A deep look into animation framework in ue4

It seems that Epic prefers telling developers how to use animation blueprint in Unreal Engine 4. But me, as a programmer, still needs to get to know what the animation system actually works underlying.

Relevant Data Structure

USkeletalMeshComponent

This is the where we setup our skeletal mesh in UE4. We use this component to handle game-play stuff. But it seems that UE4 want to decoupling game-play and animation system, as a result, you cannot write bone transform using this component.

UAnimInstance

You may not be very familiar with this class. But you must be very familiar with its child classes — AnimBlueprint.

We can choose an Anim Class for a USkeletalMeshComponent. As we just mentioned that UE4 want to decoupling game-play and animation system. And since USkeletalMeshComponent focus on game-play stuff, it needs UAnimInstance to handle animation stuff.

UAnimInstance* USkeletalMeshComponent::GetAnimInstance() const
{
    return AnimScriptInstance;
}

AnimBlueprint always inherits from UAnimInstance. I am pretty sure that you have played with Blueprint Update Animation event… We always setup our animation data in this event.

This event is called in UAnimInstance::UpdateAnimation() function, which is called in USkeletalMeshComponent::TickPose(), which is called in USkeletalMeshComponent::TickComponent.

By going through UAnimInstance::UpdateAnimation function, it’s easy to know what is happening in an AnimBlueprint.

1. By PreUpdateAnimation, the UAnimInstance would call PreUpdate function of its AnimInstanceProxy. And an AnimNode uses PreUpdate to update its game-play data.
2. Then montages would be updated. Montage needs to be updated BEFORE node update or Native Update, so that node knows where montage is.
3. Next it is time to gather game-play data for the AnimInstance. For C++ stuff, NativeUpdateAnimation is called. For Blueprint, BlueprintUpdateAnimation is called. And this is exactly Update Event in Anim Blueprint.
4. Then PostUpdateAnimation is called. At this point, notifies are handled.

Seems simple, isn’t it?

FAnimInstanceProxy

FAnimInstanceProxy is :

the proxy object passed around during animation tree update in lieu of a UAnimInstance.

This class contains almost everything you need for your AnimGraph. You can handle bones, notifies, state machines, pose snapshot, etc, in this struct.

FAnimInstanceProxy has the access to the root node of the AnimGraph. UE4 does a recursive update or evaluation starting from the root node.