  Particle Image Velocimetry (PIV) is the most commonly used optical technique
for measuring 2D velocity fields. However, improving the spatial resolution of
instantaneous velocity fields and having access to the velocity field in real
time remains a challenge. Optical Flow veolcimetry makes it possible to meet
these challenges. In this study, we show that it is possible to access dense
velocity fields (1 vector per pixel) in real-time using an appropriate seeding
concentration adapted to optical flow algorithms and no longer to
cross-correlation PIV algorithms. The influence of concentration on the quality
of velocity fields is demonstrated using synthetic images generated for a
Rankine vortex. We thus demonstrate that it is possible to precisely measure
small vortices using optical flow provided that the seeding is suitable. The
notion of "Active Pixels" is also introduced in order to define a seeding
optimization criterion adapted to experimental measurements. This criterion is
finally successfully applied to the flow downstream of a cylinder leading to a
spatial resolution down to one vector per pixel.
