Addressed negative reflectance values from fusion.

4-fusion.py

@@ -117,7 +117,11 @@ def compute_gcc_from_refl(refl_dir: Path, gcc_dir: Path) -> None:
     """Derive GCC from ``REFL_YYYYMMDD.tif`` files (BtI fusion output).
 
     Reads every ``REFL_*.tif`` and writes a co-located single-band
-    ``GCC_YYYYMMDD.tif``.  Zero pixels remain zero.
+    ``GCC_YYYYMMDD.tif``.  Pixels where any reflectance band is negative (which
+    can arise from EFAST's temporal correction) are written as NaN; the GCC
+    ratio is undefined there because the denominator B+G+R is near-zero or
+    negative due to band cancellation.  All-zero pixels (cloud mask) remain
+    zero.
     """
     gcc_dir.mkdir(parents=True, exist_ok=True)
     for src_path in sorted(refl_dir.glob("REFL_*.tif")):
@@ -128,9 +132,10 @@ def compute_gcc_from_refl(refl_dir: Path, gcc_dir: Path) -> None:
             b, g, r = src.read(1), src.read(2), src.read(3)
             profile = src.profile
         total = b + g + r
-        gcc = g / (total + 1e-10)
-        gcc[total == 0] = 0
-        profile.update(count=1)
+        invalid = (b < 0) | (g < 0) | (r < 0)
+        gcc = np.where(invalid, np.nan, g / (total + 1e-10))
+        gcc[total == 0] = np.nan
+        profile.update(count=1, dtype="float32")
         with rasterio.open(out_path, "w", **profile) as dst:
             dst.write(gcc[np.newaxis].astype("float32"))