Feet slide within a clip

A planted foot drifts along the ground during stance, so the character looks like it is skating.

Checks: foot-slide

Why it happens

During stance the foot must move consistently with the clip's declared travel: at speed_mps relative to the character for an in-place clip, or planted in the world for a root-motion clip. Retargeting, resampling, a wrong speed pin, or hand-keyed contacts break that relationship, and runtime IK and blend band-aids exist to hide the result.

What AnimSmith measures

The synthetic gait below has a left foot that skates during stance. The report samples stance intervals with the run's contact threshold and shades them, so the slide is visible as foot motion inside a shaded interval.

Open the interactive report to scrub the exact frames the checks judged; it opens on the first of the two foot-slide findings. This clip declares a loop and a speed_mps pin, so the gait panel's caption says its curves should end where they began and that the shaded bands are the stance intervals foot-slide judged: a foot that moves horizontally inside its band is the slide.

What the finding looks like

$ animsmith --config examples/report-comparison.animsmith.toml lint examples/assets/report-comparison-before.glb
examples/assets/report-comparison-before.glb:
  error[loop-closure] clip 'acceptance-matrix' bone 'left_foot' @1.000s: loop does not close in position: bone 'left_foot' is 0.4000 m from its first-frame model-space position (cap 0.0100 m) (measured 0.4000, expected 0.0100)
  error[loop-seam-vel] clip 'acceptance-matrix' bone 'left_foot' @1.000s: loop velocity changes at the seam: bone 'left_foot' differs by 1.3416 m/s between the incoming and outgoing model-space velocities (cap 0.1000 m/s) (measured 1.3416, expected 0.1000)
  warning[foot-slide] clip 'acceptance-matrix' bone 'left_foot' @0.250s: left foot skates during stance: speed deviates 0.72 m/s from the expected 1.00 m/s (cap 0.30) — foot plants will slip at runtime (measured 0.7200, expected 0.3000)
  warning[foot-slide] clip 'acceptance-matrix' bone 'right_foot' @1.000s: right foot skates during stance: speed deviates 0.88 m/s from the expected 1.00 m/s (cap 0.30) — foot plants will slip at runtime (measured 0.8800, expected 0.3000)
  note[constant-track] clip 'acceptance-matrix' bone 'hand': rotation track has 5 keys but never moves — export bloat
  coverage[bind-pose] insufficient_rotation_evidence ×1 (scopes: first_frame_rest_delta; subjects: acceptance-matrix): only 1 usable first-frame rotation track(s); at least three are required
2 error(s), 2 warning(s), 1 note(s), 1 coverage gap(s), 0 available prediction facet(s), 0 required-unavailable prediction facet(s)   # exits 1

What the repair looks like

The after clip below is an authored repair: both sides come from this repository's fixture generator, and the after side is the same gait with those defects repaired. It is not the output of an AnimSmith transform, because AnimSmith does not re-author contacts.

Open the interactive comparison to see the two side by side; press play beside the shared phase to run them together, or tick Overlay after on before to put the two skeletons in one pane. The skeleton and the root path are the same on both sides, so what the panels show is the stance repair.

What AnimSmith itself does here is the measuring and one narrow transform: it reports the slide with the stance intervals it judged, and transform --gait-anchor can rotate an eligible in-place cycle's phase to choose a better stride cut. Anchoring changes which frame the cycle starts on, not how fast a planted foot travels, so it does not repair this finding. Contact cleanup stays DCC work.

What to do

1. Check the speed pin first. Run animsmith measure on the clip and compare the measured horizontal root speed with the declared speed_mps; a stale pin produces a slide finding on a clean clip.
2. Re-author the contacts in the DCC. Plant the stance foot (or move it at the declared speed for an in-place cycle), then re-export and re-lint.
3. Do not paper over it with runtime IK before the source is clean; the check is there so the band-aid stays optional.

Who fixes it: the artist. AnimSmith can report the slide and, for an eligible in-place gait, re-anchor the cycle, but contact cleanup and blend timing are DCC and runtime work. The gate closes when the clip re-lints clean under the declared contract and stance behaves in the actual blend.

Config

[rig]
roles = { root = "root", hips = "hips", left_foot = "left_foot", right_foot = "right_foot" }

[clips.acceptance-matrix]
loop = true
speed_mps = { value = 1.0, tolerance = 0.1 }
movement_owner_xz = "gameplay"

[checks.foot-slide]
contact_height_m = 0.03
max_slide_mps = 0.3