PROCITY/pipeline/strip_transmission.py

#!/usr/bin/env python3
"""PROCITY Lane E — strip_transmission.py  (R39, the transmission pre-pass)

Remove `KHR_materials_transmission` from a GLB **without touching one byte of geometry**.

WHY THIS IS NOT A COSMETIC FIX. three.js renders a **transmission pre-pass** — a second full render
of the opaque scene into a render target — as soon as ANY material in the scene has
`transmission > 0` (`WebGLRenderer`'s transmissive-object list). Every opaque draw call in that
scene is therefore issued **twice**. Lane C measured it in R39 by zeroing the bookshelf's:

    opshop/hall 191 -> 104 draws (-46%)   ·   opshop/wide 142 -> 80
    book/hall   101 ->  51 draws (-50%)   ·   dept/hall   116 -> 116 (control, no bookshelf)

Three PROCITY assets carry `transmissionFactor: 1`, and only one of them has ever been wired:

  procity_fit_bookshelf_01.glb    mtl_10218_Bookshelves_v1     SHIPPED — the one C measured
  procity_street_longbench_01.glb lambert3SG            + ior 1  NOT WIRED — the 5.86 m arcade bench
  procity_street_streetlight_01.glb Streetlight_HighResSG3 + ior 1 NOT WIRED — one per ~18 m of street

The two unwired ones are the worse finding. They are STREET assets, LANE_E_NOTES R38 recommends both
to Lane B, and the street budget is 292/300 with **eight draws of margin** — a transmission pre-pass
there does not cost eight draws, it costs another ~292. Nobody looked because a bookshelf, a bench
and a streetlight are not glass; this is an exporter default that rode in with the source meshes.

    python3 pipeline/strip_transmission.py FILE.glb [FILE ...] [--in-place] [--dry-run]

The JSON chunk is rewritten and the BIN chunk is copied through untouched — the tool prints the
sha1 of the BIN chunk before and after so "geometry unchanged" is a checked claim, not a promise.
`KHR_materials_ior` is deliberately LEFT ALONE: ior alone does not trigger the pre-pass, and
removing more than the defect widens the diff for no measured gain.
"""
import hashlib
import json
import os
import struct
import sys

EXT = "KHR_materials_transmission"


def read(path):
    d = open(path, "rb").read()
    assert d[:4] == b"glTF", f"{path}: not a GLB"
    off, gltf, binc = 12, None, b""
    while off < len(d):
        clen, ctype = struct.unpack("<II", d[off:off + 8])
        chunk = d[off + 8: off + 8 + clen]
        if ctype == 0x4E4F534A:
            gltf = json.loads(chunk.decode("utf-8"))
        elif ctype == 0x004E4942:
            binc = chunk
        off += 8 + clen
    return gltf, binc


def write(path, gltf, binc):
    j = json.dumps(gltf, separators=(",", ":")).encode("utf-8")
    j += b" " * ((4 - len(j) % 4) % 4)
    b = binc + b"\0" * ((4 - len(binc) % 4) % 4)
    total = 12 + 8 + len(j) + (8 + len(b) if b else 0)
    out = bytearray()
    out += b"glTF" + struct.pack("<II", 2, total)
    out += struct.pack("<II", len(j), 0x4E4F534A) + j
    if b:
        out += struct.pack("<II", len(b), 0x004E4942) + b
    open(path, "wb").write(bytes(out))


def main():
    files = [a for a in sys.argv[1:] if not a.startswith("--")]
    dry = "--dry-run" in sys.argv
    for f in files:
        gltf, binc = read(f)
        before = hashlib.sha1(binc).hexdigest()
        hits = []
        for i, m in enumerate(gltf.get("materials", [])):
            ex = m.get("extensions") or {}
            if EXT in ex:
                hits.append((i, m.get("name"), ex[EXT]))
                del ex[EXT]
                if not ex:
                    m.pop("extensions", None)
        if not hits:
            print(f"{os.path.basename(f)}: clean, nothing to do")
            continue
        # prune extensionsUsed/Required only if no material still declares it
        still = any(EXT in (m.get("extensions") or {}) for m in gltf.get("materials", []))
        for key in ("extensionsUsed", "extensionsRequired"):
            if not still and EXT in (gltf.get(key) or []):
                gltf[key] = [e for e in gltf[key] if e != EXT]
                if not gltf[key]:
                    del gltf[key]
        for i, name, val in hits:
            print(f"{os.path.basename(f)}: material[{i}] '{name}' — removed {EXT} {val}")
        if dry:
            print("  (--dry-run, not written)")
            continue
        write(f, gltf, binc)
        _, binc2 = read(f)
        after = hashlib.sha1(binc2).hexdigest()
        ok = "BIN IDENTICAL" if after == before else "*** BIN CHANGED ***"
        print(f"  wrote {f}  {os.path.getsize(f)} B   {ok}  sha1(BIN) {after[:12]}")
        if after != before:
            sys.exit(2)


main()