should not have a transform as Node
+* Directs the +Y axis from the parent joint to the child joint
+* +X rotation bends the joint like a muscle contracting
+
+These rules are convenient definitions for blend animation and Inverse Kinematics (IK).
+If your model does not match this definition, you need to fix it with these options.
+
+Apply Node Transform
+^^^^^^^^^^^^^^^^^^^^
+
+If the asset is not exported correctly for sharing, the imported Skeleton may have
+a transform as a Node. For example, a glTF exported from Blender with no Apply Transform
+executed is one such case. It looks like the model matches the definition,
+but the internal transforms are different from the definition.
+This option fixes such models by applying transforms on importer.
+
+.. note:: If the imported scene contains objects other than Skeletons, this option may have a bad effect.
+
+Normalize Position Tracks
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Position track is used mostly for model movement, but sharing the moving animation
+between models with different heights may cause the appearance of slipping
+due to the difference in stride length. This option normalizes the Position track values
+based on the ``scale_base_bone`` height. The ``scale_base_bone`` height is stored in the Skeleton,
+and the normalized Position track values is multiplied by that value on playback.
+If this option is disabled, the Position tracks is not normalized
+and the Skeleton's ``motion_scale`` is always imported as ``1.0``.
+
+With SkeletonProfileHumanoid, ``scale_base_bone`` is Hips, therefore the Hips' height is used as the Motion Scale.
+
+Overwrite Axis
+^^^^^^^^^^^^^^
+
+Unifies the models' Bone Rest by overwriting it to match the Reference Pose defined in the SkeletonProfile.
+
+.. note:: This is the most important option for sharing animations in Godot 4.0+,
+          but be aware that this option can produce horrible results **if the original Bone Rest set externally is important**.
+          If you want to share animations with keeping the original Bone Rest,
+          consider to use the `Realtime Retarget Module <https://github.com/TokageItLab/realtime_retarget>`__.
+
+Fix Silhouette
+^^^^^^^^^^^^^^
+
+Attempts to make the model's silhouette match that of the reference pose defined in the SkeletonProfile,
+such as T-Pose. This cannot fix silhouettes which are too different, and it may not work for fixing bone roll.
+
+With SkeletonProfileHumanoid, this option does not need to be enabled for T-pose models,
+but should be enabled for A-pose models. However in that case, the fixed foot results
+may be bad depending on the heel height of the model, so it may be necessary to add
+the SkeletonProfile bone names you do not want fixed in the Filter array, as in the below example.
+
+.. image:: img/retargeting4.webp
+
+Also, for models with bent knees or feet, it may be necessary to adjust the ``scale_base_bone`` height.
+For that, you can use Base Height Adjustment option.