UE4 source code reading_ Bone model and animation system_ Animation process

0. Write it at the front

This article is a collection of personal learning notes , If there is a mistake , I hope you can point out .

This rough description UE4 The main process of each frame animation calculation in , Relevant codes involved in this process , Because of the diversity of animation settings , There is also quite a lot of branch code .

This article only describes the most basic animation of the character , from SkeletalMeshComp The process of calculating the pose of the animation playback node in the animation blueprint .

1. SkeletalMeshComp technological process

【AnimInstance】

Animation examples , This class is the core of controlling animation logic , You can think of it as one AnimInstance The object corresponds to an animation blueprint .

SkeletalMeshComp Three kinds of AnimInstance object ：

• AnimScriptInstance： The currently active animation blueprint object
• PostProcessAnimInstance
• LinkedInstances

【 entrance 】

1）SkinnedMeshComp->TickComponent

SkinnedMeshComp Medium TickComponent Is the interface that will be called every frame , This interface is also the entrance of animation update
Two important functions ：

• TickPose： Deal with non parallel parts of animation computation
• RefreshBoneTransforms： Deal with the multi-threaded parallel part of animation Computing , And by AnimInstanceProxy Handle

notes ：UE4 The animation blueprint is set to multi-threaded update by default , Most of the data calculation in animation calculation is done by AnimInstanceProxy Be responsible for parallel processing in multithreading .

2）SkeletalMeshComp->TickPose：

• Calculation tick Time interval of DeltaTimeForTick
• call TickAnimation(DeltaTimeForTick, bNeedsValidRootMotion) Update three animation instances
  • call AnimInstance Of UpdateAnimation, Here we deal with animation in the part of game thread calculation
  • The update order is ： To update LinkedInstances, Update again AnimScriptInstance, The last update PostProcessAnimInstance

3）SkeletalMeshComp->RefreshBoneTransforms Interface ：

• DispatchParallelEvaluationTasks, Responsible for initiating multi-threaded computing ：
  • SwapEvaluationContextBuffers：
    Exchange context data in buffer before calculation , hold SkeletalMeshComp Some animation data cached in is exchanged to AnimEvaluationContext.
    AnimEvaluationContext（FAnimationEvaluationContext）： The variable is in SkeletalMeshComp Is responsible for caching all data when calculating the current animation pose of the character .
  • Launch two Task：
    • FParallelAnimationEvaluationTask： The Task call ParallelAnimationEvaluation
      • PerformAnimationProcessing： This function is mainly responsible for adjusting the Update and Evaluate Interface , These two interfaces will update the animation data node by node , Then update the animation pose . Finally, apply animation pose data .
      • ParallelDuplicateAndInterpolate：
    • FParallelAnimationCompletionTask： The Task call CompleteParallelAnimationEvaluation
      • SwapEvaluationContextBuffers： take AnimEvaluationContext Data exchange back SkeletalMeshComp Medium cache , Wait for the rendering thread to call
      • PostAnimEvaluation(AnimEvaluationContext)：

2. AnimInstance&&Proxy technological process

【 Game threads 】

It was said that SkeletalMeshComp Can tune AnimInstance Of UpdateAnimation Interface handles the logic of the animation blueprint in the game thread calculation .

• UpdateAnimation
  • Update before the animation node logic updates Montage
    UpdateMontage(DeltaSeconds)
    UpdateMontageSyncGroup
    UpdateMontageEvaluationData
  • BlueprintUpdateAnimation, Update the entry of various animation logic in the blueprint , The specific implementation is the animation tree of the blueprint
  • Determine whether the animation needs to be updated immediately , If you don't need to update the animation immediately , Will be called by multiple threads Proxy Parallel computation
    const bool bWantsImmediateUpdate = bNeedsValidRootMotion || NeedsImmediateUpdate(DeltaSeconds);
    
    If you need to update immediately , Such as RootMotion Can't compute in parallel , Then call the update interface under this function
    ParallelUpdateAnimation();
    PostUpdateAnimation();

【 Multithreading 】

SkeletalMeshComp Of PerformAnimationProcessing Is responsible for calling AnimInstance Of Update and Evaluate, Update blueprint data , And pose data update .

Several key functions ：

• About Update：
  AnimInstance->ParallelUpdateAnimation
• About Evaluate：
  EvaluateAnimation
  EvaluatePostProcessMeshInstance
  FinalizePoseEvaluationResult
  FillComponentSpaceTransforms

1）ParallelUpdateAnimation

• call Stack , Called by other threads
  

• This function calls Proxy Of UpdateAnimation Interface , Look directly at Proxy：

  	FAnimationUpdateSharedContext SharedContext;
  	FAnimationUpdateContext Context(this, CurrentDeltaSeconds, &SharedContext);
  	UpdateAnimation_WithRoot(Context, RootNode, NAME_AnimGraph);
  

  Context： Create a context object
  UpdateAnimation_WithRoot： From the root node of the animation blueprint （ The last node , That is, animation output pose node ） Go back , Traverse all the animation nodes, and make Update.

2）Proxy->UpdateAnimation_WithRoot
Mainly do a few things ：

• CacheBones

• Proxy->UpdateAnimationNode(InContext)

  • By calling the... Of the root node Update_AnyThread Interface , Start at the root node , And keep going back , Update blueprint data
  • FAnimNode_Root->Update_AnyThread(Context)
    Find your previous node through the connection line N, Then call N Node Update_AnyThread
    Keep going back and forth , Until you reach the end node , There is no longer a connecting line .
    FPoseLink： Connect line objects .
    FPoseLink->LinkedNode： The node pointed by the connecting line

  The root node ：

The specific update logic needs to go deep into each different animation blueprint node logic , Tentatively .

3）SkeletalMeshComp->EvaluateAnimation

• AnimInstance->ParallelEvaluateAnimation
  call Stack ：
  
  Create a temporary EvaluationContext Context object
  After the calculation , take Curve and Pose Assign the result to Comp Of AnimEvaluationContext

  	FPoseContext EvaluationContext(&Proxy);
  	EvaluationContext.ResetToRefPose();
  	Proxy.EvaluateAnimation(EvaluationContext);
  	OutCurve.CopyFrom(EvaluationContext.Curve);
  	OutPose.CopyBonesFrom(EvaluationContext.Pose);
  

• Proxy.EvaluateAnimation(EvaluationContext)
• Proxy.EvaluateAnimation_WithRoot(Output, RootNode)
  • CacheBones
  • EvaluateAnimationNode_WithRoot(Output, InRootNode)
    • and Update Similar process , It starts from the root node , Trace back through the connecting line , Until the end of the node .
    • But inside the specific node Evaluate Logic should determine and Update The number of nodes called is different

4）SkeletalMeshComp->EvaluatePostProcessMeshInstance

EvaluatePostProcessMeshInstance Also called EvaluateAnimation, The same update process , Just different objects ：PostProcessAnimInstance

5）SkeletalMeshComp->FinalizePoseEvaluationResult

• Calculation TArray& OutBoneSpaceTransforms
  The bone index will be compressed （CompactBoneIndex） Turn to mesh bone index （MeshBoneIndex）
  notes ： Three bone indexes ：CompactBoneIndex、MeshBoneIndex、SkeletalBoneIndex
• Output FVector& OutRootBoneTranslation：RootBone Relative displacement

6）SkeletalMeshComp->FillComponentSpaceTransforms

Get translation 、 rotate 、 Scaling vector , To update the pose matrix of each bone
Each bone matrix applies the local matrix of the parent bone one by one starting from the root bone , That is, the output of this function is a global pose

• OutComponentSpaceTransforms ：AnimEvaluationContext.CachedComponentSpaceTransforms, The final output global pose matrix
  InBoneSpaceTransforms： from Evaluate Calculate the input joint pose matrix
• Calculate the global pose
  FTransform::Multiply(SpaceBase, LocalTransformsData + BoneIndex, ParentSpaceBase);
  among SpaceBase yes OutTrans,LocalTransformData+BoneIndex It's based on index Indexed InTrans
  Multiply(FTransform* OutTransform, const FTransform* A, const FTransform* B);

3. AnimNode technological process

Animation playback node ：FAnimNode_SequencePlayer

The specific logic of animation playback is UAnimSequence object

1）UAnimSequence->GetBonePose (FCompactPose& OutPose, FBlendedCurve& OutCurve, const FAnimExtractContext& ExtractionContext, bool bForceUseRawData=false) const;

• initialization OutPose
  According to the settings , Use Retargeting Source Of T-Pose, Or its own T-Pose data , Initialize first OutPose
• EvaluateCurveData： Extract curve data
• DecompressPose： Decompress the compressed animation data , complete OutPose The calculation of

4. Reference material

https://docs.unrealengine.com/4.27/zh-CN/AnimatingObjects/SkeletalMeshAnimation/Optimization