In this paper, we develop an on-line inspecting system for invoice printing and introduce some techniques for automatic recognition of printed digits on invoices. The poor quality of an invoice image, the textured background, and the low resolution of each printed digit make the research challenging. To overcome these problems, a robust method based on a minimal gray-level analysis is proposed for preprocessing the invoice images. The preprocessing includes the number block extraction, textured background removal, and digit image enhancement. For each digit image of 8 * 8 pixels, a row-based method is developed to extract the features of the digit, which is then applied to a tree classifier for recognition. For the prototype system developed in the laboratory, a recognition rate of 95% can be achieved.
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