Strange behavior after AddAnimation on Empty track under special conditions

abuki

Hi,

I am here again with problems partially related to the SkeletonAnimation.Update(0) call.
I checked the documentation here https://esotericsoftware.com/spine-api-reference#AnimationState-addAnimation2 and understand that AddAnimation should behave like SetAnimation in this case, but that does not explain following behavior. See the comments in example code:

private void Update ()
{
    if (Input.GetKeyDown(KeyCode.K))
        StartCoroutine(Testing());
}

private IEnumerator Testing()
{
    skeletonAnimation.AnimationState.SetAnimation(0, animationReference, false);
    yield return new WaitForSeconds(1f);

    skeletonAnimation.AnimationState.SetEmptyAnimation(0, 0);
    skeletonAnimation.Update(0); // Observe the change in behavior when there is Update or not
    yield return null;
    yield return null; // For bug to appear it is also required to wait at least 2 frames
            
    //Debug.Break();
    skeletonAnimation.AnimationState.AddAnimation(0, animationReference, false, 0.2f);
    // If Update(0) is not called, we see empty track progressing up to 0.2s delay, then mixing to new animation - this is correct
    // If Update(0) is called, the new animation is set directly but not progressing time up until 0.2s delay - this is wrong, why?
            
    // Also, you can change SetEmptyAnimation and Update lines for:
    // skeletonAnimation.AnimationState.ClearTrack(0);
    // Which produce the same behavior
            
    // You can also try removing the very first call to SetAnimation, then it works bad even without calling Update(0)
    // So there are multiple options how to produce the problem
}

abuki

Reading it again in the morning, it seems to me that the "correct" behavior is also wrong as the documentation states that If the track is empty, it is equivalent to calling setAnimation. But we can see empty progress to delay, so it looks like actually addAnimation. Maybe there is a difference between track playing empty animation and track which is empty = cleared? That should probably be more differentiated in the docs.

Note: progressing the empty to delay and then mixing to new animation looks as good preferred behavior to me.

Harald

abuki Reading it again in the morning, it seems to me that the "correct" behavior is also wrong as the documentation states that If the track is empty, it is equivalent to calling setAnimation.
Maybe there is a difference between track playing empty animation and track which is empty = cleared? That should probably be more differentiated in the docs.

Yes, this makes all the difference! These are two completely different things, and it's very important to understand the differece. If you assumed they were the same, this explains your confusion.

The empty animation is an animation where nothing is keyed. It is an animation, which leads to mixing in and mixing out. If you clear a track or have no animations on a track, or mixing has ended, you have no animation playing. When nothing is playing, no mixing happens.

Harald

abuki That should probably be more differentiated in the docs.

Thanks for the feedback, we will consider if we can improve the docs.

However IMO the documentation is already rather differentiated, looking at SetEmptyAnimation:

"Sets an empty animation for a track, discarding any queued animations, and sets the track entry's mixDuration. An empty animation has no timelines and serves as a placeholder for mixing in or out."

IMO that would be very strange and complicated wording if it meant the same as "clears the track". An animation is an animation, the empty animation is not no animation 🙂.

Also AnimationState reads:

"Applies animations over time, queues animations for later playback, mixes (crossfading) between animations, and applies multiple animations on top of each other (layering)."

Harald

@abuki From your comments on what you consider correct I assume that you have a misconception about how SetEmptyAnimation shall be used for mixing in and mixing out.

If you take a look at the SetEmptyAnimation documentation you can see:

Mixing out is done by setting an empty animation with a mix duration using either setEmptyAnimation, setEmptyAnimations, or addEmptyAnimation. [..] A mix duration of 0 still mixes out over one frame.
[..]
Mixing in is done by first setting an empty animation, then adding an animation using addAnimation with the desired delay (an empty animation has a duration of 0) and on the returned track entry, set the MixDuration.

Now from your line saying:

If Update(0) is called, the new animation is set directly but not progressing time up until 0.2s delay - this is wrong, why?

If no animation is set on the track, no mixing-in happens, animations are just played normally.

You first set the empty animation on the track with mixDuration 0 which mixes out the main animation over one frame. Then you're allowing multiple calls to Update happen, but want to receive mixing-in on the subsequent call to AddAnimation. Note that to mix in you need to immediately queue the empty animation before the animation, or have another animation playing on the track already.

The corner-case is the two frames where the empty animation stays on the track due to first mixing out the main animation for one frame (the first Update(0) or yield statement), then updating again to advance the <empty> track entry for a frame to discard it next Update. But before the next (3rd) Update, you're calling AddAnimation when the empty animation is still there, so you receive mixing-in. With the additional Update(0) call the empty animation is already removed before adding new animations.

In other words: the <empty> animation does not stay on the track, it is removed one frame after mixing out or after mixing in is finished. And if no animation (empty or other animation) is on a track, no mixing-in will happen. If you want mixing-in, you need to call SetEmptyAnimation without letting several frames pass.

I hope this makes sense and is not too confusing.

Harald

This documentation page is also rather explicit about when mixing happens and what the empty animation is for:
https://esotericsoftware.com/spine-applying-animations/#Empty-animations

abuki

Hi, thank you very much, I think I am starting to understand what is happening on our side.

Regarding the docs, I have two points:

https://esotericsoftware.com/spine-api-reference#AnimationState-addAnimation2 - wording of If the track is empty, it is equivalent to calling setAnimation. was misleading to me as I could easily think you mean trackentry which is playing empty animation. Now, I got that you really mean empty track itself (= cleared)
https://esotericsoftware.com/spine-api-reference#AnimationState-setEmptyAnimation might be nice to add link to https://esotericsoftware.com/spine-applying-animations/#Empty-animations as I wasn't aware again of this and your example there (set empty, add after) is probably what I will need to do in my case.

I will now deep dive into my code and report back when I solve all the issues.