Towards More Effective Distance Functions for Word Image Matching

摘要

Matching word images has many applications in document recognition and retrieval systems. Dynamic Time Warping (DTW) is popularly used to estimate the similarity between word images. Word images axe represented as sequences of feature vectors, and the cost associated with dynamic programming based alignment is considered as the dissimilarity between them. However, such approaches are computationally costly when compared to fixed length matching schemes. In this paper, we explore systematic methods for identifying appropriate distance metrics for a given database or language. This is achieved by learning query specific distance functions which can be computed online efficiently. We show that a weighted Euclidean distance can outperform DTW for matching word images. This class of distance functions are also ideal for scalability and large scale matching. Our results are validated with mean Average Precision (mAP) on a fully annotated data set of 160K word images. We then show that the learnt distance functions can even be extended to a new database to obtain accurate retrieval.