Image correlation is the key step of Particle Imaging Velocimetry. This is where successive images are evaluated and displacements are calculated. How does it work?
The images are separated into numerous smaller sub-areas or interrogation windows. It is the interrogation windows that are correlated not the entire images. The correlation of an interrogation image with another in the successive image yields one single vector, assigned to the point at the center of the interrogation area. When the image is separated into NxM interrogation areas along its x and y axes, we obtain NxM vectors.
To obtain a denser velocity grid, the interrogation areas may be overlapping. A common value is 50% overlap, but I have used up to 75% when employing a single pass procedure.