Editing captured motion as data

A two-performer capture runs 170.17 s and has a 53-second dead middle. Cut it to 52.9 s with a few hundred bytes of edit list — the capture itself is never modified.

1 Register the pair

Two performers captured in one metric frame are only coherent together. Registering them as a group makes every edit apply to both, atomically — same spans, same blend windows.

const sparring = $M.group('sparring', ['#a', '#b']);
sparring.collision();      // baseline over the full capture

The baseline report says how close the two hips ever get on the floor. For the capture used here: min 0.80 m, never under the 0.6 m floor for non-contact sparring.

2 Find the exchanges

In the measured capture, the close approaches cluster into 18 exchanges. They are front-loaded: ten land between 2.6 s and 43.2 s, then nothing from 43.2 s to 98.5 s. Padding each cluster by −1.0 s / +1.2 s and merging gaps under 0.4 s gives eight spans:

That removes 44.4 – 97.5 s (53.1 s) and keeps 52.9 s — 31% of the source.

3 Splice them

splice joins the spans. A blend is mandatory: over the transition window (0.25 s ≈ 8 frames by default) every rotation is slerped from the outgoing pose to the incoming one, and the root is offset so the hips continue from where they were — otherwise the body pops or teleports at every cut.

sparring.splice([
  [1.6, 9.0], [14.6, 22.1], [30.8, 44.4], [97.5, 103.4],
  [104.8, 110.8], [113.7, 117.6], [134.7, 140.6], [158.4, 161.1],
].map(([from, to]) => ({ from, to })), { blend: 0.25 });

editor.timeline.duration;   // 52.9 (once the cameras are remapped — step 5)
sparring.asCommand();       // the declarative form of this cut

4 Check for collisions

No renderer will warn you that the fighters now overlap. The collision report measures the hip-to-hip floor distance over the edited timeline and flags every span under the floor:

console.log(sparring.collision());   // { min, minAt, ok, violations: [...], spans: [...] }
sparring.collision({ floor: 0.6 }); // the default floor, in metres

For this cut: ok: true, no span under 0.6 m. The closest approach sits inside a blend window (0.74 m) — the blend interpolates the two performers' positions, so it can bring them closer than either source span does; the check is what tells you.

5 Remap the cameras

Shots authored against the 170 s source need the new timebase. remapShots() keeps every event that lies wholly inside one kept span, moves it to the cut's clock, and drops (and reports) the rest:

const r = sparring.remapShots();
r.kept; r.dropped;          // what survived the cut
editor.timeline.duration;   // 52.9

6 Slow a beat, hold a pose

Everything is composed from the same span primitive and applies to both performers:

sparring.hold(20, 0.4);                       // freeze the pose at cut-time 20 s for 0.4 s
sparring.retime([[30.8, 1], [38, 0.5]]);      // from source 38 s on, run at half speed
sparring.rootYaw(15);                         // turn the whole performance 15° about its centre
sparring.rootOffset([0.5, 0, 0]);             // move it in the world, same vector for both

7 Undo, clear, commit

1. Every call above is one Undo step — the same stack as the Animations tab.
2. sparring.clear() drops the edit list; the full 170.17 s capture plays again, untouched.
3. Open the Animations tab: the cut is one teal block per performer with a tick at every span boundary, and a Collision check / Clear cut bar underneath. Drag a block and both performers move together; the code panel shows the same cut as a $M('sparring').cut(...) line you can edit and save.
4. Commit from the Git tab. The scene is written compact (no pretty-printing) with the animation floats quantized — see below.