The image processing part of the workshop focuses on the Particle Image Velocimetry (PIV) technique. While this method is widely used for flow visualization, it can be also adapted for tracking discrete particles in granular flows. PIV yields instantaneous velocity field measurements in a cross-section of the flow. We start from the basic principles underlying PIV. The goal is to learn how to develop an own PIV image processing code, avoid the possible sources of errors, and adapt it for tracking discrete passive/active particles in biological environments. The techniques developed for volumetric PIV will be also discussed.