Abstract: Under the premise of ensuring the quality of the marking, the online marking system not only greatly reduces the workload of the teachers, but also lowers the requirement of high quality exam paper and saves energy. However, the results produced by automatic judgment by the online marking system for objective questions heavily depends on the image quality of high-speed scanning and layout of the exam answer sheets. This article proposes a robust recognition algorithm of objective questions for exam answer sheets. Firstly, considering the possibility that the user may deviate from the filling area, or the possible error in the match between the present image and template image, a sliding window strategy is proposed to relocate the practical filling area in order to eliminate the related deviation. Then the histogram of each option is calculated, and a weighted average intensity is introduced to remove the effect of the uneven filling between different options. The comparison between each option for the same question enables the recognition algorithm to have strong local adaptability. At the same time, this strategy overcomes the difficulty of parameter selection caused by the global recognition strategy. The experimental results show that owing to good compatibility, our algorithm is suitable for different typesetting types of exam answer sheets in the recognition of objective questions. In addition, the algorithm is characteristic of high recognition accuracy as well as strong robustness, thus applicable to exam answer sheets of varying scanning quality and different filling quality.

Key words:  online marking system, recognition algorithm of objective questions, sliding window, weighted average intensity