ModelTranslator: 修复 base 贴图双重 sRGB 编码变亮（well10k 内嵌贴图案例）

实测 blender 5.0：Image.pixels 对 8-bit 图返回原始字节值（不做色彩变换），
sRGB PNG 读出已是编码值，再 linear->sRGB 就双重 gamma（均值 0.33->0.60）。
现仅对线性来源编码：float 图（EXR）与 Principled 常量色。
端到端验证：well10k 重转后 base 与源贴图平均差 0.0013。

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

Tools/ModelTranslator/bl_convert.py

@@ -39,11 +39,13 @@ def linear_to_srgb(c):
                     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)
+def encode_base_texture_for_png(rgba, rgb_is_linear):
+    """Base RGB is stored for Unity sRGB import; alpha keeps AO/opacity data.
+    Image.pixels 对 8-bit 图返回原始字节值（已是 sRGB 编码，实测 blender 5.0），
+    仅线性来源（float 图 / Principled 常量色）需要 linear->sRGB，否则双重 gamma 变亮。"""
+    out = np.clip(np.array(rgba, dtype=np.float32, copy=True), 0.0, 1.0)
+    if rgb_is_linear:
+        out[..., :3] = linear_to_srgb(out[..., :3])
     return out
 
 
@@ -280,8 +282,11 @@ def _convert_material(mat, model_name, outdir, max_size, used_snames, src_dir):
     base = np.empty((th, tw, 4), dtype=np.float32)
     if maps["basecolor"]:
         base[..., :3] = _scaled_pixels(maps["basecolor"][0], tw, th)[..., :3]
+        # 8-bit 图 pixels 已是 sRGB 编码值；float 图（EXR 等）才是线性
+        rgb_is_linear = bool(maps["basecolor"][0].is_float)
     else:
         base[..., :3] = np.array(consts["basecolor"][:3], dtype=np.float32)
+        rgb_is_linear = True  # Principled 常量色是线性值
     if maps["alpha"]:
         img, ch = maps["alpha"]
         base[..., 3] = _channel(_scaled_pixels(img, tw, th), ch)
@@ -330,7 +335,7 @@ 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, encode_base_texture_for_png(base))
+    _save_png(base_png, encode_base_texture_for_png(base, rgb_is_linear))
     _save_png(mix_png, mix)
 
     return {