UE中的MotionWarping

FTransform URootMotionModifier_SkewWarp::ProcessRootMotion(const FTransform& InRootMotion, float DeltaSeconds)
{
    // 这里提取上一次采样时间到结束时间的总变换，以及当前时间片内的变换
    FTransform RootMotionTotal = ExtractRootMotionFromAnimation(Animation.Get(), PreviousPosition, EndTime);
	FTransform RootMotionDelta = ExtractRootMotionFromAnimation(Animation.Get(), PreviousPosition, FMath::Min(CurrentPosition, EndTime));

    // 处理Locaiton的Warp
    if (bWarpTranslation)
	{
        // 获取当前的Root位置
        const FVector CurrentLocation = (CharacterOwner->GetActorLocation() - UpVector() * CapsuleHalfHeight);
        const FVector DeltaTranslation = RootMotionDelta.GetLocation(); // 动画当前位移
		const FVector TotalTranslation = RootMotionTotal.GetLocation(); // 动画剩余总位移	
        const FTransform MeshTransform = ... // 获取Mesh变换矩阵
        // 得到Mesh空间下的TargetLocation
		TargetLocation = MeshTransform.InverseTransformPositionNoScale(TargetLocation);

        // 执行位置的Warp
		FVector WarpedTranslation = WarpTranslation(DeltaTranslation, TotalTranslation, TargetLocation);
		FinalRootMotion.SetTranslation(WarpedTranslation);
    }
}

FVector URootMotionModifier_SkewWarp::WarpTranslation(const FVector& DeltaTranslation, const FVector& TotalTranslation, const FVector& TargetLocation)
{
    FVector CurrentLocation = FVector::Zero; // CurrentTransform.GetLocation();
    FVector FutureLocation = CurrentLocation + TotalTranslation;
    
    // 计算期望目标点的位移差，和动画目标位移差
    FVector CurrentToWorldOffset = TargetLocation - CurrentLocation;
    FVector CurrentToRootOffset = FutureLocation - CurrentLocation; 
    
    // 核心：这里UE构建了一个ToRootSyncSpace矩阵，用于将所有的位置变换到“当前位置LookAt动画目标点的”的空间，相当于先动画位移方向作为正方向构建了坐标系
    
    // Put everything into RootSyncSpace.
    FVector RootMotionInSyncSpace = ToRootSyncSpace.InverseTransformVector(DeltaTranslation);
    FVector CurrentToWorldSync = ToRootSyncSpace.InverseTransformVector(CurrentToWorldOffset);
    FVector CurrentToRootMotionSync = ToRootSyncSpace.InverseTransformVector(CurrentToRootOffset);

    // 这里将期望的位移投影到了动画位移的方向，计算了缩放比
    float ProjectedScale = FVector::DotProduct(CurrentToWorldSync, CurrentToRootMotionSyncNorm) / CurrentToRootMotionSync.Size();
    if (ProjectedScale != 0.0f)
    {
        // 计算X轴正方向的缩放，因为X正方向是LookAtRootDir，所以上面计算的ProjectedScale就是
        // X轴的缩放
        FMatrix ScaleMatrix;
        ScaleMatrix.SetIdentity();
        ScaleMatrix.SetAxis(0, FVector(ProjectedScale, 0.0f, 0.0f));

        // 计算Y轴的缩放
        // 计算实际位移方向和X轴（CurrentToRootDir）的夹角，然后乘上Tan(angle)
		FVector FlatToWorldDir = FlatVectorNorm(CurrentToWorldSyncNorm);
		FVector FlatToRootDir = FlatVectorNorm(CurrentToRootMotionSyncNorm);
		float AngleAboutZ = FMath::Acos(
            FVector::DotProduct(FlatToWorldDir, FlatToRootDir));
		float AngleAboutZNorm = FMath::DegreesToRadians(
            FRotator::NormalizeAxis(FMath::RadiansToDegrees(AngleAboutZ)));	
        if (FlatToWorld.Y < 0.0f)
			AngleAboutZNorm *= -1.0f;
    
        FMatrix ShearXAlongY;
        ShearXAlongY.SetIdentity();
        ShearXAlongY.SetAxis(0, FVector(1.0f, FMath::Tan(AngleAboutZNorm), 0.0f));
		
        // Skew and scale the Root motion.
        FMatrix ScaledSkewMatrix = ScaleMatrix * ShearXAlongY;
        TargetRootMotion = ScaledSkewMatrix.TransformVector(RootMotionInSyncSpace);
    }
    else
    {
        // 如果期望的位移完全和动画位移方向正交,是最简单的情况，计算实际位移和动画位移的比例，
        // 移动方向直接使用当前位置到目标位置的朝向
        // Figure out ratio between remaining Root and remaining World. Then project scaled length of current Root onto World.
        float Scale = CurrentToWorldSync.Size() / CurrentToRootMotionSync.Size();
        float StepTowardTarget = RootMotionInSyncSpace.ProjectOnTo(RootMotionInSyncSpace).Size();
 		TargetRootMotion = CurrentToWorldSyncNorm * (Scale * StepTowardTarget)
    }
    
    return TargetRootMotion;
}