Hold pixel count in mean boundary RAG edges

This value is needed to combine edges when merging nodes.

skimage/future/graph/rag.py

@@ -406,6 +406,8 @@ def rag_boundary(labels, edge_map, connectivity=2):
     rag = RAG()
     rag.add_weighted_edges_from(_edge_generator_from_csr(graph_matrix),
                                 weight='weight')
+    rag.add_weighted_edges_from(_edge_generator_from_csr(count_matrix),
+                                weight='count')
 
     for n in rag.nodes():
         rag.node[n].update({'labels': [n]})

skimage/future/graph/tests/test_rag.py

@@ -215,3 +215,4 @@ def test_rag_boundary():
     assert set(g.edges()) == set([(1, 2), (1, 3), (2, 4), (3, 4)])
     assert g[1][3]['weight'] == 0.25
     assert g[2][4]['weight'] == 0.34375
+    assert g[1][3]['count'] == 16