ModelTranslator: base 贴图写 PNG 前做 linear->sRGB 编码

Blender Image.pixels 对 sRGB 贴图返回线性值，直接写 Non-Color PNG 会偏暗；
现按 Unity sRGB 导入预期编码 RGB，alpha（AO/透明）保持原值。附单元测试。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Tools/ModelTranslator/bl_convert.py

@@ -33,6 +33,20 @@ def decode_sphere_map(enc01):
     return np.concatenate([xy * 2.0, (l * 2.0 - 1.0)[..., None]], axis=-1)
 
 
+def linear_to_srgb(c):
+    c = np.clip(c, 0.0, 1.0)
+    return np.where(c <= 0.0031308, c * 12.92,
+                    1.055 * np.power(c, 1.0 / 2.4) - 0.055)
+
+
+def encode_base_texture_for_png(rgba):
+    """Base RGB is stored for Unity sRGB import; alpha keeps AO/opacity data."""
+    out = np.array(rgba, dtype=np.float32, copy=True)
+    out[..., :3] = linear_to_srgb(out[..., :3])
+    out[..., 3] = np.clip(out[..., 3], 0.0, 1.0)
+    return out
+
+
 # ---------------- 以下仅在 Blender 内执行 ----------------
 
 
@@ -316,7 +330,7 @@ def _convert_material(mat, model_name, outdir, max_size, used_snames, src_dir):
     stem = model_name if sname == model_name else "%s_%s" % (model_name, sname)
     base_png = os.path.abspath(os.path.join(outdir, "%s_base.png" % stem))
     mix_png = os.path.abspath(os.path.join(outdir, "%s_mix.png" % stem))
-    _save_png(base_png, base)
+    _save_png(base_png, encode_base_texture_for_png(base))
     _save_png(mix_png, mix)
 
     return {