72 lines
2.3 KiB
Python
72 lines
2.3 KiB
Python
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"""Strip axial roll (twist about the bone axis) from a hand-pose JSON, keeping the curl.
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The pose delta vs the body's rest is swing-twist decomposed per bone; the twist factor is
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dropped and the pose rebuilt as rest*swing. Thumb chains are left untouched (their roll is
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functional opposition). usage: handpose_detwist.py body.glb pose_in.json pose_out.json"""
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import json, math, struct, sys
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from pathlib import Path
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body, pose_in, pose_out = sys.argv[1:4]
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data = Path(body).read_bytes()
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jlen = struct.unpack_from("<I", data, 12)[0]
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gltf = json.loads(data[20:20 + jlen].decode("utf-8"))
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nodes = gltf["nodes"]
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byname = {n.get("name"): i for i, n in enumerate(nodes)}
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def qmul(a, b):
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ax, ay, az, aw = a
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bx, by, bz, bw = b
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return [aw * bx + ax * bw + ay * bz - az * by,
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aw * by - ax * bz + ay * bw + az * bx,
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aw * bz + ax * by - ay * bx + az * bw,
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aw * bw - ax * bx - ay * by - az * bz]
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def qinv(q):
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return [-q[0], -q[1], -q[2], q[3]]
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def qnorm(q):
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m = math.sqrt(sum(v * v for v in q))
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return [v / m for v in q]
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def bone_axis(i):
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for c in nodes[i].get("children", []):
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t = nodes[c].get("translation")
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if t:
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m = math.sqrt(sum(v * v for v in t))
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if m > 1e-8:
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return [v / m for v in t]
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return None
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pose = json.loads(Path(pose_in).read_text())["bones"]
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out = {}
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report = []
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for name, p in pose.items():
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i = byname.get(name)
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if i is None or name.startswith("thumb"):
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out[name] = p
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continue
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a = bone_axis(i)
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if a is None: # leaf tips: twist is invisible, keep as-is
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out[name] = p
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continue
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r = nodes[i].get("rotation", [0, 0, 0, 1])
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d = qmul(qinv(r), p) # delta in the bone's rest-local frame
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dot = d[0] * a[0] + d[1] * a[1] + d[2] * a[2]
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twist = qnorm([dot * a[0], dot * a[1], dot * a[2], d[3]])
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swing = qmul(d, qinv(twist))
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out[name] = [round(v, 6) for v in qnorm(qmul(r, swing))]
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deg = 2 * math.degrees(math.atan2(abs(dot), abs(d[3])))
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if deg > 1.0:
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report.append((deg, name))
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Path(pose_out).write_text(json.dumps({"bones": out}, indent=1))
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report.sort(reverse=True)
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print("wrote %s (%d bones, thumbs untouched)" % (pose_out, len(out)))
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for deg, name in report[:6]:
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print(" stripped %5.1f deg %s" % (deg, name))
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