#!/usr/bin/env python3
"""Derive tangent-space normal maps from albedo textures (Pillow only).

    python3 tools/make_normal_maps.py assets/textures/ai/game_ready/brick_facade.png ...

Writes <stem>_normal.png next to each input. The height signal is the
luminance, so mortar/aggregate recesses catch the light; edges wrap for tiling.
"""

import sys
from pathlib import Path

from PIL import Image

STRENGTH = 2.0


def normal_map(source: Path) -> Path:
    image = Image.open(source).convert("L")
    width, height = image.size
    pixels = list(image.getdata())
    out = bytearray(width * height * 3)

    def lum(x: int, y: int) -> float:
        return pixels[(y % height) * width + (x % width)]

    for y in range(height):
        row = y * width
        for x in range(width):
            dx = (lum(x + 1, y) - lum(x - 1, y)) / 255.0
            dy = (lum(x, y + 1) - lum(x, y - 1)) / 255.0
            nx = -dx * STRENGTH
            ny = -dy * STRENGTH
            nz = 1.0
            length = (nx * nx + ny * ny + nz * nz) ** 0.5
            offset = (row + x) * 3
            out[offset] = int((nx / length * 0.5 + 0.5) * 255)
            out[offset + 1] = int((ny / length * 0.5 + 0.5) * 255)
            out[offset + 2] = int((nz / length * 0.5 + 0.5) * 255)
    normal = Image.frombytes("RGB", (width, height), bytes(out))
    target = source.with_name(source.stem + "_normal.png")
    normal.save(target)
    print(f"wrote {target}")
    return target


def main(argv=None) -> int:
    paths = [Path(p) for p in (argv if argv is not None else sys.argv[1:])]
    if not paths:
        print(__doc__, file=sys.stderr)
        return 2
    for path in paths:
        normal_map(path)
    return 0


if __name__ == "__main__":
    sys.exit(main())