Center a binary STL file so that the average coordinate is (0, 0, 0)

center_stl.py

# public domain

import struct, sys

if len(sys.argv) != 3:
    print("[!] Usage:", sys.argv[0], "<in>", "<out>")
    exit(1)

ax, ay, az = 0, 0, 0
with open(sys.argv[1], "rb") as fd:
    fd.read(80)
    count = int.from_bytes(fd.read(4), "little")
    print(f"[*] {count} triangles")

    for i in range(0, count):
        fd.read(12)
        for j in range(0, 3):
            x, y, z = struct.unpack("fff", fd.read(12))
            ax += x
            ay += y
            az += z

        fd.read(2)

ax /= count * 3
ay /= count * 3
az /= count * 3

print(f"[*] Global offset is ({ax}, {ay}, {az})")

with open(sys.argv[1], "rb") as ifd:
    with open(sys.argv[2], "wb") as ofd:
        ofd.write(ifd.read(84))

        for i in range(0, count):
            ofd.write(ifd.read(12))
            for j in range(0, 3):
                x, y, z = struct.unpack("fff", ifd.read(12))
                x -= ax
                y -= ay
                z -= az
                ofd.write(struct.pack("fff", x, y, z))

            ofd.write(ifd.read(2))