Animation Tree fakes default keyframes for non-keyframed values

[jcarlosrc]

Godot version

4.0 alpha 5 and alpha 6

System information

Windows 10

Issue description

Tracks that are not keyframed should not be animated. However, Animation Tree is using default values (at least for position and color) as they were keyframed values, causing animation issues.

In the videos: First, The expected behavior (Godot 4.0 Alpha 4 and all previous version): Sphere does not move because it is not keyframed in the one shot animation. Second, Alpha 5 and 6: the sphere moves to its default (0,0,0) position, however there is not keyframed at all at that position.

This issue happens also with color. To see it, make the Color Rect node visible and play the game. Transparency goes back to 0 causing scene not to be visible. This can not be an on-purpose behavior, because each animation should contain a track for every single node and position to behave properly, which does not make sense.

A4Behavior.mp4

A6Behavior.mp4

Steps to reproduce

Play the game with alpha 4 and alpha 6 versions. Pressing "Enter" activates the one shot animation, showing the bug.

Minimal reproduction project

BugReportAnimTreeAlpha6.zip

[TokageItLab]

I say again, as written in #59474 (comment). At least the #59474 issue is a bug, but the missing key animation is now blended with the initial value, which is an ideal change to make the blending result always consistent. This change was necessary to solve the bigger problem #46038. I proposed an implementation that use ResetTrack as the default value, but that was rejected in discussion with @reduz.
However, I am concerned that the inability to use Rest as InitValue may be a problem for 2DSkeleton, which does not have a TRS track like 3D. We will discuss that later with reduz.

[jcarlosrc]

First, the color rect behavior is not an isolated bug, is the result of the same behavior.
Secondly. As I can see it, it does not make sense. If we have an animation with just one track and need to mix it with another animation having 100 tracks. We want it just to mix the one track. However, we will need to copy all the 100 tracks and its keyframes to have the desired effect? In the videos avobe, I will need to copy all the keyframes for the sphere position to make it still. In this case is only one position, but it could be much more complex.This was never an issue before and should not become one now.
Third: Keyframing values the user never intented to keyframe can not be an ideal solution at all. If we keyframe a value and only one value is because we want that behavior. Mixing with a default value is the same as keyframing a value that was never intended to be keyframed.

[TokageItLab]

As a result of the fix in #59490, the missing Color track is indeed blended with the transparent value, but it is not a bug. It is just blended with the init value (zero). The bug in #59474 is due to the fact that the default Alpha value for Color is actually 1 instead of 0. This breaks the alpha channel blend even when the Color track is not missing. You must first understand the difference.

If we don't want to blend with the 0 value, we need at least some intended default value similar to the ResetTrack solution.

@reduz Do you think AnimationLibrary will solve this?

[jcarlosrc]

@TokageItLab Please consider the following minimal situation: We have two properties p and q, let's say legs and arms We have animations A1 and A2 with keyframes for both legs and arms (idle an walk). And a third animation B with only keyframes for arms (say hello). We want to mix A1 or A2 with B, replacing only arms. We only want to replace arms, but let legs unaltered to move. How could we achieve this under the current behavior?

• If we mix A1 or A2 with B, the engine will create a default keyframe for legs in B, causing legs to change (as seen in the videos).
• If we "complete" animation B to include legs, we will need to decide how. If we copy A1, it will mess up animation A2 when we want to mix B with A2. The only solution is creating another B, to match with each A. This is not a solution, because we could have many A, and will need to create as many B, leading to an exponential number of copies just for a simple behavior.

The solution is just not mixing legs at all, which does not have keyframes in B. In this way, only keyframed tracks (arms) get mixed, which makes sense.

[jcarlosrc]

As a result of the fix in #59490, the missing Color track is indeed blended with the transparent value, but it is not a bug. It is just blended with the init value (zero). The bug in #59474 is due to the fact that the default Alpha value for Color is actually 1 instead of 0. This breaks the alpha channel blend even when the Color track is not missing. You must first understand the difference.

If we don't want to blend with the 0 value, we need at least some intended default value similar to the ResetTrack solution.

@reduz Do you think AnimationLibrary will solve this?

Yes, the color is a bug because it has not a missing track. My fault.