Minimum-risk training for semi-Markov conditional random fields with application to handwritten Chinese/Japanese text recognition

• We present a minimum-risk (MR) training method for semi-CRFs.
• MR aims at minimizing the character error rate rather than the string error rate.
• Three non-0/1 cost functions are compared with the conventional 0/1 cost.
• Lattice edge selection is investigated in MR to reduce the training complexity.
• Six learning criteria are evaluated on handwritten Chinese text recognition tasks.

Semi-Markov conditional random fields (semi-CRFs) are usually trained with maximum a posteriori (MAP) criterion which adopts the 0/1 cost for measuring the loss of misclassification. In this paper, based on our previous work on handwritten Chinese/Japanese text recognition (HCTR) using semi-CRFs, we propose an alternative parameter learning method by minimizing the risk on the training set, which has unequal misclassification costs depending on the hypothesis and the ground-truth. Based on this framework, three non-uniform cost functions are compared with the conventional 0/1 cost, and training data selection is incorporated to reduce the computational complexity. In experiments of online handwriting recognition on databases CASIA-OLHWDB and TUAT Kondate, we compared the performances of the proposed method with several widely used learning criteria, including conditional log-likelihood (CLL), softmax-margin (SMM), minimum classification error (MCE), large-margin MCE (LM-MCE) and max-margin (MM). On the test set (online handwritten texts) of ICDAR 2011 Chinese handwriting recognition competition, the proposed method outperforms the best system in competition.