In this article, a technique to relax particle image velocimetry (PIV) based on the analysis of motion blurring of particles is proposed in order to reduce the frame rate required for measurement of particle velocities. Another advantage is the relaxation of the requirements of the measurement setup while retaining the principle of measurement analysis. This technique is based on convolutions with 5×5 spatial filters that can be computed in linear time, independently from the contents of the image. Due to multi-scale processing, the technique adapts its parameters autonomously, according to the image and particle properties to provide accurate results. The accuracy of the results is guaranteed by integrating the filter parameters, the measured angle and eventually the measured size of the particles into a linear model. The low complexity and high parallelizability of this method enable the online measurement of particles' velocities.