Bugfix: mask combination if both masks are specified.

src/tirl/costs/cost.py

@@ -455,15 +455,21 @@ class Cost(TIRLObject):
         if not sm and not tm:
             return None
         # Return target mask if source mask is not defined
-        elif tm:
+        elif tm and not sm:
             return target_mask
         # Return source mask if target mask is not defined
-        elif sm:
+        elif sm and not tm:
             return source_mask
 
         # Create composite mask if both masks are defined
         else:
-            mm = self.metaparameters.get("maskmode")
+            # Enforce the "and" match mode if need to mask missing data. With
+            # the "or" method, the mask values may end up 0.5 instead of 0.
+            if ts.TIMAGE_MASK_MISSING_DATA:
+                mm = "and"
+            else:
+                mm = self.metaparameters.get("maskmode")
+
             if mm == "and":
                 # Negative values are impossible, hence NaNs are not expected.
                 return (source_mask * target_mask) ** 0.5