A review on self transition in animation graph

[shrinktofit]

In animation graph, self transitions are vague edge case. Here we take about two kinds of self transitions: state transition and interruption.

State transition

Our animation graph now allows self transitions. It means the following constructions may appear in animation graph, where A is a motion state and SM is a state machine containing A:

• A -> A

  In this case, user connected a transition from A to itself.

• SM/A -> SM/Exit -> SM/Entry -> SM/A

  In this case, user connected a transition from A to Exit, and the SM has a transition on itself, and the SM/Entry select to transition into A again.

As for the "indirect" self transition, we won't talk the topic in this section since if no interruption happened, only one transition can happen at a time. We will talk about that in next section.

I had considered this case in early stage of design. My answer once was: the self transitions are valid and have definite sementic. For example, if a motion state A was proceeded at 0.3s, a self transition cause the whole animation graph (layer) to be proceeded as $transitionBlend(A_{0.3s}, A_{0.0s})$ and then, as time elapsed, $transitionBlend(A_{0.4s}, A_{0.1s})$...etc .

Well, I said yes to this, is because I think there may be a case: the game character is skill ACTing and when the user press a key, that skill will be reset to its beginning animation.

No matter if the requirement exists in real world, I allowed them in current version though. To achieve this, I internally implemented the self-transition as: a motion state may have two ports: a "from" port and a "to" port. Both port sample the same animation sequence(clip or animation blend) but each port has its own timer. When a transition happen, the from port of transition source and the to port transition destination are sampled. You can see the relavent code here: https://github.com/cocos/cocos-engine/blob/v3.6.2/cocos/core/animation/marionette/graph-eval.ts#L1633

But as of now, I'm gonna blame on the "self-transition" for the reason listed as follow.

• Hard to teach.

  Because self transitions, a state is actually divided into two internal states("from" and "to").

• Make it hard to implement function states

  TODO