2. The method of claim 1 , wherein the graph comprises a weighted graph and edges in the weighted graph are assigned weights measuring similarities between document images represented by nodes connected by the edges.
2. The method of claim 1 , wherein the graph comprises a weighted graph and edges in the weighted graph are assigned weights measuring similarities between document images represented by nodes connected by the edges. 3. The method of claim 2 , wherein a weight assigned to an edge in the weighted graph comprises a combination of component scores measuring similarities between image features detected in images represented by nodes connected by the edge, and the method further comprises: training a machine-learning engine using labeled image pairs; and generating the combination of component scores by the machine-learning engine based on the component scores.