参考：Animation Blend Profile in Unity
参考：Unreal Engine - How to make a Blend Profile
参考：blend-masks-and-blend-profiles-in-unreal-engine

什么是Blend Profile

Animation Blend Profile is a UE4 animation system feature that enables us to set blend speed of each bone separately.

这个概念是在UE4的动画系统里提出的feature，可以让我们设置每个Bone的blend speed。如果没有这玩意儿，正常动画Blend时，所有的Bone的Blend Speed都是相同的。

这个功能可以有效解决movement动画之间的transition造成的滑步问题，同时保证上半身动画的流畅性。

这种动画的转态，比如Idle和Moving的切换，悬空和落地的切换，都是极度需要Blend Profile功能的，我估计这个原理是让上半身的Blend Speed和下半身的BlendSpeed不同，从而保证上半身动画流畅的同时，解决下半身的滑步问题。

举个细节的例子，从动画A到动画B的Transition Time为0.5s，如果是线性Blend，那么正常Joint X在Blend时的A的权重为1，B的权重为0，那么0.5s后，A的权重会从1线性减少到0，B的权重从0线性增加到1。如果此时我把Joint Y的Blend Speed设置为X的两倍，那么Y会在0.25s内从A动画过渡到B，如下图所示：

UE的动画过渡里就可以使用Blend Profile，如下图所示：

而且Blend Space里也可以设置Blend Profile，因为Blend Space的本质就是处理动画的Blend，如下图所示是在Blend Space里的设置：

Blend Mask是什么

在UE的源码里，Blend Mask类型为EBlendProfileMode的一种，也就是说UE认为它是一种Blend Profile：

enum class EBlendProfileMode : uint8
{// The bone's transition time is a factor based on the transition time. // For example 0.5 means it takes half the time of the transition.// Values should be between 0 and 1. They will be clamped if they go out of this range.// A bone value of 0 means the bone will instantly transition into the target state.TimeFactor = 0,// 值必须<=1// The bone's transition weight is multiplied by this factor.// For example 2.0 means the bone's blend weight is twice as high as the transition's blend weight.// Values should typically be equal or greater than 1.0.// If you want certain bones to instantly transition into the target state// the Time Factor based method might be a better choice.WeightFactor,// 值必须>=1// Used for blend masks. Per bone alphaBlendMask UMETA(Hidden),
};

感觉UE里认为，能够改变joint影响Blend时的joint的权重设置的东西，都属于Blend Profile，所以它才把Blend Mask也归类为Blend Profile的一种。Blend Mask跟其他类型的Blend Profile不同，它并不会加速blend过程中各个joint的权重值的改变，它只能决定各个joint的权重值的改变是否随着MainWeight改变，改变的程度是多少(范围在[0, 1]区间)，类似于Unity的AvatarMask

Blend Mask的用法

创建和编辑的操作跟其他类型的Blend Profile是差不多的：

默认Blend Mask的每个Bone的BlendScale值为0，可供选择的范围在[0, 1]区间，0代表此Bone完全不受Blend Profile影响

具体使用时要通过动画蓝图的Layered Blend Per Bone节点实现，如下图所示：

蓝图连接逻辑如下图所示，其实这里的Blend Mask很像Unity里的Avatar Mask

Blend Profile的使用场景

Blend Mask应该是只能用在Layered Blend Per Bone里，而Weight Factor和Time Factor类型的Blend Profile会被用在这五个地方：

Blend Poses by bool.
Blend Poses by int.
Blend Poses by enum.
State Machine Transitions.
Animation Montages.

BlendSpace里有类似的设置，不过不是BlendProfile，而是叫Target Weight Interpolation Speed Per Sec。

这里的五种节点都会最终调用UBlendProfile::CalculateBoneWeight函数，计算每个Bone在Blend过程中的权重值，无非调用地点有所区别：

Blend Poses by bool，Blend Poses by int和Blend Poses by enum都继承于FAnimNode_BlendListBase类，会在里面的Update_AnyThread函数里被调用
State Machine Transitions则是在FAnimationActiveTransitionEntry::Update里被调用
Animation Montages则是通过AnimationSlot，在FAnimInstanceProxy::SlotEvaluatePoseWithBlendProfiles里被调用的，当AnimationSlotNode在其Evaluate函数里发现了处于Blend状态下的Montage，且设置了BlendProfile时，会调用SlotEvaluatePoseWithBlendProfiles函数

UE相关源码记录

这里拿动画状态机里BlendProfile的应用举例，我在动画的转态上加了个Blend Profile，源码里的CallStack顺序如下：

图中的展示顺序表示，最终调用的UBlendProfile::UpdateBoneWeights的caller是statieMachine对应的AnimNode（FAnimNode_StateMachine::Update_AnyThread函数），对应的状态机节点会遍历每个动画Transition的对象，然后根据是否存在BlendProfile改变骨骼权重：

void FAnimationActiveTransitionEntry::Update(const FAnimationUpdateContext& Context, int32 CurrentStateIndex, bool& bOutFinished)
{bOutFinished = false;// Advance timeif (bActive){ElapsedTime += Context.GetDeltaTime();Blend.Update(Context.GetDeltaTime());// If non-zero, calculate the query alphafloat QueryAlpha = 0.0f;if (CrossfadeDuration > 0.0f){QueryAlpha = ElapsedTime / CrossfadeDuration;}Alpha = FAlphaBlend::AlphaToBlendOption(QueryAlpha, Blend.GetBlendOption(), Blend.GetCustomCurve());if (Blend.IsComplete()){bActive = false;bOutFinished = true;}// Update state blend data (only when we're using per-bone)if (BlendProfile){for (int32 Idx = 0 ; Idx < 2 ; ++Idx){const bool bForwards = (Idx == 0);StateBlendData[Idx].TotalWeight = bForwards ? Alpha : 1.0f - Alpha;BlendProfile->UpdateBoneWeights(StateBlendData[Idx], Blend, 0.0f, StateBlendData[Idx].TotalWeight, !bForwards);}FBlendSampleData::NormalizeDataWeight(StateBlendData);}}
}

Montage里的Blend Profile

相关设置不在LayeredBlendPerBone或者AnimationSlotNode的蓝图Details面板上，而是在Montage资产里，如下图所示，可以选择在Blend In Montage和Blend Out Montage时使用Blend Profile：

三种模式下的BlendProfile

UE的枚举叫EBlendProfileMode:

/** The mode in which the blend profile should be applied. */
UENUM()
enum class EBlendProfileMode : uint8
{// The bone's transition time is a factor based on the transition time. // For example 0.5 means it takes half the time of the transition.// 注意这里设置0.5并不是0.5秒的意思, 而是指的TransitionTime的一半// Values should be between 0 and 1. They will be clamped if they go out of this range.// A bone value of 0 means the bone will instantly transition into the target state.TimeFactor = 0,// The bone's transition weight is multiplied by this factor.// For example 2.0 means the bone's blend weight is twice as high as the transition's blend weight.// Values should typically be equal or greater than 1.0.// If you want certain bones to instantly transition into the target state// the Time Factor based method might be a better choice.WeightFactor,// Used for blend masks. Per bone alphaBlendMask UMETA(Hidden),
};

TimeFactor和WeightFactor模式的区别在于权重的计算方法不同，其他基本没任何区别，WeightFactor算法其实很简单，直接根据原本的PoseWeight乘以对应的BlendProfile即可：

float UBlendProfile::CalculateBoneWeight(float BoneFactor, EBlendProfileMode Mode, const FAlphaBlend& BlendInfo, float BlendStartAlpha, float MainWeight, bool bInverse)
{switch (Mode){// The per bone value is a factor of the transition time, where 0.5 means half the transition time, 0.1 means one tenth of the transition time, etc.case EBlendProfileMode::TimeFactor:{// Most bones will have a bone factor of 1, so let's optimize that case.// Basically it means it will just follow the main weight.if (BoneFactor >= 1.0f - ZERO_ANIMWEIGHT_THRESH){return !bInverse ? MainWeight : 1.0f - MainWeight;}// Make sure our input values are valid, which is between 0 and 1.const float ClampedFactor = FMath::Clamp(BoneFactor, 0.0f, 1.0f);// Calculate where blend begin value is for this specific bone. So where did our blend start from?// Note that this isn't just the BlendInfo.GetBlendedValue() because it can be different per bone as some bones are further ahead in time.// We also need to sample the actual curve for this to get the real value.const float BeginValue = (ClampedFactor > ZERO_ANIMWEIGHT_THRESH) ? FMath::Clamp(BlendStartAlpha / ClampedFactor, 0.0f, 1.0f) : 1.0f;const float RealBeginValue = FAlphaBlend::AlphaToBlendOption(BeginValue, BlendInfo.GetBlendOption(), BlendInfo.GetCustomCurve());// Calculate the current alpha value for the bone.// As some bones can blend faster than others, we basically scale the current blend's alpha by the bone's factor.// After that we sample the curve to get the real alpha blend value.const float LinearAlpha = (ClampedFactor > ZERO_ANIMWEIGHT_THRESH) ? FMath::Clamp(BlendInfo.GetAlpha() / ClampedFactor, 0.0f, 1.0f) : 1.0f;const float RealBoneAlpha = FAlphaBlend::AlphaToBlendOption(LinearAlpha, BlendInfo.GetBlendOption(), BlendInfo.GetCustomCurve());// Now that we know the alpha for our blend, we can calculate the actual weight value.// Also make sure the bone weight is valid. Values can't be zero because this could introduce issues during normalization internally in the pipeline.const float BoneWeight = RealBeginValue + RealBoneAlpha * (BlendInfo.GetDesiredValue() - RealBeginValue);const float ClampedBoneWeight = FMath::Clamp(BoneWeight, ZERO_ANIMWEIGHT_THRESH, 1.0f);// Return our calculated weight, depending whether we'd like to invert it or not.return !bInverse ? ClampedBoneWeight : (1.0f - ClampedBoneWeight);}// The per bone value is a factor of the main blend's weight.case EBlendProfileMode::WeightFactor:{if (!bInverse){return FMath::Max(MainWeight * BoneFactor, ZERO_ANIMWEIGHT_THRESH);}// We're inversing.const float Weight = (BoneFactor > ZERO_ANIMWEIGHT_THRESH) ? MainWeight / BoneFactor : 1.0f;return FMath::Max(Weight, ZERO_ANIMWEIGHT_THRESH);}// Handle unsupported modes.// If you reach this point you have to add another case statement for your newly added blend profile mode.default:{checkf(false, TEXT("The selected Blend Profile Mode is not supported (Mode=%d)"), Mode);break;}}return MainWeight;
}

这里面没有对BlendMask这种模式的处理，因为BlendMask是专门用于 Layered Blend Per Bone节点上的，

这里的TimeFactor和WeightFactor的BlendProfile的使用机制是差不多的，但UE的文档告诉我，这里weightFactor模式下的值应该>=1，而TimeFactor下的值应该<=1，如下所示：

这说明Blend Profile只能加速动画转态里的特定joint的Blend速度，而不能减速，顺便看了下代码里面，也是这么个逻辑。感觉也比较合理，毕竟动画转态时间外部已经给定了，所以只能加速部分joints的转态，而不能减速。

附录

Unity里实现Blend Profile

首先，既然是逐Bone Blend，那么需要有个编辑的地方，去设置每个Bone的Blend速度，可以选择在AvatarMask界面里编辑，如下图所示（注意这里的时间至少是1， otherwise there would be some problem with transition time）：

然后在动画状态机的Transition里设置加上对应的Blend Profile借助的Avatar Mask即可：

Unity的BlendTree里使用Blend Profile

其实就跟UE的Blend Space里用Blend Profile是一样的，UE里Blend Space有这么个设置：

也是类似的，在Blend Tree的Inspector界面里添加Avatar Mask即可：

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UE的Blend Profile