Fix lookup table enhancement removing transparency (NaNs)

AUTHORS.md

@@ -115,3 +115,4 @@ The following people have made contributions to this project:
 - [Albert Brotzer](https://github.com/albertbrotzer)
 - [Alexandra Melzer](https://github.com/armelzer)
 - [Francesc Lucas Carbó (cesclc)](https://github.com/cesclc)
+- [Göte Kleringer (Grukank)](https://github.com/Grukank)

satpy/enhancements/colormap.py

@@ -34,7 +34,12 @@
 def lookup(img, **kwargs):
     """Assign values to channels based on a table."""
     luts = np.array(kwargs["luts"], dtype=np.float32) / 255.0
-    return _lookup_table(img.data, luts=luts)
+    # Preserve NaNs
+    nans = np.isfinite(img.data)
+    _lookup_table(img.data, luts=luts)
+    # Replace lost NaNs
+    img.data = img.data.where(nans, np.nan)
+    return
 
 
 @exclude_alpha

satpy/tests/enhancement_tests/test_colormap.py

@@ -42,16 +42,16 @@ def test_lookup(self):
         """Test the lookup enhancement function."""
         from satpy.enhancements.colormap import lookup
         expected = np.array([[
-            [0., 0., 0., 0.333333, 0.705882],
+            [np.nan, 0., 0., 0.333333, 0.705882],
             [1., 1., 1., 1., 1.]]])
         lut = np.arange(256.)
         run_and_check_enhancement(lookup, self.ch1, expected, luts=lut)
 
-        expected = np.array([[[0., 0., 0., 0.333333, 0.705882],
+        expected = np.array([[[np.nan, 0., 0., 0.333333, 0.705882],
                               [1., 1., 1., 1., 1.]],
-                             [[0., 0., 0., 0.333333, 0.705882],
+                             [[np.nan, 0., 0., 0.333333, 0.705882],
                               [1., 1., 1., 1., 1.]],
-                             [[0., 0., 0., 0.333333, 0.705882],
+                             [[np.nan, 0., 0., 0.333333, 0.705882],
                               [1., 1., 1., 1., 1.]]])
         lut = np.arange(256.)
         lut = np.vstack((lut, lut, lut)).T