1. In probe-based pattern matching, a method for locating patterns, the method comprising: providing a database that comprises: a probe-based pattern model having: a plurality of positive probes at selected points along a boundary of the pattern, at least one imaginary straight segment parallel to a segment of the boundary of the pattern, and a plurality of negative probes at selected points along the imaginary straight segment, each negative probe having a negative weight; and providing a computer programmed to perform the step of: using the probe-based pattern model, and probe-based pattern matching, to locate at least one instance of the pattern in an image.