Detonate rifle grenades on impact with original Half-Life fireball

tools/extract_explosion_sprite.py

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+"""Convert the original GoldSrc additive explosion into a portable PNG atlas.
+
+Read-only on the Half-Life installation. No native SPR importer needed at runtime.
+"""
+import argparse
+import hashlib
+import json
+import struct
+from pathlib import Path
+
+from PIL import Image
+
+ROOT = Path(__file__).resolve().parents[1]
+
+
+def extract(source: Path, destination: Path) -> dict:
+    raw = source.read_bytes()
+    ident, version, orientation, fmt, radius, width, height, count, beam, sync = (
+        struct.unpack_from('<4siiifiiifi', raw)
+    )
+    assert (ident, version, fmt) == (b'IDSP', 2, 1), 'Expected GoldSrc additive SPR v2'
+    palette_count = struct.unpack_from('<H', raw, 40)[0]
+    assert 1 <= palette_count <= 256 and count == 15 and (width, height) == (64, 64)
+    palette = raw[42:42 + palette_count * 3]
+    offset = 42 + len(palette)
+    columns, rows, gutter = 5, 3, 2
+    tile_w, tile_h = width + 2 * gutter, height + 2 * gutter
+    atlas = Image.new('RGB', (columns * tile_w, rows * tile_h))
+    origins = []
+    for index in range(count):
+        kind, x, y, w, h = struct.unpack_from('<5i', raw, offset)
+        assert kind == 0 and (w, h) == (width, height), 'Unsupported frame/group'
+        offset += 20
+        pixels = raw[offset:offset + w * h]
+        assert len(pixels) == w * h and max(pixels) < palette_count
+        frame = Image.frombytes('P', (w, h), pixels)
+        frame.putpalette(palette)
+        # Preserve RGB: black contributes zero under additive blending, NOT alphatest.
+        atlas.paste(frame.convert('RGB'),
+                    ((index % columns) * tile_w + gutter, (index // columns) * tile_h + gutter))
+        origins.append([x, y])
+        offset += w * h
+    assert offset == len(raw), 'Trailing or truncated sprite data'
+    destination.mkdir(parents=True, exist_ok=True)
+    output = destination / 'zerogxplode_atlas.png'
+    atlas.save(output, optimize=True)
+    manifest = {
+        'source': 'Half-Life/valve/sprites/zerogxplode.spr',
+        'source_sha256': hashlib.sha256(raw).hexdigest(),
+        'destination': str(output.relative_to(ROOT)),
+        'destination_sha256': hashlib.sha256(output.read_bytes()).hexdigest(),
+        'format': 'IDSP v2 / SPR_ADDITIVE', 'frame_size': [width, height],
+        'frame_count': count, 'columns': columns, 'rows': rows, 'gutter': gutter,
+        'origins': origins, 'fps': 15,
+        'fps_source': 'https://github.com/ValveSoftware/halflife/blob/master/dlls/ggrenade.cpp',
+    }
+    (destination / 'source_manifest.json').write_text(json.dumps(manifest, indent=2) + '\n')
+    return manifest
+
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument('--source', type=Path, default=Path.home() /
+                        '.steam/steam/steamapps/common/Half-Life/valve/sprites/zerogxplode.spr')
+    args = parser.parse_args()
+    print(json.dumps(extract(args.source, ROOT / 'assets/effects/explosion'), indent=2))