KLT is an implementation, in the C programming language, of a feature tracker for the computer vision community. The tracker is based on the early work of Lucas and Kanade [1], was developed fully by Tomasi and Kanade [2], and was explained clearly in the paper by Shi and Tomasi [3]. Later, Tomasi proposed a slight modification which makes the computation symmetric with respect to the two images -- the resulting equation is derived in the unpublished note by myself [4].
Briefly, good features are located by examining the minimum eigenvalue of each 2 by 2 gradient matrix, and features are tracked using a Newton-Raphson method of minimizing the difference between the two windows. Multiresolution tracking allows for relatively large displacements between images.
The affine computation that evaluates the consistency of features between non-consecutive frames [3] was implemented by Thorsten Thormaehlen several years after the original code and documentation were written. To use this feature, please see the comments and declarations in the source code.
Note: KLT is now in the public domain. Stanford's Office of Technology Licensing has removed the non-commercial restriction.