I’m using UE 5.5.
FIRST PROBLEM:
When I toggle time dilation between normal and slow motion (like 1/10th speed), dead character rigid bodies will flex and move spontaneously, and start flying around my level. I think it’s a bug with the physics simulation where angle constraints are loaded more in higher dt, but my efforts to hack phys code to account for it failed so far.
UE5.5 SOLUTION:
When we upgraded, I enabled async physics (using 1/30 for fixed step, 4 max substeps also set to 1/30, in case more phys ticks were needed to catch up to low frame rate < 30fps). Behold, the death ragdolls stopped coming to life and flying around!
NEW PROBLEM:
Our alive character have some simulated physics bits, like tails or big floppy ears. With async physics, especially with my low tick rate, the simulated bits stretch behind the kinematic parts they are attached to. I’m guessing if there’s a long round trip from anim update → write to async phys or async phys reads from it → phys updates constraints → mesh update from async phys → render… I can see that stacking up the latency (and idk if there are buffer frames introducing more latency or what).
NOW WHAT:
I briefly investigated trying to tick some bodies synchronously, and other asynchronously, but it doesn’t appear to be supported. I tried manually enforcing phys bodies length constraints (like projection) in post phys tick, but it didn’t play nice with the async physics (I might try this again in the asynch phys callback). My other best hope is to keep async physics enabled, and write a custom simpl constraint chain solver that runs synchronously (no collisions).
Does anyone have anything smart to help prevent me going down a bad path? Thanks in advance!