Xuefeng Bai Tiejun Zhang Xianhua Song Xiamu Niu Playfield Detection Using Color Ratio and Local Entropy. 356-359 2011 IIH-MSP http://doi.ieeecomputersociety.org/10.1109/IIHMSP.2011.84 conf/iih-msp/2011 db/conf/iih-msp/iih-msp2011.html#BaiZSN11