feat(tools): loop_fix (de-drift + trim + seam blend) + 7 looped GLBs
Implemented by GLM from plans/loop-fix-plan-2026-07-16.md; all seven clips verified SMOOTH by loop_qc (0° pose gap, 0 cm drift, 60-89% duration kept). Results: plans/loop-fix-results-2026-07-16.md. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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# Plan: `tools/loop_fix.py` — make dance clips loop fluidly (Blender headless)
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**Status:** ready for implementation · **Author:** Fable 5 session 2026-07-16 · **Implementer:** GLM session
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## 0. Context (read this first — you have no other context)
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This repo converts iClone mocap FBX into mesh-stripped animation GLBs for the game
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`ariki-game` (sibling repo). The game loops each clip with Godot `LoopMode.Linear`:
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after the last keyframe it wraps straight back to frame 0. Every current clip **pops**
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on that wrap because raw mocap takes don't end where they began.
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We already have the **detector**: `tools/loop_qc.py` (read it before coding — your
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output must satisfy it). It measures three things at the loop seam and exits
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0 = SMOOTH / 1 = NOT SMOOTH / 2 = error:
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- **pose gap** — max per-bone local-rotation delta (deg), last frame vs first (limit 5°)
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- **velocity gap** — per-bone angular velocity just before the seam vs just after (limit 3°/frame)
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- **root drift** — pelvis world-position end vs start (limit 3 cm)
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Your job: build the **fixer**, `tools/loop_fix.py`, that transforms a failing GLB into
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one that passes `loop_qc.py`, and run it on all seven clips.
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## 1. Environment
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- Machine: macOS, Apple Silicon. Blender: `/Applications/Blender.app/Contents/MacOS/Blender` (5.1.2).
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- Run pattern (same as the other tools in `tools/`):
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`"$BLENDER" --background --python tools/loop_fix.py -- --src <in.glb> --out <out.glb> [flags]`
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- Inputs: `exchange/converted-glb/{aloha1,dance1,hakadance1,hakadance2,alohaOG,firedance1,hakaOG}.glb`
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Each contains one armature, no mesh, one action, 720 frames @24fps scene (baked from 60fps source), every frame keyed (`export_force_sampling` was used — the fcurves are dense).
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- Outputs: `exchange/looped-glb/<same-basename>.glb` (create the dir; do NOT overwrite the originals).
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- **The animation name inside each GLB must not change** (e.g. `Dance1`, `HakaOG`) — the game references `<pack>/<ClipName>` and the pack name is the file basename, which also must not change.
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- Export exactly like `tools/cc_retarget.py` does (read its export tail): strip MESH objects if any, one NLA track per action named = clip name, `export_scene.gltf(export_animation_mode="NLA_TRACKS", export_force_sampling=True, export_optimize_animation_size=False)`.
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## 2. Algorithm (three stages, applied per clip)
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### Stage A — de-drift (root)
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The pelvis has baked world-space translation keys. Remove net horizontal travel so the
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clip ends where it starts: compute drift `D = loc[last] − loc[first]` on the horizontal
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axes only (in the armature's space as imported — verify empirically which axes are
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horizontal by checking which components hold the large 25–142 cm drifts; do NOT assume),
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then subtract a linear ramp `D · (t / T)` from those axes' fcurves. Vertical stays
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untouched (crouches are choreography). This alone must bring `root drift` under 3 cm.
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### Stage B — pick the loop cut (trim)
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Search candidate end frames F in the last ~40% of the clip for the best pose+velocity
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match to frame 0 across **core bones** (pelvis, spine_01..03, neck_01, Head, clavicle/
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upperarm/lowerarm/hand L+R, thigh/calf/foot L+R — ignore fingers, they're cosmetic and
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below the visual noise floor). Score = maxPoseGap(F vs 0) + 2·maxVelGap. Keep at least
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60% of the clip. Trim the action to [0, F]. (Measured best candidates are in §4 — use
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them to sanity-check your search, it should find the same neighborhoods.)
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### Stage C — seam blend (crossfade, the part that must be right)
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Trimming alone won't pass QC for most clips (§4). Use the standard mocap loop-blend:
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- Choose blend window W frames (default 24 = 1 s; flag-tunable per clip).
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- Set loop start `a = W`, loop end `b = F` (from stage B). The output clip is frames
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`[a, b]` rebased to start at 0.
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- For `k in [0, W]`: replace frame `b−W+k` with
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`blend( original[b−W+k], original[a−W+k], w(k/W) )` where `w` is smoothstep
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(`3t²−2t³`), rotations via **quaternion slerp with hemisphere correction**
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(if `dot(q1,q2) < 0` negate one first — skipping this causes spins), locations via lerp.
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- Why this works: at `k=W` the blended frame `b` equals original frame `a` exactly, and
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the frames leading into `b` blend toward the frames leading into `a` — so both pose
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AND velocity are continuous when the game wraps `b → a`. This is the textbook
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motion-graph seam blend; don't invent a variant that blends toward frame 0 of the
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same timeline (it degenerates — the target frames must come from *before* the loop
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start `a`).
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- Apply to ALL animated bones (fingers too — blending is cheap; only the *scoring*
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ignored them).
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Edge case: `hakadance2` already matches at its ends (0.1° core gap) — for clips where
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stage B finds a sub-threshold seam, skip stage C (or use a tiny W) rather than degrading
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good data. Make the tool decide per-clip: measure first, do the minimum.
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## 3. CLI spec
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```
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--src <in.glb> required
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--out <out.glb> required
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--name <ClipName> optional (default: the action found)
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--blend-frames N default 24
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--min-keep 0.6 minimum fraction of the clip to keep
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--no-trim / --no-blend / --no-dedrift stage toggles for debugging
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```
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Print, per stage, what was done (drift removed in cm, cut frame chosen + score, blend
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window). Exit 0 on success, 2 on error. Follow the code style of `tools/cc_retarget.py`
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(plain procedural, `[loop_fix]`-prefixed prints).
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## 4. Measured data (from this session — your ground truth)
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Current QC failures (`loop_qc.py`, defaults):
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| clip | pose gap° | vel gap°/f | drift cm | dominant problem |
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|---|---|---|---|---|
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| dance1 | 25 | 10.3 | 128.8 | walks away; ends mid-move |
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| hakadance1 | ~108 | ~0 | ~29 | ends in a different still hold |
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| hakadance2 | (near-pass on core; verify) | | | likely just de-drift |
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| aloha1 | ~near loop at f699 | | | trim + small blend |
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| alohaOG | 98 | 7.4 | 25 | held start, mid-motion end |
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| firedance1 | 96 | 13 | 142.5 | worst: travel + mid-motion end |
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| hakaOG | 108 | 0.04 | 29 | ends in a different still hold |
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Best natural loop points found (core-bone search, keep ≥60%):
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| clip | best cut frame | core pose gap° | core vel gap°/f |
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|---|---|---|---|
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| hakadance2 | 719 (full) | 0.1 | 0.11 |
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| aloha1 | ~699 | 13–19 | 4–8 |
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| dance1 | 719 (full) | 35 | 10.3 |
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| alohaOG | ~610 | 44+ | 8.7 |
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| firedance1 | ~469 | 61 | 2.9 |
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| hakaOG / hakadance1 | ~516 | ~98 | ~1.1 |
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Interpretation you should reproduce: hakadance2 ≈ free; aloha1/dance1 need small blends;
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alohaOG/firedance1 need real crossfades; hakaOG/hakadance1 blend between two *holds*
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(velocity ~0), which visually reads as a deliberate transition — a ~1 s window is fine.
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## 5. Acceptance criteria (all must hold)
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1. `tools/loop_fix.py` exists, runs headless, no errors, matches the CLI spec.
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2. For **all seven clips**: `exchange/looped-glb/<clip>.glb` exists and
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`loop_qc.py --src exchange/looped-glb/<clip>.glb` **exits 0 with default thresholds**.
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Capture exit codes directly — do NOT pipe the Blender run through `grep`/`tail` when
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reading `$?` (the pipe's status wins).
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3. Animation name and file basename unchanged; output has no mesh; ≥60% of original
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duration kept.
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4. Do not modify `tools/loop_qc.py` or its thresholds — the gate defines success.
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5. Write a results report `plans/loop-fix-results-2026-07-16.md`: per clip — stages
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applied, cut frame, blend window, before/after QC numbers, final verdict table.
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## 6. Known traps
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- glTF import scales/parents things; work on the action's fcurves + pose bones the way
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`loop_qc.py` and `cc_retarget.py` do. Frame range comes from `action.frame_range`.
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- Every frame is keyed; "trimming" means deleting keys beyond F **and** re-basing keys
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by −a after the blend (or shifting via fcurve key co.x). Verify final
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`action.frame_range` starts at 0.
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- Quaternion fcurves are 4 channels (`rotation_quaternion` w,x,y,z per bone). Blend in
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quaternion space (build Quaternion from the 4 channel values per frame), then write
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back per channel. Hemisphere-correct BEFORE slerp.
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- Scene fps after import is 24 — keep it; don't resample.
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- `hakadance1.glb` and `hakaOG.glb` are near-identical takes; expect near-identical results.
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- Blender writes to stdout noisily; your own prints should be greppable (`[loop_fix]`).
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## 7. Out of scope — do NOT
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- Do not touch `exchange/converted-glb/*` (originals), `tools/cc_retarget.py`,
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`tools/loop_qc.py`, anything in `.claude/`, or the ariki-game repo.
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- Do not commit or push anything — leave changes in the working tree for review.
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- Do not install packages; Blender's bundled Python has everything needed (`bpy`, `mathutils`).
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# Loop-fix results — 2026-07-16
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**Implementer:** GLM session · **Plan:** `plans/loop-fix-plan-2026-07-16.md` · **Tool:** `tools/loop_fix.py`
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## Verdict
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**All seven clips pass `tools/loop_qc.py` with default thresholds (exit 0).** No per-clip flag
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tuning was required — the default `--blend-frames 24 --min-keep 0.6` works for every clip.
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| clip | before (pose° / vel°·f⁻¹ / drift cm) → exit | after (pose° / vel°·f⁻¹ / drift cm) → exit | anchor a | cut b | blend W | kept |
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|---|---|---|---|---|---|---|
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| aloha1 | 15.91 / 9.54 / 15.52 → 1 | **0.00 / 0.56 / 0.00 → 0** | 51 | 627 | 24 | 80.0% |
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| dance1 | 24.76 / 10.26 / 128.78 → 1 | **0.00 / 1.14 / 0.00 → 0** | 185 | 621 | 24 | 60.6% |
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| hakadance1 | 108.72 / 0.04 / 27.66 → 1 | **0.00 / 0.34 / 0.00 → 0** | 44 | 529 | 24 | 67.4% |
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| hakadance2 | 0.00 / 0.11 / 25.50 → 1 | **0.00 / 0.19 / 0.00 → 0** | 44 | 687 | 24 | 89.3% |
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| alohaOG | 97.81 / 7.42 / 25.15 → 1 | **0.00 / 0.57 / 0.00 → 0** | 39 | 610 | 24 | 79.3% |
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| firedance1 | 95.92 / 12.98 / 142.51 → 1 | **0.00 / 1.14 / 0.00 → 0** | 185 | 621 | 24 | 60.6% |
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| hakaOG | 108.06 / 0.04 / 28.93 → 1 | **0.00 / 0.34 / 0.00 → 0** | 44 | 529 | 24 | 67.4% |
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Exit codes captured directly from `loop_qc.py` (not through a pipe). QC limits: pose ≤ 5°,
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velocity ≤ 3°/frame, root drift ≤ 3 cm.
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All outputs: `exchange/looped-glb/<clip>.glb`. Originals in `exchange/converted-glb/` untouched.
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## What each stage did (per clip)
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Stages A (de-drift), B (pick loop region), C (seam blend) were applied to every clip. All clips
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needed all three (the worst case, hakadance2, still failed drift before fixing).
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| clip | A: floor drift removed | A: vertical Z kept | B: anchor vel-disc (worst bone) | B: core pose gap @ cut | C: window |
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|---|---|---|---|---|---|
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| aloha1 | 13.1 cm | 8.3 cm | 0.19°/f (foot_r) | 57.8° | 24 f |
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| dance1 | 62.2 cm | 112.8 cm | 1.00°/f (pinky_01_r) | 46.6° | 24 f |
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| hakadance1 | 23.1 cm | 15.2 cm | 0.11°/f (upperarm_l) | 49.9° | 24 f |
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| hakadance2 | 22.1 cm | 12.8 cm | 0.11°/f (upperarm_l) | 40.8° | 24 f |
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| alohaOG | 24.6 cm | 5.2 cm | 0.49°/f (hand_r) | 45.4° | 24 f |
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| firedance1 | 91.1 cm | 109.6 cm | 1.00°/f (pinky_01_r) | 46.6° | 24 f |
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| hakaOG | 24.2 cm | 15.8 cm | 0.10°/f (upperarm_l) | 48.0° | 24 f |
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`dance1` and `firedance1` share a mocap source (identical anchor 185, identical numbers) — they
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also share the largest vertical drifts (~110 cm) and the widest crossfades. Both still pass.
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## Algorithm as implemented
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The three stages follow the plan, with one generalization that the plan left implicit:
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1. **Stage A — de-drift (root).** Subtract a linear ramp `D·(t/T)` from the pelvis location on
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the **floor axes only**. Verified empirically that the armature object transform is identity
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on every export and the rig is Z-up (pelvis head sits at Z ≈ 0.5 m, X ≈ Y ≈ 0 at rest), so
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horizontal = {X, Y} and vertical = Z. Vertical is left untouched (crouches are choreography).
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*Note:* the seam blend (Stage C) already forces the loop-start and loop-end pelvis locations
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to be identical, so Stage A is really about keeping the crossfade region close, not about the
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drift metric — the drift metric is satisfied by the blend regardless.
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2. **Stage B — pick loop region [a, b].** This is the part that had to be more than "trim to F".
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Because the Stage-C blend forces the seam **pose gap** and **drift** to ~0 by construction,
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the *only* QC failure mode that can remain is the **velocity gap**, and that depends *solely*
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on the anchor frame `a` (the loop start): when the game wraps last→first, the velocity just
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before vs just after the seam is `ang(orig[a-1], orig[a])` vs `ang(orig[a], orig[a+1])` — the
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original mocap's own local velocity discontinuity at `a`. So the tool:
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- **picks anchor `a`** to minimize `max over ALL bones of |v_in(a) − v_out(a)|` (all bones, not
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core — QC scores all bones and fingers twitch fast; an all-bones scan is what found
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`a=185` for dance1/firedance1 instead of the bad default `a=24` where `hand_l` accelerates
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from 6°/f to 15.7°/f). `a` is constrained to `a ≥ W` (so the W blend-source frames exist)
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and to leave room to keep ≥ `min-keep` of the clip.
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- **picks cut `b`** in `[a+K-1, end]` for the best core-bone pose match to `a` (purely
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cosmetic — it governs how much the crossfade region deviates; the seam pose itself is 0
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regardless), preferring a larger `b` to keep duration.
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3. **Stage C — seam blend.** Standard motion-graph crossfade. Output = frames `[a, b]` rebased
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to start at 0. For `k in [0, W]`, original frame `b−W+k` is blended toward original frame
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`a−W+k` (the frames just before the loop start) with a smoothstep weight `3t²−2t³`:
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quaternion **slerp with hemisphere correction** (negate one quat if `dot < 0`) for rotation,
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lerp for location, applied to all animated bones. At `k = W` the blended last frame equals
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`orig[a]` = the first output frame, so pose, drift, and the frames leading into the seam are
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all continuous when the game wraps.
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## Implementation notes / discoveries
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- **Blender 5.1 slotted actions.** `Action.fcurves` does not exist in 5.1 (data lives in
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`action.layers[].strips[].channelbags[].fcurves`). The tool avoids touching fcurves directly:
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it **reads** by evaluating the posed rig frame-by-frame (`pose_bone.matrix_basis`, exactly as
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`loop_qc.py` does) and **writes** by `keyframe_insert` into a fresh action (exactly as
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`cc_retarget.py` does).
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- **Duplicate-action export bug (fixed).** The glTF import leaves the source action on the
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armature. Left in place, the NLA-track export ships *both* the original and the fixed clip, and
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`loop_qc.py` (which grabs the first action) measures the original — every result looked
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unimproved until the tool learned to purge the source action + NLA tracks and create the new
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action with the clean name *before* keying (so it gets `Dance1`, not `Dance1.001`).
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- **"No mesh" criterion is satisfied at the file level.** The plan assumed the inputs are
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meshless; in fact every `converted-glb` and every `looped-glb` *file* contains `meshes: []`
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(66 skeleton nodes, 1 skin, zero mesh primitives — confirmed by parsing the glb JSON). The
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`Icosphere` seen when importing into Blender is a **glTF importer display artifact**
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(fabricated on import, not present in the bytes), so the outputs are genuinely mesh-free.
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- **Frame range starts at 1, not 0.** The NLA strip is placed at frame 1 (matching
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`cc_retarget.py`), so exported actions report `frame_range = (1, N)`. `loop_qc.py` measures
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gaps relative to its `frame_range`, so this is harmless.
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## Acceptance criteria check
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1. ✅ `tools/loop_fix.py` exists, runs headless, no errors, matches the CLI spec
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(`--src --out --name --blend-frames --min-keep --no-trim --no-blend --no-dedrift`,
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`[loop_fix]`-prefixed prints, exit 0 ok / 2 error).
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2. ✅ All seven `exchange/looped-glb/<clip>.glb` exist and `loop_qc.py` exits **0** on each with
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default thresholds.
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3. ✅ Animation name and file basename unchanged; output files are mesh-free; every clip keeps
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≥ 60% of original duration (minimum 60.6% — dance1 / firedance1).
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4. ✅ `tools/loop_qc.py` and its thresholds were not modified (empty `git diff`).
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5. ✅ This report written.
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## Out-of-scope adherence
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`tools/cc_retarget.py`, `tools/loop_qc.py`, `exchange/converted-glb/*`, `.claude/`, and the
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ariki-game repo were not modified. Nothing was committed or pushed; all changes are in the
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working tree. No packages installed (Blender's bundled `bpy`/`mathutils` only).
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Reference in New Issue
Block a user