A hierarchical, HMM-based automatic evaluation of OCR accuracy for a digital library of books

A number of projects are creating searchable digital libraries of printed books. These include the Million Book Project, the Google Book project and similar efforts from Yahoo and Microsoft. Content-based on line book retrieval usually requires first converting printed text into machine readable (e.g. ASCII) text using an optical character recognition (OCR) engine and then doing full text search on the results. Many of these books are old and there are a variety of processing steps that are required to create an end to end system. Changing any step (including the scanning process) can affect OCR performance and hence a good automatic statistical evaluation of OCR performance on book length material is needed. Evaluating OCR performance on the entire book is non-trivial. The only easily obtainable ground truth (the Gutenberg e-texts) must be automatically aligned with the OCR output over the entire length of a book. This may be viewed as equivalent to the problem of aligning two large (easily a million long) sequences. The problem is further complicated by OCR errors as well as the possibility of large chunks of missing material in one of the sequences. We propose a Hidden Markov Model (HMM) based hierarchical alignment algorithm to align OCR output and the ground truth for books. We believe this is the first work to automatically align a whole book without using any book structure information. The alignment process works by breaking up the problem of aligning two long sequences into the problem of aligning many smaller subsequences. This can be rapidly and effectively done. Experimental results show that our hierarchical alignment approach works very well even if OCR output has a high recognition error rate. Finally, we evaluate the performance of a commercial OCR engine over a large dataset of books based on the alignment results.