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engine: physics steps before motion in each fixed step, so every followed body chases last step's rig #845
Description
The latency. Behaviours run their fixed_update in the order they were added (engine.run_fixed_updates), and physics.attach adds the physics behaviour before motion.on_figure adds motion's. So within each fixed step:
- the bodies are stepped toward the anchors' targets, which are the rig as motion drew it in the previous step;
- only then does motion work out this step's pose and draw the rig.
Every followed body (ACTING, ANIMATED) therefore chases a target one step stale. Unity, and PuppetMaster on it, run the scripts' FixedUpdate, with the animation read there, before the physics step for this reason. Unreal's physical animation sets its targets before the physics tick.
Measured (the #843 probe). The box man sat on a 0.38 m log raises a bowl to its mouth, the hand moving at up to 0.94 m/s, with its arms' anchors pinned at 30 Hz. The forearm bodies end up 4.9 cm behind the grips the drawn hands are on. A critically damped 30 Hz anchor should trail a 0.94 m/s target by about 1.0 cm. Stiffening the pin shows the floor:
| pin | worst lag mid-bite |
|---|---|
| 30 Hz | 4.9 cm |
| 60 Hz | 3.7 cm |
| 120 Hz | 2.7 cm |
The lag tends toward a floor of a step or more of motion (1.6 cm a step at that speed) rather than toward the spring's own lag.
Wanted. The physics behaviour steps after every other behaviour's fixed_update in the same fixed step, so the bodies chase this step's targets. This shifts every motion test's timing by a step, so it needs the full motion gate, the plaza and the showcase against main, and a pass over the gains tuned with the lag in place.
Activity
nicolas-maman commentedon Oct 21, 2026
Measured with a knob that runs the physics behaviour's fixed_update after every other behaviour's, in the same fixed step. The patch is build/scratch/physics_last_knob.patch:
X_PHYSICS_LAST=1, matched on the behaviour named "physics".The hold probe (#843's case, with #846's arm pins):
The 1.4 cm gained is about one step of the hand's motion, as expected.
The motion gate: 41 of 42 suites pass unchanged. The one that moves is test_falls, "forwards: the knees are down first, then the hands, in at least 29". It goes to 28 of 30: two protected forward falls land a hand first. The protective fall's arms take hold a step sooner, which may be the arms being quicker rather than a defect.
For the PR:
nicolas-maman commentedon Oct 22, 2026
Moved under the animation epic #853, milestone #858 (the physics asset and physical animation layer).