Hatch patterns are commonly used in graphics software to highlight distinct areas of a map or drawing. There are three general techniques for filling in polygonal areas on a software canvas:
This page is dedicated to the discussion of the repeated vector pattern.
Official http://docs.autodesk.com/ACD/2010/ENU/AutoCAD%202010%20User%20documentation/index.html?url=WS73099cc142f4875513fb5cd10c4aa30d6b-7e57.htm,topicNumber=d0e368209%|%documentation%|% from the Autodesk website for the popular commercial hatch definition format.
Hatch pattern definitions are defined in ASCII text .pat file. There are many free definition files available at various places (here , for example) around the web, though there doesn't seem to be any truly "open source" .pat files available.
Examples of hatch patterns, rendered with tcl and pdf4tcl .