Refactor subtract_dark filter to subtract, change implementation to avoid type casting

bob/io/stream/stream.py

@@ -528,7 +528,7 @@ class StreamReproject(StreamFilter):
                                 image_points=image_points)
 
 ### subtract dark frame ###
-@stream_filter('subtract_dark')
+@stream_filter('subtract')
 class StreamSubtractDark(StreamFilter):
 
     def __init__(self, dark, name, parent):
@@ -540,14 +540,10 @@ class StreamSubtractDark(StreamFilter):
         self.dark.set_source(src)
 
     def process(self, data, indices):
-        dtype = data.dtype
-        data = data.astype(np.float)
         dark_data = self.dark.load(indices)
         assert data.shape == dark_data.shape
-        dark_data = dark_data.astype(np.float)
-        data = data - dark_data
-        data[data < 0.] = 0.
-        return data.astype(dtype)
+        # if data > dark_data: return data - dark-data, else return 0  (substraction of uint can be problematic)
+        return np.where(data > dark_data, data - dark_data, 0)
 
 
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