Nicholas ButzkeandClaude Opus 5
c8b0337aa7
WEAPON MOUNT, all models. set_weapon() seated the gun with a fixed rotation_degrees = (0, 90, -90). A bone attachment is expressed in the BONE's axes and no two rigs agree on those, so one constant mounted the weapon differently on every character. It never needed to be right: the pose layer aims the gun by rotating the wrist until the weapon's forward lies on the aim line, so the identity means "forward is the hand bone's -Z", which is true on any rig, and the wrist absorbs the roll. The grip now sits at the bone origin, so the gun is in the hand rather than at an offset from a differently-oriented bone. Verified by render on kiyoko: rifle shouldered, both hands on it. FACING. flatten_and_scale() now measures toes-versus-ankles and snaps the character to face Blender -Y, the convention the runtime's blanket flip is built around. Kiyoko was 180 degrees off. Snapped to the nearest quarter turn so splayed feet in a rest pose are not read as a turned character. MIKU. Re-imported without --grow-cloth. Her grown hair chains were the cause of the stretching: cloth_bones.py clears a vertex's body weights and re-assigns it to the fitted polyline, so a poor fit does not degrade to "stiff", it degrades to "torn". She now has no hair simulation — stiff but correct — until that tool blends against the weights it replaces instead of destroying them. ARIA'S SKIRT IS NOT SIMULATED. Worth recording plainly, because it looks better than Taila's and the obvious conclusion is the wrong one: aria has ZERO cloth chains. Her skirt never clips because it is rigidly skinned and follows the legs it is weighted to. There is nothing to port to Taila except switching her simulation off. Co-Authored-By: Claude Opus 5 <[email protected]>
243 lines
12 KiB
Markdown
243 lines
12 KiB
Markdown
# How imports fail, and why the checks did not catch it
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Seven characters shipped with "All checks passed" and four of them were visibly
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broken in game — lying on their backs, seven times too big, facing backwards,
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holding a gun that floated near their chest. Nothing in the verification suite
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was wrong. It just never asked the questions that mattered.
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**The generalised lesson, which is the whole of this page:**
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> The suite verified that the character was *well-formed* — skeleton attached,
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> weights authored, clips non-frozen, cloth unkeyed. It never verified that the
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> character was *correct*: the right size, the right way up, the right way round,
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> and reachable by the runtime. Structural validity and usable output are
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> different properties, and a pipeline that only checks the first will ship the
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> second broken every time a source deviates from the one it was written against.
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Every check below is cheap. None of them existed.
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---
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## 1. Up is not always +Z — measure it, never assume it
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**Symptom:** the character is enormous and lying on their back.
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**Hit:** aria, momo, hikari. **Confidence: certain** — measured, not inferred.
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`flatten_and_scale()` sets the character's real-world size with
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```python
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height = hi.z - lo.z # Blender Z is up
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s = target_height / height
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```
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which is right for a model that arrives Z-up in Blender, and catastrophic for one
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that does not. If the character is actually lying along Blender's Y, `hi.z - lo.z`
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measures their **thickness** — about 0.25 m — so `s = 1.75 / 0.25 ≈ 7`. The model
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is scaled seven-fold *and* left on its back. One wrong assumption, both symptoms.
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The bind-pose bounding boxes say it plainly. A correct character is tall on Y and
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narrow on X and Z:
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| | X | Y | Z | |
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|---|---|---|---|---|
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| taila | 1.24 | **1.75** | 0.69 | correct |
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| kiyoko | 1.50 | **1.75** | 0.32 | correct |
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| mannequin | 1.86 | **1.75** | 0.35 | correct |
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| aria | 6.12 | 1.75 | **7.48** | tall axis is Z — lying down, ~7x too big |
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| momo | 6.89 | 1.75 | **7.95** | same |
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| hikari | 6.23 | 1.75 | **13.01** | same, and worse |
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The 1.75 lands on Y for everyone because the exporter maps Blender Z to glTF Y.
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That is exactly what makes the bug invisible: **the number you normalised always
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comes out right, whether or not it was the right number.** A check on "is the
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height 1.75" passes on all seven of these.
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**Rule:** derive the up axis from the SKELETON, and rotate the model upright
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before scaling anything. `flatten_and_scale()` now does this.
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**Measure it from the FEET to the HIPS, not from the hips to the head.** The head
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is not a reliable landmark: the spine walk ends on whatever the last non-cosmetic
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bone in the chain is, and on a rig with a facial skeleton that can be a bone
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sitting BELOW the hips. Momo's did, so the first version of this fix stood her
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neatly on her head — correct size, correct proportions, upside down. Feet cannot
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be mistaken; they are the bottom of a standing character on every rig, and
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`foot.L/R` have resolved on every source met so far.
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**And compare it to WORLD up, not to the model's own proportions.** The obvious
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test — "is the spine the longest axis of the bounding box?" — catches nothing
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here, because a model rotated as a whole is internally consistent: aria's spine
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*is* her longest axis, she is just lying down. She passes that test comfortably.
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The question is whether the character stands up in the world the game runs in,
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which means asserting the spine runs along Blender +Z, full stop.
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Two more numbers are worth asserting for free: the other two extents should be
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under about 2.5 m, and the height itself should land in a human range. Those
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three together are what separated the four good characters from the three broken
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ones on the first run.
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**Where this comes from:** all three casualties have bone names like `Hips`,
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`Left leg`, `Upper Chest`, `Breast_L` — a VRM that someone imported into Blender,
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renamed, and re-exported. Kiyoko kept raw VRoid `J_Bip_*` names and was fine. A
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**Blender round-trip can bake an axis rotation into the export**, and that family
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of files is common on Sketchfab. Treat "the bone names have been humanised" as a
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signal to check the axes.
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---
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## 2. The runtime must look bones up by ROLE, not by name
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**Symptom:** the gun is not in the hands — it floats near the chest, and can
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point backwards. **Hit:** aria, momo, kiyoko, hikari.
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**Confidence: certain** — measured.
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`SkinnedPlayerModel.set_weapon()` finds the hand with
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```gdscript
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var hand_idx := _find_bone(["RightHand", "Hand_R", "hand.R"])
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```
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Three hardcoded spellings. Against the shipped roster:
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| Skin | `hand.R` resolved in the sidecar | matched by `_find_bone` |
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| taila, miku, mannequin | `DEF-hand.R` | yes |
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| kiyoko | `J_Bip_R_Hand` | **no** |
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| aria, momo, hikari | `Right wrist` | **no** |
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When it misses, `set_weapon` falls back to parenting the weapon to the model root
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at a fixed chest-height offset. The gun is then not attached to the character at
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all; it hangs in space near the torso and inherits none of the arm's motion.
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This is the same class of bug as `verify_character.py` looking for legs by the
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substrings `thigh`/`shin`. **`tools/rig_map.py` exists precisely so that nothing
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downstream has to guess a bone name, and the resolved roles are written to
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`<model>.rig.json` for exactly this purpose — but only some consumers read them.**
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**Rule:** every bone lookup anywhere in the runtime or the tools goes through the
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sidecar roles, with a name heuristic only as a last-resort fallback. Grep for
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`find_bone`, `findn(`, and any tuple of bone-name spellings; each one is a rig
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this project has not met yet.
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---
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## 3. Facing is inferred and never verified
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**Symptom:** the character runs backwards. **Hit:** kiyoko.
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**Confidence: probable** — the mechanism is understood, the specific cause is not
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yet isolated.
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Two independent things decide which way a character ends up pointing:
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`retarget.py::facing_correction()` computes a yaw to align the character's rest
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pose with the library's, and `SkinnedPlayerModel.facing_flip` then applies a
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blanket 180° because "glTF forward is +Z; players face -Z". If the source already
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faces the other way, the two compose to a character running backwards — and
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nothing anywhere measures the finished result.
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**Rule:** facing is measurable from the skeleton — the toes are forward of the
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ankles. `flatten_and_scale()` now snaps that to Blender -Y, the convention the
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runtime flip is built around, so every character leaves the pipeline pointing the
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same way whatever the source did. Kiyoko was 180 degrees off and is now correct.
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Snap to the nearest QUARTER TURN, not to the measured angle: a rest pose with the
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feet slightly splayed is not a character who is 7 degrees turned, and correcting
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it as one puts a permanent yaw on the whole skeleton.
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---
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## 4. Generated cloth chains must be validated against the geometry they drive
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**Symptom:** hair stretches wildly during animation.
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**Hit:** miku. **Confidence: probable.**
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`tools/cloth_bones.py` grows chains for a costume that has none, then **clears
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each vertex's existing body weights** and re-assigns it to the fitted chain,
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keeping the original only over the first 22%. That is correct when the polyline
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actually follows the clump. When it does not — a large or forked island, a
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mis-picked root end — vertices land on a bone travelling somewhere else entirely,
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and linear-blend skinning turns that into stretching.
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The tool reports how many chains it grew. It never checks whether they *work*.
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**Rule:** after growing chains, verify per vertex that its assigned bone stays
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near it — pose the chain a few degrees and assert the vertex moves with its bone
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rather than away from it. And never destroy the original weights without a
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fallback: a generated chain should blend against the body weight it replaced, so
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a bad fit degrades to "stiff" rather than to "torn".
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---
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## 5. A single-piece rig still needs its arms checked
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**Symptom:** the gun is held, but not convincingly — the stock is not in the
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shoulder and the hands are not on the grip. **Hit:** mannequin.
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**Confidence: uncertain** — its hand bone resolves, so this is not #2.
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The rifle hold places the weapon from the shoulder joint and the aim direction,
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then solves both arms onto the grip and foregrip with two-bone IK. It is tuned
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against proportions like Taila's. A rig with different arm lengths, a different
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rest pose (A-pose vs T-pose) or a different bone roll will put the hands
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somewhere plausible for the maths and wrong for the eye.
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**The real cause turned out to be #2's sibling, and it affected every model.**
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`set_weapon` mounted the gun with a fixed `rotation_degrees = (0, 90, -90)`. A
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bone attachment is expressed in the BONE's axes, and no two rigs agree on those,
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so a constant there mounts the weapon differently on every character.
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It does not need to be right at all. The pose layer aims the gun by rotating the
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WRIST until the weapon's forward axis lies on the aim line, so handing it the
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IDENTITY means "the gun's forward is the hand bone's -Z" — true by construction
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on any rig — and the wrist absorbs whatever roll that bone happens to have. The
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grip is placed at the bone's origin, so the gun sits *in* the hand instead of at
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a fixed offset from a differently-oriented bone.
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**Rule:** never express a mount as a constant in a bone's local frame. Either
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derive the frame from the skeleton, or hand the downstream solver an identity and
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let it do the work — it already knows where the gun must point.
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---
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## 6. Some sources are not salvageable, and the gate should say so
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**Hit:** hikari. She failed every way at once — stretched and warped, tiny, far
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away, gun backwards. Her rig has been through at least two toolchains: her
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cosmetic bones are zero-length terminators, she carried a second armature with
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its own clips, and her feet and her spine disagree about which way is up, so the
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stand-up correction cannot resolve her either. She is now REJECTED by the gate
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and removed from the roster rather than shipped broken.
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**Rule:** a source that fails several unrelated checks is not a tuning problem,
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it is a bad file. Spend the effort on finding a cleaner source, not on repairing
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this one — and make sure the gate blocks it, because the failure mode before this
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work was that everything passed and the breakage was only visible in game.
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The vetting snippet in `separation.md` catches most of these before download:
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several meshes, 50+ joints, cosmetic bones present. Add a look for duplicate
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armatures and for bone chains whose bones are all at the same position.
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## 7. Known-unsolved, and honestly so
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Taila's legs still clip through the front of her skirt in a run, slide and dash
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(~95 mm). See `cloth-and-hair.md`. The solver sees the contact and pushes on it
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every iteration; what remains is a standing fight between the collision and the
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garment's shape constraints, not a missing check.
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---
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## The check that would have caught most of this
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One pass over the finished GLB, before it is ever registered:
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```
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POSTURE tallest axis of the bind-pose AABB == the hips->head axis,
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and the other two are under ~2.5 m (catches #1)
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SCALE height within a few percent of --height (catches #1)
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FACING toes forward of ankles, along the world forward the game
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expects, AFTER facing_flip (catches #3)
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REACHABLE every role the runtime looks up — hands, head, spine — resolves
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through the sidecar and not by name (catches #2)
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CLOTH every chain has measurable extent, and its vertices track it (#4)
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```
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None of these needs Blender or the engine; the bind-pose AABB and the inverse
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bind matrices in the GLB are enough for the first four.
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