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This commit adds support for *masks* to the animation graph. A mask is a
set of animation targets (bones) that neither a node nor its descendants
are allowed to animate. Animation targets can be assigned one or more
*mask group*s, which are specific to a single graph. If a node masks out
any mask group that an animation target belongs to, animation curves for
that target will be ignored during evaluation.
The canonical use case for masks is to support characters holding
objects. Typically, character animations will contain hand animations in
the case that the character's hand is empty. (For example, running
animations may close a character's fingers into a fist.) However, when
the character is holding an object, the animation must be altered so
that the hand grips the object.
Bevy currently has no convenient way to handle this. The only workaround
that I can see is to have entirely separate animation clips for
characters' hands and bodies and keep them in sync, which is burdensome
and doesn't match artists' expectations from other engines, which all
effectively have support for masks. However, with mask group support,
this task is simple. We assign each hand to a mask group and parent all
character animations to a node. When a character grasps an object in
hand, we position the fingers as appropriate and then enable the mask
group for that hand in that node. This allows the character's animations
to run normally, while the object remains correctly attached to the
hand.
Note that even with this PR, we won't have support for running separate
animations for a character's hand and the rest of the character. This is
because we're missing additive blending: there's no way to combine the
two masked animations together properly. I intend that to be a follow-up
PR.
The major engines all have support for masks, though the workflow varies
from engine to engine:
* Unity has support for masks [essentially as implemented here], though
with layers instead of a tree. However, when using the Mecanim
("Humanoid") feature, precise control over bones is lost in favor of
predefined muscle groups.
* Unreal has a feature named [*layered blend per bone*]. This allows for
separate blend weights for different bones, effectively achieving masks.
I believe that the combination of blend nodes and masks make Bevy's
animation graph as expressible as that of Unreal, once we have support
for additive blending, though you may have to use more nodes than you
would in Unreal. Moreover, separating out the concepts of "blend weight"
and "which bones this node applies to" seems like a cleaner design than
what Unreal has.
* Godot's `AnimationTree` has the notion of [*blend filters*], which are
essentially the same as masks as implemented in this PR.
Additionally, this patch fixes a bug with weight evaluation whereby
weights weren't properly propagated down to grandchildren, because the
weight evaluation for a node only checked its parent's weight, not its
evaluated weight. I considered submitting this as a separate PR, but
given that this PR refactors that code entirely to support masks and
weights under a unified "evaluated node" concept, I simply included the
fix here.
A new example, `animation_masks`, has been added. It demonstrates how to
toggle masks on and off for specific portions of a skin.
This is part of #14395, but I'm going to defer closing that issue until
we have additive blending.
[essentially as implemented here]:
https://docs.unity3d.com/560/Documentation/Manual/class-AvatarMask.html
[*layered blend per bone*]:
https://dev.epicgames.com/documentation/en-us/unreal-engine/using-layered-animations-in-unreal-engine
[*blend filters*]:
https://docs.godotengine.org/en/stable/tutorials/animation/animation_tree.html
## Migration Guide
* The serialized format of animation graphs has changed with the
addition of animation masks. To upgrade animation graph RON files, add
`mask` and `mask_groups` fields as appropriate. (They can be safely set
to zero.)
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Bevy Animation
