tools/convert_loadingscreen_dds.py

#!/usr/bin/env python3
"""Convert a source image into a DXT1 (BC1) DDS loading screen.

Center-crops the source to the target aspect ratio, resizes to WxH with
Lanczos, then compresses to DXT1 via Microsoft's texconv (DirectXTex, MIT).
DXT1 is 8:1 compression (half of DXT5's 4:1) with identical color quality for
opaque images; loading screens need no alpha. The DDS stores sRGB-encoded
bytes directly (no linear round-trip), matching how vanilla EU4 loading
screens are encoded.

Usage:
    python convert_loadingscreen_dds.py <input_image> <output_dds> <W> <H>

Requires tools/bin/texconv.exe next to this script.
"""
import os
import subprocess
import sys
import tempfile

from PIL import Image

HERE = os.path.dirname(os.path.abspath(__file__))
TEXCONV = os.path.join(HERE, "bin", "texconv.exe")


def center_crop_to_aspect(im: Image.Image, w: int, h: int) -> Image.Image:
    sw, sh = im.size
    target_ar = w / h
    src_ar = sw / sh
    if src_ar > target_ar:
        # Source is wider than target: trim sides, keep full height.
        new_w = round(sh * target_ar)
        x0 = (sw - new_w) // 2
        box = (x0, 0, x0 + new_w, sh)
    else:
        # Source is taller than target: trim top/bottom, keep full width.
        new_h = round(sw / target_ar)
        y0 = (sh - new_h) // 2
        box = (0, y0, sw, y0 + new_h)
    return im.crop(box)


def main() -> int:
    if len(sys.argv) != 5:
        print(__doc__)
        return 2
    src, dst, w_s, h_s = sys.argv[1], sys.argv[2], sys.argv[3], sys.argv[4]
    w, h = int(w_s), int(h_s)
    if not os.path.isfile(TEXCONV):
        print(f"error: texconv not found at {TEXCONV}", file=sys.stderr)
        return 1

    im = Image.open(src)
    if im.mode not in ("RGB", "RGBA"):
        im = im.convert("RGB")
    print(f"source: {im.size[0]}x{im.size[1]} {im.mode} ({os.path.basename(src)})")

    im = center_crop_to_aspect(im, w, h)
    im = im.resize((w, h), Image.LANCZOS)
    if im.mode == "RGBA":
        # Loading screens are opaque; drop alpha so DXT1 has no transparency artifacts.
        im = im.convert("RGB")
    print(f"resized: {im.size[0]}x{im.size[1]}")

    with tempfile.TemporaryDirectory() as tmp:
        stage = os.path.join(tmp, "stage.png")
        im.save(stage, format="PNG")
        # -f DXT1  : BC1/DXT1 - 8:1 compression, opaque, half the size of DXT5
        # -m 1     : top level only, no mip chain (loading screens never minify)
        # -ft dds  : DDS output
        # -o tmp   : write into the scratch dir, then rename to the final name
        #            (keeps texconv on ASCII paths; the final unicode rename is
        #            handled by Python).
        cmd = [TEXCONV, "-f", "DXT1", "-m", "1", "-ft", "dds", "-y", "-o", tmp, stage]
        print("texconv:", " ".join(cmd))
        r = subprocess.run(cmd, capture_output=True, text=True)
        if r.returncode != 0:
            sys.stdout.write(r.stdout)
            sys.stderr.write(r.stderr)
            return r.returncode
        produced = os.path.join(tmp, "stage.dds")
        if not os.path.isfile(produced):
            print("error: texconv produced no output", file=sys.stderr)
            sys.stderr.write(r.stderr)
            return 1
        os.replace(produced, dst)

    print(f"wrote: {dst} ({os.path.getsize(dst)} bytes)")
    return 0


if __name__ == "__main__":
    raise SystemExit(main())