Automatic Rule Based Transition For Back-playing Animation

What is Automatic Rule Based Transition?

Automatic rule based transition is used, in most cases, if you want an auto-transition from source state to target state when the source animation ends.

For example, consider a state transition from State Jump_Land to Idle:

An alternative way to implement state transition condition is to use Get Relevant Time Remaining node like this:

But this is, to some extent, dirty and expensive. We cannot use Animation variable with it, even worse, Fast Path does not work with this way.

---

The proper method in this case is to check Automatic Rule Based on SequencePlayer in State of this transition:

Transition would start when there is 0.2 second left in the source state animation. This is clean and elegant.

Problem With Back-Playing Sequence

For the official UE4 engine, Automatic Rule Based Transition does not work with a back-playing sequence. Because transition only cares about the accumulated time of an animation asset.

if (UAnimationAsset* AnimAsset = RelevantPlayer->GetAnimAsset())
{
	const float AnimTimeRemaining = AnimAsset->GetMaxCurrentTime() - RelevantPlayer->GetAccumulatedTime();
	const FAnimationTransitionBetweenStates& TransitionInfo = GetTransitionInfo(TransitionRule.TransitionIndex);
	bCanEnterTransition = (AnimTimeRemaining <= TransitionInfo.CrossfadeDuration);
}

As a result, for a back-playing animation sequence, it does not work as expected. Hence some modifications are needed:

// FAnimNode_StateMachine::FindValidTransition
if (UAnimationAsset* AnimAsset = RelevantPlayer->GetAnimAsset())
{
	// Noah Zuo: Here we need to make it work with an asset player of minus play rate. 
	const bool AnimReversePlay = RelevantPlayer->GetEffectivePlayRate() < 0.f;
	const float AnimTimeRemaining = AnimReversePlay ? RelevantPlayer->GetAccumulatedTime() : AnimAsset->GetMaxCurrentTime() - RelevantPlayer->GetAccumulatedTime();

	const FAnimationTransitionBetweenStates& TransitionInfo = GetTransitionInfo(TransitionRule.TransitionIndex);
	// For a back-running playing asset player, we need to flip condition. 
	bCanEnterTransition = (AnimTimeRemaining <= TransitionInfo.CrossfadeDuration);
}

We need to add a function for each FAnimNode_AssetPlayerBase to get real play rate:

// FAnimNode_AssetPlayerBase
virtual float GetEffectivePlayRate() { return 1.f; }

And override this function in FAnimNode_SequencePlayer:

float FAnimNode_SequencePlayer::GetEffectivePlayRate()
{
	const float SequencePlayRate = (Sequence ? Sequence->RateScale : 1.f);
	const float AdjustedPlayRate = PlayRateScaleBiasClamp.ApplyTo(FMath::IsNearlyZero(PlayRateBasis) ? 0.f : (PlayRate / PlayRateBasis), 0.f);
	const float EffectivePlayrate = SequencePlayRate * AdjustedPlayRate;
	return EffectivePlayrate; 
}

And that’s it! Enjoy this feature!