diff --git a/gnes/preprocessor/image/simple.py b/gnes/preprocessor/image/simple.py
index 6b2c3bd3..83776ecb 100644
--- a/gnes/preprocessor/image/simple.py
+++ b/gnes/preprocessor/image/simple.py
@@ -34,6 +34,7 @@ def __init__(self, window_size: int = 64,
         self.stride_wide = stride_wide
 
     def apply(self, doc: 'gnes_pb2.Document'):
+        super().apply(doc)
         if doc.raw_bytes:
             img = np.array(Image.open(io.BytesIO(doc.raw_bytes)))
             image_set = self._get_all_sliding_window(img)
@@ -87,7 +88,18 @@ def _get_all_chunks_weight(self, image_set) -> List[float]:
 class WeightedSlidingPreprocessor(BaseSlidingPreprocessor):
 
     def _get_all_chunks_weight(self, image_set) -> List[float]:
-        raise NotImplementedError
+        weight = np.zeros([len(image_set)])
+        # n_channel is usually 3 for RGB images
+        n_channel = image_set[0].shape[-1]
+        for i in range(len(image_set)):
+            # calcualte the variance of histgram of pixels
+            weight[i] = sum([np.histogram(image_set[i][:, :, _])[0].var()
+                             for _ in range(n_channel)])
+        weight = weight / weight.sum()
+
+        # normalized result
+        weight = np.exp(- weight * 10)
+        return weight / weight.sum()
 
 
 class SegmentPreprocessor(BaseImagePreprocessor):
diff --git a/gnes/service/encoder.py b/gnes/service/encoder.py
index 56ed8d64..9afb4d68 100644
--- a/gnes/service/encoder.py
+++ b/gnes/service/encoder.py
@@ -50,7 +50,7 @@ def _handler_train(self, msg: 'gnes_pb2.Message'):
             chunks = self.get_chunks_from_docs(msg.request.train.docs)
             self.train_data.extend(chunks)
             msg.response.train.status = gnes_pb2.Response.PENDING
-            raise BlockMessage
+            # raise BlockMessage
         if msg.request.train.flush:
             self._model.train(self.train_data)
             self.logger.info('%d samples is flushed for training' % len(self.train_data))
diff --git a/gnes/service/indexer.py b/gnes/service/indexer.py
index 8829195d..7322ae44 100644
--- a/gnes/service/indexer.py
+++ b/gnes/service/indexer.py
@@ -77,9 +77,9 @@ def _handler_chunk_search(self, msg: 'gnes_pb2.Message'):
     @handler.register(gnes_pb2.Response.QueryResponse)
     def _handler_doc_search(self, msg: 'gnes_pb2.Message'):
         if msg.response.search.level == gnes_pb2.Response.QueryResponse.DOCUMENT_NOT_FILLED:
-            doc_ids = [r.doc.doc_id for r in msg.response.topk_results]
+            doc_ids = [r.doc.doc_id for r in msg.response.search.topk_results]
             docs = self._model.query(doc_ids)
-            for r, d in zip(msg.response.topk_results, docs):
+            for r, d in zip(msg.response.search.topk_results, docs):
                 if d is not None:
                     # fill in the doc if this shard returns non-empty
                     r.doc.CopyFrom(d)