Python script to convert a plaintext STL (read from stdin) to LDraw part format (printed to stdout)

stl_to_ldraw.py

# read from stdin
import sys
lines = sys.stdin.readlines()

# extract the points from the STL
points = []
for line in lines:
    split = line.split()
    if split[0] == "vertex":
        (p1, p2, p3) = (float(split[1]), float(split[2]), float(split[3]))
        points += [(p1, p2, p3)]

# scale coordinates to match ldraw part sizes
# NOTE: assumes input STL units are mm (1 LDraw unit = 0.4 mm)
# if the input units are in inches, each coordinate should instead be multiplied by 64 (1 LDU = 1/64 in)
points = [(point[0]/0.4, point[1]/0.4, point[2]/0.4) for point in points]

# get sets of 3 points that form triangles
triangles = []
for i in range(0, len(points), 3):
    triangles += [(points[i], points[i+1], points[i+2])]

# file format reference: https://www.ldraw.org/article/218.html
for t in triangles:
    print("3 16 {} {} {} {} {} {} {} {} {}".format(t[0][0], t[0][1], t[0][2], t[1][0], t[1][1], t[1][2], t[2][0], t[2][1], t[2][2],))