Papay-Shooter/.claude/skills/character-pipeline/references/failure-modes.md

# How imports fail, and why the checks did not catch it

Seven characters shipped with "All checks passed" and four of them were visibly
broken in game — lying on their backs, seven times too big, facing backwards,
holding a gun that floated near their chest. Nothing in the verification suite
was wrong. It just never asked the questions that mattered.

**The generalised lesson, which is the whole of this page:**

> The suite verified that the character was *well-formed* — skeleton attached,
> weights authored, clips non-frozen, cloth unkeyed. It never verified that the
> character was *correct*: the right size, the right way up, the right way round,
> and reachable by the runtime. Structural validity and usable output are
> different properties, and a pipeline that only checks the first will ship the
> second broken every time a source deviates from the one it was written against.

Every check below is cheap. None of them existed.

---

## 1. Up is not always +Z — measure it, never assume it

**Symptom:** the character is enormous and lying on their back.
**Hit:** aria, momo, hikari. **Confidence: certain** — measured, not inferred.

`flatten_and_scale()` sets the character's real-world size with

```python
height = hi.z - lo.z          # Blender Z is up
s = target_height / height
```

which is right for a model that arrives Z-up in Blender, and catastrophic for one
that does not. If the character is actually lying along Blender's Y, `hi.z - lo.z`
measures their **thickness** — about 0.25 m — so `s = 1.75 / 0.25 ≈ 7`. The model
is scaled seven-fold *and* left on its back. One wrong assumption, both symptoms.

The bind-pose bounding boxes say it plainly. A correct character is tall on Y and
narrow on X and Z:

| | X | Y | Z | |
|---|---|---|---|---|
| taila | 1.24 | **1.75** | 0.69 | correct |
| kiyoko | 1.50 | **1.75** | 0.32 | correct |
| mannequin | 1.86 | **1.75** | 0.35 | correct |
| aria | 6.12 | 1.75 | **7.48** | tall axis is Z — lying down, ~7x too big |
| momo | 6.89 | 1.75 | **7.95** | same |
| hikari | 6.23 | 1.75 | **13.01** | same, and worse |

The 1.75 lands on Y for everyone because the exporter maps Blender Z to glTF Y.
That is exactly what makes the bug invisible: **the number you normalised always
comes out right, whether or not it was the right number.** A check on "is the
height 1.75" passes on all seven of these.

**Rule:** derive the up axis from the SKELETON, and rotate the model upright
before scaling anything. `flatten_and_scale()` now does this.

**Measure it from the FEET to the HIPS, not from the hips to the head.** The head
is not a reliable landmark: the spine walk ends on whatever the last non-cosmetic
bone in the chain is, and on a rig with a facial skeleton that can be a bone
sitting BELOW the hips. Momo's did, so the first version of this fix stood her
neatly on her head — correct size, correct proportions, upside down. Feet cannot
be mistaken; they are the bottom of a standing character on every rig, and
`foot.L/R` have resolved on every source met so far.

**And compare it to WORLD up, not to the model's own proportions.** The obvious
test — "is the spine the longest axis of the bounding box?" — catches nothing
here, because a model rotated as a whole is internally consistent: aria's spine
*is* her longest axis, she is just lying down. She passes that test comfortably.
The question is whether the character stands up in the world the game runs in,
which means asserting the spine runs along Blender +Z, full stop.

Two more numbers are worth asserting for free: the other two extents should be
under about 2.5 m, and the height itself should land in a human range. Those
three together are what separated the four good characters from the three broken
ones on the first run.

**Where this comes from:** all three casualties have bone names like `Hips`,
`Left leg`, `Upper Chest`, `Breast_L` — a VRM that someone imported into Blender,
renamed, and re-exported. Kiyoko kept raw VRoid `J_Bip_*` names and was fine. A
**Blender round-trip can bake an axis rotation into the export**, and that family
of files is common on Sketchfab. Treat "the bone names have been humanised" as a
signal to check the axes.

---

## 2. The runtime must look bones up by ROLE, not by name

**Symptom:** the gun is not in the hands — it floats near the chest, and can
point backwards. **Hit:** aria, momo, kiyoko, hikari.
**Confidence: certain** — measured.

`SkinnedPlayerModel.set_weapon()` finds the hand with

```gdscript
var hand_idx := _find_bone(["RightHand", "Hand_R", "hand.R"])
```

Three hardcoded spellings. Against the shipped roster:

| Skin | `hand.R` resolved in the sidecar | matched by `_find_bone` |
|---|---|---|
| taila, miku, mannequin | `DEF-hand.R` | yes |
| kiyoko | `J_Bip_R_Hand` | **no** |
| aria, momo, hikari | `Right wrist` | **no** |

When it misses, `set_weapon` falls back to parenting the weapon to the model root
at a fixed chest-height offset. The gun is then not attached to the character at
all; it hangs in space near the torso and inherits none of the arm's motion.

This is the same class of bug as `verify_character.py` looking for legs by the
substrings `thigh`/`shin`. **`tools/rig_map.py` exists precisely so that nothing
downstream has to guess a bone name, and the resolved roles are written to
`<model>.rig.json` for exactly this purpose — but only some consumers read them.**

**Rule:** every bone lookup anywhere in the runtime or the tools goes through the
sidecar roles, with a name heuristic only as a last-resort fallback. Grep for
`find_bone`, `findn(`, and any tuple of bone-name spellings; each one is a rig
this project has not met yet.

---

## 3. Facing is inferred and never verified

**Symptom:** the character runs backwards. **Hit:** kiyoko.
**Confidence: probable** — the mechanism is understood, the specific cause is not
yet isolated.

Two independent things decide which way a character ends up pointing:
`retarget.py::facing_correction()` computes a yaw to align the character's rest
pose with the library's, and `SkinnedPlayerModel.facing_flip` then applies a
blanket 180° because "glTF forward is +Z; players face -Z". If the source already
faces the other way, the two compose to a character running backwards — and
nothing anywhere measures the finished result.

**Rule:** facing is measurable from the skeleton — the toes are forward of the
ankles, and the nose/head is forward of the spine. Assert it on the OUTPUT, after
every transform has been applied. A blanket constant like `facing_flip` is a
guess about the source that must be replaced by a measurement.

---

## 4. Generated cloth chains must be validated against the geometry they drive

**Symptom:** hair stretches wildly during animation.
**Hit:** miku. **Confidence: probable.**

`tools/cloth_bones.py` grows chains for a costume that has none, then **clears
each vertex's existing body weights** and re-assigns it to the fitted chain,
keeping the original only over the first 22%. That is correct when the polyline
actually follows the clump. When it does not — a large or forked island, a
mis-picked root end — vertices land on a bone travelling somewhere else entirely,
and linear-blend skinning turns that into stretching.

The tool reports how many chains it grew. It never checks whether they *work*.

**Rule:** after growing chains, verify per vertex that its assigned bone stays
near it — pose the chain a few degrees and assert the vertex moves with its bone
rather than away from it. And never destroy the original weights without a
fallback: a generated chain should blend against the body weight it replaced, so
a bad fit degrades to "stiff" rather than to "torn".

---

## 5. A single-piece rig still needs its arms checked

**Symptom:** the gun is held, but not convincingly — the stock is not in the
shoulder and the hands are not on the grip. **Hit:** mannequin.
**Confidence: uncertain** — its hand bone resolves, so this is not #2.

The rifle hold places the weapon from the shoulder joint and the aim direction,
then solves both arms onto the grip and foregrip with two-bone IK. It is tuned
against proportions like Taila's. A rig with different arm lengths, a different
rest pose (A-pose vs T-pose) or a different bone roll will put the hands
somewhere plausible for the maths and wrong for the eye.

**Rule:** the hold has measurable success criteria — the distance from each hand
bone to the grip point it was solved onto. Assert those, per character, rather
than judging by eye.

---

## 6. Some sources are not salvageable, and the gate should say so

**Hit:** hikari. She failed every way at once — stretched and warped, tiny, far
away, gun backwards. Her rig has been through at least two toolchains: her
cosmetic bones are zero-length terminators, she carried a second armature with
its own clips, and her feet and her spine disagree about which way is up, so the
stand-up correction cannot resolve her either. She is now REJECTED by the gate
and removed from the roster rather than shipped broken.

**Rule:** a source that fails several unrelated checks is not a tuning problem,
it is a bad file. Spend the effort on finding a cleaner source, not on repairing
this one — and make sure the gate blocks it, because the failure mode before this
work was that everything passed and the breakage was only visible in game.

The vetting snippet in `separation.md` catches most of these before download:
several meshes, 50+ joints, cosmetic bones present. Add a look for duplicate
armatures and for bone chains whose bones are all at the same position.

## 7. Known-unsolved, and honestly so

Taila's legs still clip through the front of her skirt in a run, slide and dash
(~95 mm). See `cloth-and-hair.md`. The solver sees the contact and pushes on it
every iteration; what remains is a standing fight between the collision and the
garment's shape constraints, not a missing check.

---

## The check that would have caught most of this

One pass over the finished GLB, before it is ever registered:

```
POSTURE   tallest axis of the bind-pose AABB == the hips->head axis,
          and the other two are under ~2.5 m           (catches #1)
SCALE     height within a few percent of --height       (catches #1)
FACING    toes forward of ankles, along the world forward the game
          expects, AFTER facing_flip                    (catches #3)
REACHABLE every role the runtime looks up — hands, head, spine — resolves
          through the sidecar and not by name           (catches #2)
CLOTH     every chain has measurable extent, and its vertices track it (#4)
```

None of these needs Blender or the engine; the bind-pose AABB and the inverse
bind matrices in the GLB are enough for the first four.