Fang Dai Nanning Zheng Jianru Xue Integrating EMD and Gradient for Generating Primal Sketch of Natural Images. 429-432 2006 conf/icpr/2006 ICPR (2) http://doi.ieeecomputersociety.org/10.1109/ICPR.2006.717 db/conf/icpr/icpr2006-2.html#DaiZX06