MatPIV is a free Matlab-toolbox for Particle Image Velocimetry.
MatPIV is one of a number of free, open source PIV toolboxes for Matlab. It also happens to be one of the oldest and most widely used.
Particle Image Velocimetry is based on the principles of pattern matching. The displacement of local patterns in an image may be found by calculating the (local) similarity (or minimal difference) between two successive images.