ShouldMarkSyncPhases causes large pose discontinuities when sync leader changes

The issue is caused by the way that we calculate the leader sample's Time vs the follower sample’s Time and how that calculation switches on the frame when a follower sample becomes a lead sample. On that frame, we continue to update LeaderSampleData.Time and PreviousTime using the same logic that we did on previous frames when the leader didn’t change. ie. the following code:

float CurrentTime = NormalizedCurrentTime * NewAnimLength;
LeaderSampleData.PreviousTime = CurrentTime;
                        	
FAnimationRuntime::AdvanceTime(Instance.bLooping, NewDelta, /*inout*/ CurrentTime, NewAnimLength);
                        	
LeaderSampleData.Time = CurrentTime;
NormalizedCurrentTime = NewAnimLength ? (CurrentTime / NewAnimLength) : 0.0f;

This uses NormalizedCurrentTime (based on the time accumulator of the BS) to calculate the new PreviousTime and Time of the lead sample (Time is set again in ResetBlendSamples). But this new time bears no relation to the time of the sample on the previous frame when it was a follower and was being updated via the follower branch logic (ie. by UBlendSpace::TickFollowerSamples). So we can get pops from large jumps forwards, or backwards, in the playback time.

Note that while the intention with this update mode is that it’s used with a motion matching or blendstack node, the layer blending when the BS parameters change only partially hides the issue.

• Open the attached project [Link Removed]
• In the Developers folder, navigate to the C_26035818 folder and open the blendspace asset
• Hold Alt to drive the BS parameters while hovering the mouse over the grid UI
  • Notice how you move the input parameter position across the halfway point between the two samples, there’s a visible pop
  • This is also shown in the video, note how there is no pop until ShouldMarkSyncPhases is set