Fast image template and dictionary matching algorithms

Abstract

Given a large text image and a small template image, the Template Matching Problem is that of finding every location within the text which looks like the pattern. This problem, which has recieved attention for low-level image processing, has been formalized by defining a distance metric between arrays of pixels and finding all subarrays of the large image which are within some threshold distance of the template. These so-called metric methods tends to be too slow for many applications, since evaluating the distance function can take too much time.

We present a method for quickly eliminating most positions of the text from consideration as possible matches. The remaining candidate positions are them evaluated one by one against the template for a match. We are still guaranteed to find all matching positions, and our method gives significant speed-ups.

Finally, we consider the problem of matching a dictionary of templates against a text. We present methods which are much faster than matching the templates individually against the input image.