preview.py

#!/usr/bin/env python3
"""
Render a wireframe preview of a (quad) OBJ to a PNG, so you can eyeball the
topology without opening Blender.

    python preview.py samples/blob_clean.obj -o samples/blob_clean.png

Edges are drawn per-face, so quads read as quads. No textures, no shading --
just the mesh flow.
"""
import argparse
import numpy as np

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d.art3d import Line3DCollection


def load_obj(path):
    verts, faces = [], []
    with open(path) as f:
        for line in f:
            if line.startswith("v "):
                verts.append([float(x) for x in line.split()[1:4]])
            elif line.startswith("f "):
                idx = [int(p.split("/")[0]) - 1 for p in line.split()[1:]]
                faces.append(idx)
    return np.array(verts, dtype="float64"), faces


def main():
    ap = argparse.ArgumentParser(description=__doc__,
                                 formatter_class=argparse.RawDescriptionHelpFormatter)
    ap.add_argument("input")
    ap.add_argument("-o", "--output", required=True)
    ap.add_argument("--elev", type=float, default=22)
    ap.add_argument("--azim", type=float, default=-60)
    ap.add_argument("--size", type=int, default=1100)
    a = ap.parse_args()

    V, F = load_obj(a.input)
    quads = sum(1 for f in F if len(f) == 4)
    ratio = 100.0 * quads / max(len(F), 1)

    # build unique edge segments
    segs = set()
    for f in F:
        for i in range(len(f)):
            e = (f[i], f[(i + 1) % len(f)])
            segs.add((min(e), max(e)))
    lines = [[V[a_], V[b_]] for a_, b_ in segs]

    fig = plt.figure(figsize=(a.size / 100, a.size / 100), dpi=100)
    ax = fig.add_subplot(111, projection="3d")
    lc = Line3DCollection(lines, colors="#19c37d", linewidths=0.35, alpha=0.85)
    ax.add_collection3d(lc)

    c = V.mean(axis=0)
    r = np.abs(V - c).max()
    for setlim in (ax.set_xlim, ax.set_ylim, ax.set_zlim):
        pass
    ax.set_xlim(c[0] - r, c[0] + r)
    ax.set_ylim(c[1] - r, c[1] + r)
    ax.set_zlim(c[2] - r, c[2] + r)
    ax.set_box_aspect((1, 1, 1))
    ax.view_init(elev=a.elev, azim=a.azim)
    ax.set_axis_off()
    ax.set_facecolor("#0d1117")
    fig.patch.set_facecolor("#0d1117")
    ax.set_title(f"{a.input}\n{len(F):,} faces  -  {ratio:.0f}% quads",
                 color="#e6edf3", fontsize=11)
    fig.savefig(a.output, facecolor="#0d1117", bbox_inches="tight")
    print(f"wrote {a.output}  ({len(F):,} faces, {ratio:.0f}% quads)")


if __name__ == "__main__":
    main()