Handle cases when none of the annotations are valid

bob/pad/face/preprocessor/FrameDifference.py

@@ -21,9 +21,13 @@ import bob.ip.color
 
 import bob.ip.facedetect
 
+import logging
+
 #==============================================================================
 # Main body:
 
+logger = logging.getLogger(__name__)
+
 
 class FrameDifference(Preprocessor):
     """
@@ -414,6 +418,10 @@ class FrameDifference(Preprocessor):
         selected_frames, selected_annotations = self.check_face_size(
             frames, annotations, self.min_face_size)
 
+        if not len(selected_annotations):
+            logger.warn("None of the annotations are valid.")
+            return None
+
         diff = self.comp_face_bg_diff(
             frames=selected_frames,
             annotations=selected_annotations,